Download Forces

Forces
Table of Contents
The Nature of Force
Friction and Gravity
Newton’s First and Second Laws
Newton’s Third Law
Rockets and Satellites
Forces
Learning Objectives
1. Describe what a force is.
2. Describe how balanced and unbalanced forces are
related to an object’s motion.
Forces
What is a Force? (No lab write-up)
Goal: Determine more than one way to create a force reading using
the force sensor.
Results: What were the ways you used to create a force? How did
or would you create a weak force? A stronger force? Can forces be
negative? How did you get a negative force?
Conclusions: Based upon these ways, what is a force? Does a
force depend on direction? How do you know from the experiment?
How else is a force described outside of direction?
Forces - The Nature of Force
Combining Forces
(Real World Examples + Tug-of-War Challenge)
Do forces usually act alone? The combination of all forces
acting on an object is called the net force.
Forces - The Nature of Force
Unbalanced Forces
Unbalanced forces acting on an object result in a net force
and cause a change in the object’s motion.
Forces - The Nature of Force
Balanced Forces
Balanced forces acting on an object do not change the
object’s motion.
Forces
A force is…
A.
B.
C.
D.
Always balanced.
A push or a pull in a particular direction.
A pull that pushes.
What Jedi Knights possess.
Forces
A force is measured by its
A.
B.
C.
D.
Power only.
Strength or magnitude only.
Strength or magnitude and direction.
Direction and its pull.
Forces
A sky diver experiences a 900 N downward force
and a 200 N upward force. What is the net force (in
the correct direction)?
A.
B.
C.
D.
700 N up
700 N down
1100 N up
1100 N down
Forces
Which direction will the cart below move?
A.
B.
C.
D.
The cart will not because the forces are balanced.
Left because the net force is -90 N to the left.
Right because the net force is 90 N to the right.
Up because the cart is being pulled to the left and to the
right.
Forces
The two dogs in the picture below are pulling on a
rope. The dog on the left is pulling at 200 N while
the dog on the right is pulling at 200 N. What
direction will the rope move?
A.
B.
C.
D.
South because the forces are balanced.
West because the dog on the left is pulling harder.
East because the dog on the right is stronger.
The rope will not move because the forces are balanced.
Forces - The Nature of Force
Asking Questions
Before you read, preview the red headings. In a graphic
organizer like the one below, ask a what or how question for
each heading. As you read, write answers to your questions.
Question
Answer
What is a force?
A force is a push or pull.
What happens when forces
combine?
Forces combine to produce a
net force.
Forces - The Nature of Force
Links on Force
Click the SciLinks button for links on force.
Forces
End of Section:
The Nature of
Force
Forces
Noggin Knockers/Homework- p. 39: 1b, 1c, 2b, 2c
(8 points total- 2 points each)
1 (1 point per part for 2 pts. total)- (A) Balanced forces do not
affect an object’s motion because they cancel out. (B)
Unbalanced forces will cause the object to move or change it’s
motion.
2 (2 points)- Arrow (B) because it is longer.
3 (2 points)- 120 N + 150 N = 270 N downward (strength &
direction)
4 (2 points)- 200 N – 100 N = 100 N to the right
Forces
Learning Objective
1. Describe friction and identify the factors that determine
the frictional force between 2 objects.
Forces
Friction Experiments
Goal: Determine what 2 factors affect the frictional force between two objects.
Also, determine how friction can be reduced.
Hypothesis (x3): For Part 1, determine which surface (floor or rug) will produce the
greatest frictional force. For Part 2, determine how the added weight will affect the
frictional force. For Part 3, determine how adding wheels will affect the frictional
force (w/o weights).
Procedure (Use the Digits function, not the Graph function):
Part 1- Push an upside-down go-car (wheels up) across the floor using the force
sensor to determine the frictional force that needs to be OVERCOME so the car can
move at a constant speed. Make sure you ZERO the sensor before you start
pushing. Repeat the procedure above with the rug.
Part 2- Add weight to the go cars and repeat Part 1.
Part 3- After removing the weights, flip the go-car over so that the wheels are on the
floor and repeat Part 1.
Results: Organize your results for each part of the procedure.
Forces
Friction Experiment
Conclusions (in complete sentences):
1. Which surface produced a greater frictional force? Explain why this is
the case.
2. What was the effect of extra weight on the frictional force? What was
the effect of wheels on the frictional force?
3. Make your claim/conclusion (make sure it relates to the goal below),
and back it up with evidence from the experiment. Use your
conclusions from #1 and #2 above for guidance.
Goal: Determine what 2 factors affect the frictional force between two
objects. Also, determine how friction can be reduced.
Forces
Friction Notes
For friction notes, examine your conclusions from the Friction
Experiment.
Friction is a force that resists or inhibits the motion of
objects when they move against each other.
Also, you should know that the type of surface of an object
and that how hard surfaces push together affect friction
(you increased the 2nd variable in bold by adding more
weight).
Lastly, know how wheels affect friction as well as any other
ways to reduce friction.
Forces
Markie and Suzy are moving a safe across the floor.
Which of the following describes the frictional
force?
A.
B.
C.
D.
Markie and Suzy both push the safe the same direction.
The weight of the safe exerts a downward force.
The floor pushes upward on the safe.
There is some resistance going against the direction the
safe is moving.
Forces
Which change would require more force to pull the
wooden block up the ramp?
A.
B.
C.
D.
Glue sandpaper to the surface of the ramp.
Reduce the mass of the wooden block.
Restack the books so the thinnest book is on the bottom.
Have the student use two hands.
Forces
Which of the following would produce the greatest
frictional force between 2 objects?
A.
B.
C.
D.
A 100 kg wooden block sliding across a smooth table.
A 200 kg wooden block sliding across a smooth table.
A 100 kg wooden block sliding across a rug.
A 200 kg wooden block sliding across a rug.
Forces
The diagrams below show a
person moving a 50-kilogram
object up a ramp. In which
diagram is there the least
amount of friction on the object?
A.
B.
C.
D.
Diagram 1
Diagram 2
Diagram 3
Diagram 4
Forces
The wheels and gears of machines are greased in
order to decrease
A.
B.
C.
D.
Potential energy.
Output.
Efficiency.
Friction.
Forces
Learning Objectives
1. Identify the factors that affect the gravitational force
between two objects, and describe how they affect
this force.
2. Explain why objects accelerate during free fall (by
describing how gravity affects falling objects).
Forces
Gravitational Pull Experiment- Part 1
(write conclusions in your notes)
Goal- Determine what affects the gravitational force or
attraction between 2 objects by using magnets & Planet
Data.
Background: This brief experiment is only a MODEL of the
gravitational pull between 2 objects.
Results: Determine what happens to the force of attraction
between the magnets as they get closer together.
Conclusion: What affects the gravitational pull between 2
objects?
Forces - Friction and Gravity
Gravity
The force of gravity on a person or object at the surface of a planet is
known as weight.
Right now you are experiencing 1G, which is the amount of force due to
the gravity on Earth.
When you ride on a rollercoaster, you will experience more than 1G.
Sometimes, you are feeling 3, 4, or 5 G’s due to the acceleration of the
rollercoaster.
Forces
Gravitational Pull Experiment- Part 2:
Mass & Gravitational Pull of the Planets
Name
Mass (X 1024 kg)
Gravitational force on the surface
(relative to Earth)
0.1
Pluto
0.0013
Moon
0.07
0.2
Mars
0.64
0.4
Earth
5.98
1.0
Jupiter
1900
2.4
Sun
1,989,000
7.1
Conclusion: So, what else affects the gravitational pull between 2 objects?
Forces - Friction and Gravity
Gravity
Two factors affect the gravitational attraction between
objects: mass and distance.
Forces
Which two factors determine the gravitational
attraction between two objects?
A.
B.
C.
D.
time and temperature
shape and orbital speed
color and hardness
mass and distance apart
Forces
The gravitational force between the Moon and Earth
depends on
A.
B.
C.
D.
Their masses only.
Their diameters only.
Their masses and how far apart they are.
Their diameters and how far apart they are.
Forces
Suppose three new planetary bodies are discovered
near Earth. They all have the same mass. Planet A
is 9 million miles away, Planet B is 7 million miles
away, and Planet C is 30 million miles away. Which
planet would Earth have the greatest gravitational
attraction for?
A.
B.
C.
D.
Planet A
Planet B
Planet C
Planet D
Forces
On which planet in our solar system would you feel
the most gravitational pull?
A.
B.
C.
D.
Earth
Saturn
Jupiter
Mars
Name
Mass (X 1024 kg)
Earth
Mars
Jupiter
Pluto
Moon
Sun
5.98
0.64
1900
0.0013
0.07
1,989,000
Gravitational force on the surface
(relative to Earth)
1.0
0.4
2.4
0.1
0.2
7.1
Forces
Why does the Sun have the greatest gravitational
pull?
A.
B.
C.
D.
It has the lowest mass.
It has the most mass.
It’s the closest to the Earth.
It’s gravitational force is 7.1 times that of Earth.
Name
Mass (X 1024 kg)
Earth
Mars
Jupiter
Pluto
Moon
Sun
5.98
0.64
1900
0.0013
0.07
1,989,000
Gravitational force on the surface
(relative to Earth)
1.0
0.4
2.4
0.1
0.2
7.1
Forces
Suppose a new planet was discovered that had a
mass of 4.00 (x 1024 kg). What would be a possible
gravitational force for this new planet?
A.
B.
C.
D.
0.85
0.3
1.4
8.0
Name
Mass (X 1024 kg)
Pluto
Moon
Mars
Earth
Jupiter
Sun
0.0013
0.07
0.64
5.98
1900
1,989,000
Gravitational force on the surface
(relative to Earth)
0.1
0.2
0.4
1.0
2.4
7.1
Forces
Motion Test: Commonly Missed Problems
Forces
Sneaker Traction Friction Experiment
(Just record your group’s data)
Goal: Determine which sneakers have the best traction (by
examining the frictional force).
Background: What will you be measuring that indicates good
traction?
Procedure: Same as Part 1 from the previous Friction
Experiment (minus the rug). Record the frictional force on the
SMART Board for the group member who had the shoes with
the most traction.
Results: Organize your results into a table for your group.
Add the other groups’ data into your table.
Conclusion: Make your claim (related to the goal) & back it
up with evidence and relate it to what affects the frictional
force between 2 objects from the previous experiment.
Forces
Learning Objectives
1. Explain why objects accelerate during free fall (by
describing how gravity affects falling objects).
Forces
Acceleration Due to Gravity (No lab write-up)
Goal: Determine the acceleration due to gravity for objects in free fall.
Pre-Lab Demos (Dropping Paper and a Ball): Determine if objects of different
masses accelerate slower or faster by observing when they hit the ground. Does
the mass of an object change how fast it free falls? Why or why not? So, does
the force due to gravity affect all objects on Earth the same?
Hypothesis: We will do the hypothesis as a class verbally.
Procedure:
1. Use a motion sensor from a high height (on the Digits and Acceleration settings)
& drop a ball (AFTER HITTING PLAY) to determine the Acc. due to Gravity.
2. Repeat 3-5 times and RECORD the acceleration right before the ball hits the
ground (hit play to pause the data collection).
Results/Conclusions (Make sure you can answer these questions):
1. What is the acceleration due to gravity from the experiment?
2. The actual value is
m/s/s. How close were you to the actual value?
3. What may cause the ball to not accelerate as quickly (what could slow it down)?
Forces - Friction and Gravity
Free Fall = When gravity is the only force acting on
the object
Use the graph to answer the
following questions.
Forces - Friction and Gravity
Free Fall
Calculating:
Calculate the slope of the
graph. What does the slope
tell you about the object’s
motion?
The slope is 9.8. The speed
increases by 9.8 m/s each
second. This is the actual
acceleration due to gravity on
Earth without any air
resistance.
Forces - Friction and Gravity
Free Fall
Predicting:
What will the speed of the
object be at 6 seconds?
58.8 m/s
Forces - Friction and Gravity
Free Fall
Drawing Conclusions:
Suppose another object of the
same size but with a greater
mass was dropped instead.
How would the speed values
change?
The speed values would not
change.
Forces - Friction and Gravity
Air Resistance
Falling objects with a greater surface area experience more
air resistance.
Forces
Two objects are dropped from the top of a tall
building and there is no wind. One object is a 16 lb.
bowling ball and the other is a basketball. What will
be each object’s approximate acceleration when
they are dropped?
A. 9.8 m/s/s for the basketball and 8.0 m/s/s for the bowling
ball.
B. 0.0 m/s/s for both objects.
C. 9.8 m/s/s for both objects.
D. 9.8 m/s/s for the bowling ball and 4.9 m/s/s for the
basketball
Forces
When gravity is the only force acting on an object,
then that object is
A.
B.
C.
D.
Moving at a constant velocity.
Not accelerating.
In free fall.
Moving quickly.
Forces
Suppose a large rock is dropped straight down from
a high cliff while the other is pushed out from the
top of the cliff. Which one will land first and WHY?
A. The rock that is pushed out from the cliff will hit first
because it had an extra force to make it fall faster.
B. The rock that’s dropped straight down will hit first
because it has a shorter path to travel.
C. They will both hit the ground at the same time because
gravity acts the same on all free-falling objects.
D. Neither will hit the ground.
Forces
Why will a leaf fall slowly to the ground while other
objects fall quickly?
A.
B.
C.
D.
It’s lighter.
Air resistance
Gravity
They just do.
Forces
Air resistance is a type of
A.
B.
C.
D.
band.
gravity.
friction.
Law.
Forces - Friction and Gravity
Comparing and Contrasting
As you read, compare and contrast friction and gravity by
completing a table like the one below.
Friction
Gravity
Effect on motion
Opposes motion
Pulls objects toward one
another
Depends on
Types of surfaces
involved, how hard the
surfaces push together
Mass and distance
Measured in
Newtons
Newtons
Forces - Friction and Gravity
Links on Friction
Click the SciLinks button for links on friction.
Forces - Friction and Gravity
Free Fall
Click the Video button to watch a movie about free fall.
Forces
End of Section:
Friction and
Gravity
Forces
Noggin Knockers/Hwk.- p. 50: 1b, 2b, 2c, 3a, 3b, 3c (10
pts.)
1 (a)- Push = Arrow A (1 pt.)
(b) Friction = Arrow C (1 pt.)
(c) Gravity = Arrow D (1 pt.)
(d) 300 N to the left. (2 points- 1 pt. for value, 1 pt. for direction)
2 (a) More mass = Greater Grav. Attraction (1 point)
(b) Closer together/less distance = Greater Grav. Attraction (1 point)
3 (1 point)- An object’s mass does not affect its acceleration during free
fall.
4a (1 point)- Force that changes = air resistance/friction
4b (1 point)- Force that stays constant = gravity
Forces
Learning Objectives
1. Apply Newton’s 1st Law of Motion to real world
examples.
• Key Term: Inertia
2. Apply Newton’s 2nd Law of Motion to real world
examples.
• Key Terms: Force, Mass, Acceleration
Forces
Introduction to Newton’s 1st Law
What happens to your body when a car or rollercoaster first
takes off quickly?
What happens to your body when a car or rollercoaster stops
abruptly?
Forces
Newton’s 1st Law of Motion
Objects in Motion (Pencil on a cart being stopped):
Stay in motion until a force stops them from moving.
Objects at Rest (Tablecloth Demo):
Stay at rest until an outside force causes the object to move.
Inertia:
The tendency of an object to stay in motion or stay at rest.
What does inertia depend on? Example- Which has more
inertia a toy car or a large truck?
As mass increases, inertia increases.
Newton’s 1st Law:
An object in motion will stay in motion unless acted on by an
unbalanced force. Also, an object at rest will remain at rest
unless acted on by an unbalanced force.
Forces
Why is it difficult to stop a large fast-moving
football player?
A. He has a small inertia.
B. An object at rest will stay at rest until an unbalanced force
causes it to move.
C. An object in motion will stay in motion until an unbalanced
force acts on it.
D. Because he plays for Notre Dame.
Forces
Why is it difficult to move heavy objects from rest?
A. Because they are at rest.
B. An object at rest tends to remain at rest until an
unbalanced force acts on it.
C. An object in motion will remain in motion until an
unbalanced force acts on it.
D. Because it’s light in weight.
Forces
The tendency of an object to remain at rest or
remain in motion is called ___________
A.
B.
C.
D.
Mass.
Inertia.
Newton’s 1st Law.
Density.
Forces
Which of the following affects the amount of inertia
an object possesses?
A.
B.
C.
D.
Mass
Volume
Gravity
Friction
Forces
Which of the following has the most inertia?
A.
B.
C.
D.
A table
Grandma
A Toyota Yaris
A freight train
Forces
What Affects Force? (Newton’s 2nd Law)
1. Suppose you push two people who have the same mass. You
push with more force on one person vs. the other. Which person
would accelerate faster from rest?
2. Suppose you push two people and they both accelerate at the
same rate. However, one person has much more mass than the
other. Which one required more force to accelerate?
3. Suppose you push two people with the same amount of force.
However, one person has more mass than the other. Which
person would accelerate at a faster rate?
Forces
Newton’s 2nd Law Experiment
Goal: Determine the relationship between Force, mass, and acceleration for an
object (a Go-car in this case).
Hypothesis (x 3):
1. What has to happen to the force required to move an object if acceleration
increases (& mass is constant)?
2. What would happen to the force if mass increases (& acceleration is constant)?
3. What would happen to the acceleration if mass increases (& the force is
constant)?
Procedure (use Digits):
Part 1- Keeping mass on the cart constant, determine the relationship between force
and acceleration. (Determine the force needed to accelerate the car slowly
and compare to the force needed to accelerate the car quickly)
Part 2- Keeping acceleration constant, determine the relationship between mass and
force. (Determine the force needed to move the car from rest to a constant
speed with weights and compare to force required without weights at a
constant speed). This can be tricky to achieve so as long as the acc. Is close
it’s okay.
Part 3- Keeping force constant, determine the relationship between mass and
acceleration. (Determine the acceleration with and without weights but keep
the force the same)
Results: Organize your results in a table for EACH part.
Forces
Newton’s 2nd Law Experiment
Conclusions (Using your data…)
1. What was the relationship for each part? For example, as
acceleration increased, the force (increased or decreased). Use data
from the experiment back up your claims.
a) Force and acceleration (constant mass)- from Part 1 Data
b) Force and mass (constant acceleration)- from Part 2 Data
c) Mass and acceleration (constant force)- from Part 3 Data
2. Which of the equations below represents the relationships you
discovered? Choose 2- one from (a) or (b) and one from (c) or (d).
[F is force, m is mass, and a is acceleration]
a) F = m/a
b) F =m x a
c) a = F x m
d) a = F/m
Forces
F=mxa
Acceleration due to gravity & Constant force
Force, mass, and acceleration units:
1 N = 1 kg x m/s/s
a=
=
Acc.
Due to
Gravity
Forces - Newton’s First and Second Laws
Calculating Force
A speedboat pulls a 55-kg water-skier. The force causes the skier
to accelerate at 2.0 m/s2. Calculate the net force that causes this
acceleration.
Read and Understand
What information have you been given?
Mass of the water-skier (m) = 55 kg
Acceleration of the water-skier (a) = 2.0 m/s2
Forces - Newton’s First and Second Laws
Calculating Force
A speedboat pulls a 55-kg water-skier. The force causes the skier to
accelerate at 2.0 m/s2. Calculate the net force that causes this acceleration.
Plan and Solve
What quantity are you trying to calculate?
The net force (Fnet) = __
What formula contains the given quantities and the unknown
quantity?
a = Fnet/m or Fnet = m X a
Perform the calculation.
Fnet = m X a = 55 kg X 2.0 m/s2
F = 110 kg • m/s2
F = 110 N
Forces - Newton’s First and Second Laws
Calculating Force
A speedboat pulls a 55-kg water-skier. The force causes the skier to
accelerate at 2.0 m/s2. Calculate the net force that causes this
acceleration.
Look Back and Check
Does your answer make sense?
A net force of 110 N is required to accelerate the water-skier. This
may not seem like enough force, but it does not include the force of
the speedboat's pull that overcomes friction.
Forces - Newton’s First and Second Laws
Calculating Force
Practice Problem
What is the net force on a 1,000-kg object accelerating
at 3 m/s2?
3,000 N (1,000 kg X 3 m/s2)
Forces - Newton’s First and Second Laws
Calculating Acceleration
Practice Problem
What is the acceleration of an object in free fall with a
weight of 98 N and a mass of 10 kg? Does your answer
make sense? Note that weight is the force due to gravity!
a = F/m
a = 98 N/10 kg = 9.8 m/s2
Forces - Newton’s First and Second Laws
Calculating Acceleration
Practice Problem
What is the acceleration of a 100 kg couch being pulled
across the floor with a force of 200 N?
a = F/m
a = 200 N/100 kg = 2 m/s2
Forces
Noggin Knockers
Forces - Newton’s First and Second Laws
Calculating Acceleration
Practice Problem
Determine the amount of force required to accelerate a
1000 kg roller coaster at 5 m/s/s?
F=mxa
F = 1000 kg x 5 m/s/s = 5000 N or 5000 kg x m/s/s
Forces - Newton’s First and Second Laws
Calculating Acceleration
Practice Problem
Determine the weight of 50 kg person. Hint- You should
already know the acceleration since weight is the force
of gravity acting on your body.
F=mxa
F = 50 kg x 9.8 m/s/s = 490 N
Forces - Newton’s First and Second Laws
Calculating Acceleration
Practice Problem
What is the acceleration of a 2 kg ball thrown with a
force of 20 N?
a = F/m
a = 20 N/2 kg = 10 m/s2
Forces
Which of the following best describes the
relationships between force, mass, and
acceleration?
A. As mass decreases, the net force will increase if the
acceleration remains constant.
B. As mass and acceleration increase together, so will the
net force.
C. As mass increases so will the acceleration, but force will
remain constant.
D. As acceleration increases and the mass remains
constant, the net force will decrease.
Forces
Suppose you are pushing a car with a certain
amount of force. What will happen to the car’s
acceleration if the mass of the car increases?
Assume the amount of applied force remains the
same.
A.
B.
C.
D.
The acceleration will increase.
The acceleration will decrease.
The acceleration will remain the same.
There is not enough information from the problem.
Forces
Suppose a small car strikes a wall. Which of the
following would exert the same amount of force as
the car?
A. A truck that’s twice as heavy and has twice the
acceleration of the car.
B. A truck that’s half as heavy with the same acceleration of
the car.
C. A truck that’s the same mass with half of the acceleration
of the car.
D. A truck that’s twice as heavy with half of the acceleration
of the car.
Forces
How much force does a 100 kg Notre Dame football
player exert when he accelerates at 5 m/s/s into a
Nittany Lion?
A.
B.
C.
D.
500 N
20 N
100 N
5N
Forces
Determine the amount of force exerted on a 10 kg
dumbbell in free-fall.
A.
B.
C.
D.
9.8 N
10 N
98 N
It cannot be determined.
Forces
A force of 500 N is exerted on a 100 kg safe. What
is the safe’s acceleration?
A.
B.
C.
D.
0.2 m/s/s
50,000 m/s/s
9.8 m/s/s
5 m/s/s
Forces - Newton’s First and Second Laws
Outlining
As you read, make an outline
about Newton’s first and
second laws. Use the red
headings for the main topics
and the blue headings for the
subtopics.
Newton’s First and Second Laws
I. Newton’s First Law of Motion
A. Inertia
B. Inertia Depends on Mass
II. The Second Law of Motion
A. Changes in Force and Mass
Forces - Newton’s First and Second Laws
More on Newton’s Laws
Click the PHSchool.com button for an activity
about Newton’s laws.
Forces
End of Section:
Newton’s First
and Second
Laws
Forces
Noggin Knocker Calculations
Forces
Homework: p. 54 (1a, 1c, 2 all, and 3)
1a- An object will remain at rest or remain in motion until acted on
by an unbalanced force.
1c- Your body wants to remain at rest.
2a- Examples: A force is required to get an object to accelerate, the
force an object exerts is equal to its mass multiplied its
acceleration, etc.
2b- Double the mass.
2c- Greater mass means more force is required to get the car to
accelerate.
3- F = m x a = 800 kg x 5 m/s/s = 4000 N
Forces
Noggin Knocker Calculations
Forces
Noggin Knockers/Hwk. (8 pts.- 2 pts each)
1- Your body wants to remain at rest.
2- Double the mass.
3- Greater mass means more force is required to get the car to
accelerate.
4- F = m x a = 800 kg x 5 m/s/s = 4000 N
Forces
Learning Objectives
1. Apply Newton’s 3rd Law of Motion to real world
examples.
Forces
Newton’s 3rd Law
Balloon Race Activity: Determine what makes
the balloon propel across the room based upon
the forces involved.
Newton’s 3rd Law:
For every action, there is an equal and opposite
reaction.
What are the action-reaction forces for the
following:
–Rocket taking off?
–Punching a wall?
–Jumping from a high height to the ground?
– Hammering a nail?
Forces
Forces on an object moving to the left
Reaction Force/Floor
or air pushing up
Friction
Push
or Pull
Gravity
Forces
Which of the following is an example of Newton’s
3rd Law of Motion?
A. When you do push-ups, you won’t stop until an
unbalanced force causes you to stop.
B. The net force you exert when doing a push-up is equal to
your body mass multiplied by the acceleration due to
gravity.
C. When you do a push-up, an equal and opposite force
pushes back on your body or hands.
D. What’s a push-up?
Forces
What direction will the contraption shown below go
when the string is burned?
A.
B.
C.
D.
left
right
up
down
Forces
Why will the contraption below move to the right?
A. Friction is going to the left
B. The rubber band snaps to the left, so the equal and opposite
reaction is for the wooden platform to move to the right.
C. Gravity pulls the wooden platform to the right when the rubber
band snaps.
D. The pencils cause the wooden platform to move to the right
because friction acts in the opposite direction.
Forces
If you add mass to the wooden platform, what effect
would that have on the distance it travels?
A.
B.
C.
D.
The distance would increase.
The distance would decrease.
The distance would not change.
It would travel the same distance in the opposite
direction.
Forces
Learning Objectives
1. Explain how an object’s momentum is conserved & be
able to calculate momentum.
2. State the law of conservation of momentum.
Forces
Conservation of Momentum
Momentum- Use what you think momentum
is and modify your definition based upon
what is discussed in class (if needed).
Newton’s Cradle: Is the total momentum
lost or transferred for this activity? How do
you know?
Bouncing Sports Balls: What makes the
sports balls on top bounce as high as they
do?
Colliding Cars with Tape: Is the total
momentum lost or transferred for this
activity? How do you know? Be sure to
think about how the direction of the cars
affect this experiment.
Forces - Newton’s Third Law
Conservation of Momentum
Law of the Conservation of Momentum (mass in motion):
The total momentum is conserved (or does not change)
for any group of objects, unless outside forces (such as
friction) act on the objects.
Forces
Relationship between Momentum, Mass, & Velocity
What causes an
object’s momentum
to increase?
How does velocity
affect momentum?
How does mass affect
momentum?
Momentum = m x v
Forces - Newton’s Third Law
Calculating Momentum
Which has more momentum: a 3.0-kg sledgehammer swung at 1.5
m/s or a 4.0-kg sledgehammer swung at 1.0 m/s?
Read and Understand
What information have you been given?
Mass of smaller sledgehammer = 3.0 kg
Velocity of smaller sledgehammer = 1.5 m/s
Mass of larger sledgehammer = 4.0 kg
Velocity of larger sledgehammer = 1.0 m/s
Forces - Newton’s Third Law
Calculating Momentum
Which has more momentum: a 3.0-kg sledgehammer swung at 1.5
m/s or a 4.0-kg sledgehammer swung at 1.0 m/s?
Plan and Solve
What quantities are you trying to calculate?
The momentum of each sledgehammer = __
What formula contains the given quantities and the unknown
quantity?
Momentum = Mass X Velocity
Perform the calculation.
Smaller sledgehammer = 3.0 kg X 1.5 m/s = 4.5 kg x m/s
Larger sledgehammer = 4.0 kg X 1.0 m/s = 4.0 kg x m/s
Forces - Newton’s Third Law
Calculating Momentum
Which has more momentum: a 3.0-kg sledgehammer swung at 1.5
m/s or a 4.0-kg sledgehammer swung at 1.0 m/s?
Look Back and Check
Does your answer make sense?
The 3.0-kg hammer has more momentum than the 4.0-kg one. This
answer makes sense because the 3.0-kg hammer is swung at a
greater velocity.
Forces
The total momentum before and after a collision (in
the absence of friction) is always
A.
B.
C.
D.
Constipated.
Conserved.
Different.
Reserved.
Forces
Suppose the total momentum before 2 cars collide
is 200 kg x m/s. What is total momentum after the
collision (in the absence of friction)?
A.
B.
C.
D.
100 kg x m/s
0 kg x m/s
200 kg x m/s
There is not enough information in the question.
Forces
Which of the following would have the most
momentum: a 1000 kg car moving at 10 m/s or a
1000 kg car moving at 20 m/s?
A.
B.
C.
D.
The car moving at 10 m/s.
The car moving at 20 m/s.
They have the same momentum.
The momentum cannot be determined.
Forces
Which of the following has the most momentum:
bowling ball moving at 5 m/s or a tennis ball moving
at 5 m/s?
A.
B.
C.
D.
The tennis ball.
The bowling ball.
They have the same momentum.
I was not paying attention.
Forces
What outside force causes momentum to not be
“conserved” for almost every collision?
A.
B.
C.
D.
Gravity
Friction
Normal
Weight
Forces
Which has more momentum: a 1000 kg car moving
at 20 m/s or a 2000 kg truck moving at 10 m/s?
A.
B.
C.
D.
The car
The truck
They both have the same momentum.
Football players from Notre Dame.
Forces
Forces Practice Test
1- Person shoving their friend
2- Unbalanced forces CHANGE an object’s motion.
3 & 4- Friction
5- 150 N to the right (200 N – 50 N = 150 N)
6- 50 N = Frictional Force; 300 N down = Gravitational Force; 300
N up = Reaction force (from the action of gravity pulling downward)
7- Shoes with lots of tread, sandpaper used in shop class, etc.
8- Air Resistance would change; Gravity would remain the same
9- Increase mass- Greater Grav. Pull; Lower distance- Greater Grav. Pull
10- Any number from 1.1 to 2.3 (between Earth and Jupiter’s Grav. pull)
11- Heavy objects- High Inertia and like to stay at rest more than lighter
ones & are more difficult to stop moving. Seatbelts prevent your body
from moving too far forward (wanting to stay in motion) when brakes are
used
Forces
Forces Practice Test
12- ½ the acceleration = ½ the force, so ½ of 20 N = 10 N
13- F = m x a = 50 kg x 2 m/s/s = 100 N
14- 9.8 m/s/s
15- 5 kg because it is lighter
16- Action- Striking the board; Reaction- Board pushes back onto
your hand.
17- Increase the object’s velocity and mass
18- Momentum is the same before and after (w/o friction).
19- Friction
Forces
Noggin Knockers/Hwk. (10 points- 2 pts. each)
1- You move backwards (or away).
2- Action force = you pushing the other person; Reaction
force = the other person’s body pushes back onto you.
3- 0 kg x m/s
4- 0.06 kg x m/s
5- Dolphin because it is moving faster.
Momentum of dolphin = m x v = 250 kg x 4 m/s = 1000 kg x m/s
Momentum of manatee = 350 kg x 2 m/s = 700 kg x m/s
Forces
Noggin Knockers & Hwk: p. 61- 1a, 1b, 1c, 2a, 2c, 3a,
3b, & 5
1a- For every action there is an equal and opposite reaction.
1b- The reaction force is EQUAL and OPPOSITE to the action force.
1c- You would move the direction the ball is moving.
2a- Momentum is the mass x velocity of an object; 2b- The momentum of a parked
car is 0 kg x m/s.
2c- Greater speeds means a greater momentum, which makes it more difficult to
stop.
3a- The total momentum is the same before and after a collision (w/o friction).
3b- 0.06 kg x m/s
5- Momentum of dolphin = m x v = 250 kg x 4 m/s = 1000 kg x m/s
Momentum of manatee = 350 kg x 2 m/s = 700 kg x m/s
Forces - Newton’s Third Law
Momentum Activity
Click the Active Art button to open a browser window and
access Active Art about momentum.
Forces - Newton’s Third Law
Calculating Momentum
Practice Problem
A golf ball travels at 16 m/s, while a baseball moves at 7
m/s. The mass of the golf ball is 0.045 kg and the mass
of the baseball is 0.14 kg. Which has the greater
momentum?
Golf ball: 0.045 kg X 16 m/s = 0.72 kg•m/s
Baseball: 0.14 kg X 7 m/s = 0.98 kg•m/s
The baseball has greater momentum.
Forces - Newton’s Third Law
Calculating Momentum
Practice Problem
What is the momentum of a bird with a mass of 0.018 kg
flying at 15 m/s?
0.27 kg•m/s (0.018 kg X 15 m/s = 0.27 kg•m/s)
Forces - Newton’s Third Law
Previewing Visuals
Before you read, preview Figure 18. Then write two
questions that you have about the diagram in a graphic
organizer like the one below. As you read, answer your
questions.
Conservation of Momentum
Q. What happens when two moving objects collide?
A. In the absence of friction, the total momentum is the same before and
after the collision.
Q. What is the momentum of an object?
A. Its mass multiplied by its velocity
Forces
End of Section:
Newton’s Third
Law
Forces - Rockets and Satellites
What Is a Satellite?
A projectile follows a curved path because the horizontal and
vertical motions combine.
Forces - Rockets and Satellites
What Is a Satellite?
The faster a projectile is thrown, the father it travels before it
hits the ground. A projectile with enough velocity moves in a
circular orbit.
Forces - Rockets and Satellites
What Is a Satellite?
Depending on their uses, artificial satellites orbit at different
heights.
Forces - Rockets and Satellites
Identifying Main Ideas
As you read the section “What Is a Satellite?” write the main
idea in a graphic organizer like the one below. Then write
three supporting details that further explain the main idea.
Main Idea
A satellite stays in orbit due to…
Detail
its inertia
Detail
Earth’s gravity
Detail
Earth’s shape
Forces
End of Section:
Rockets and
Satellites
Forces
Graphic Organizer
Type of Friction Occurs When
Example
Static
An object is not
moving
Friction between an
unmoving book and
desk
Sliding
Two solid surfaces
slide over each
other
Rubber pads on a
bicycle’s brakes
Rolling
An object rolls
across a surface
Ball bearings in
skateboard wheels
Fluid
A solid object moves
Air resistance
through a fluid
Forces
End of Section:
Graphic Organizer