Download 7th gd 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
Motion
Book M – Ch 2 – Section 1 (Page 36-39)
The Nature of Force
• A force is a push or a pull.
• Like velocity and acceleration, a force is
described by its strength and by the direction
in which it acts.
• The strength of a force is measured in the SI
unit called newton (N).
Motion - The Nature of Force
Book M – Ch 2 – Section 1 (Page 36-39)
Combining Forces
• The combination of all forces acting on an
object is called the net force.
Motion - The Nature of Force
Book M – Ch 2 – Section 1 (Page 36-39)
Unbalanced Forces vs. Balanced Forces
•Unbalanced forces can •Balanced forces are
cause an object to start equal forces acting on
moving, stop moving, or one object in opposite
change directions.
directions.
•Unbalanced forces
•Balanced forces acting
acting on an object
on an object do not
result in a net force and change the object’s
cause a change in the
motion.
object’s motion.
Motion - The Nature of Force
Book M – Ch 2 – Section 1 (Page 36-39)
Unbalanced Forces vs. Balanced Forces
Motion - The Nature of Force
Links on Force
Click the SciLinks button for links on force.
Motion
Book M – Ch 2 – Section 2 (Page 42-50)
Friction and Gravity
•The force that two surfaces exert on each other
when they rub against each other is called friction.
•The strength of the force of friction depends on two
factors:
1. How hard the surfaces push together.
2. The types of surfaces involved.
Motion
Book M – Ch 2 – Section 2 (Page 42-50)
Four Types of Friction
1. Static Friction – the friction that acts on
objects that are not moving.
2. Sliding Friction – occurs when two solid
surfaces slide over each other.
3. Rolling Friction – occurs when an object rolls
across a surface.
4. Fluid Friction – occurs when a solid object
moves through a fluid.
Motion
Book M – Ch 2 – Section 2 (Page 42-50)
Gravity
• Gravity is a force that pulls
objects toward each other.
• The law of universal
gravitation states that the
force of gravity acts
between all objects in the
universe.
• Two factors affect the
gravitational attraction
between objects:
1. Mass
2. Distance
Motion - Friction and Gravity
Book M – Ch 2 – Section 2 (Page 42-50)
Mass vs. Weight
• Mass is a measure of the amount of matter in
an object.
• The force of gravity on a person or object at
the surface of a planet is known as weight.
Motion - Friction and Gravity
Book M – Ch 2 – Section 2 (Page 42-50)
Free Fall
•When the only force acting on an object is
gravity, the object is said to be in free fall.
•In free fall, the force of gravity is an
unbalanced force, which causes the object
to accelerate.
• Acceleration due to gravity is 9.8 m/s².
• This means for every second an object
is falling, its velocity increases by 9.8
meters per second (m/s).
Motion - Friction and Gravity
Free Fall
Use the graph to answer the
following questions.
Motion - Friction and Gravity
Free Fall
Interpreting Graphs:
What variable is on the
horizontal axis? The vertical
axis?
Time is on the horizontal axis,
and speed is on the vertical
axis.
Motion - 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.
Motion - Friction and Gravity
Free Fall
Predicting:
What will the speed of the
object be at 6 seconds?
58.8 m/s
Motion - 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.
Motion - Friction and Gravity
Book M – Ch 2 – Section 2 (Page 42-50)
Air Resistance
• Falling objects with a greater surface area
experience more air resistance.
• This is why a leaf falls more slowly than an acorn.
• Air resistance increases
with velocity.
• The greatest velocity a
falling object reaches is
called terminal velocity.
Motion - Friction and Gravity
Book M – Ch 2 – Section 2 (Page 42-50)
Projectile Motion
An object that is thrown is called a projectile.
Motion - 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
Motion - Friction and Gravity
Links on Friction
Click the SciLinks button for links on friction.
Motion - Friction and Gravity
Free Fall
Click the Video button to watch a movie about free fall.
Motion
Motion - Newton’s First and Second Laws
Book M – Ch 2 Section 3 (pgs. 51-54)
Newton’s First Law of Motion
• States that an object at rest will remain at rest,
and an object moving at a constant velocity will
continue moving at a constant velocity, unless it
is acted upon by an unbalanced force.
• Inertia - the tendency of an
object to resist a change in
motion
• Dependent on mass - the
more mass, the more inertia
(resists change more)
Motion - Newton’s First and Second Laws
Book M – Ch 2 Section 3 (pgs. 51-54)
Newton’s Second Law of Motion
• Acceleration depends on the object’s mass
and on the net force acting on the object
• Force – measured in Newtons (N) or (kgm/s2)
• Mass – measured in kilograms (kg)
• Acceleration – measured in (m/s2)
• 1 newton has the force required to give a
1-kg mass an acceleration of 1 m/s2
Motion - Newton’s First and Second Laws
Book M – Ch 2 Section 3 (pgs. 51-54)
• What two ways can you increase
acceleration??
• Increase force
• Decrease mass
Let’s
Practice!
Motion - 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
Motion - 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 (F) = __
What formula contains the given quantities and the unknown
quantity?
a = F/m or F = m X a
Perform the calculation.
F = m X a = 55 kg X 2.0 m/s2
F = 110 kg • m/s2
F = 110 N
Motion - 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 waterskier. This may not seem like enough force, but it
does not include the force of the speedboat's pull that
overcomes friction.
Motion - 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)
Motion - Newton’s First and Second Laws
Calculating Force
Practice Problem
What net force is needed to accelerate a 25-kg cart
at 14 m/s2?
350 N (25 kg X 14 m/s2)
Motion - Newton’s First and Second Laws
More on Newton’s Laws
Click the PHSchool.com button for an activity
about Newton’s laws.
Motion - Newton’s Third Law
Book M – Ch 2 Section 4 (pgs. 55-61)
Newton’s Third Law of Motion
• States that if one object exerts a force on
another object, then the second object
exerts a force of equal strength in the
opposite direction on the first object.
• For every action there is an equal but
opposite reaction
Motion - Newton’s Third Law
Book M – Ch 2 Section 4 (pgs. 55-61)
• Motion can not always be observed – Ex. Earth’s
gravity pulling on a pencil vs. pencil pulling on the
earth.
• Action-Reaction forces do not cancel (like in Newtons
1st law of motion) because they are acting on different
objects
Motion - Newton’s Third Law
Book M – Ch 2 Section 4 (pgs. 55-61)
• Momentum – a characteristic of a moving object
that is related to the mass and the velocity of the
object.
• Mass – measured in kilograms (kg)
• Velocity – measured in meters per second (m/s)
• Momentum – measured in kilogram meters per
second (kg·m/s)
• Described by its direction as well as its quantity
• An objects momentum is the same direction as it’s
velocity
Motion - Newton’s Third Law
Book M – Ch 2 Section 4 (pgs. 55-61)
• The more momentum a moving object has, the
harder it is to stop.
• Increased mass and/or increased velocity will
increase momentum
Which has more momentum?
Baseball traveling at 20 m/s or Car traveling at 20
m/s
Which has more momentum?
2 ton car traveling at 10 m/s
or
2 ton car traveling at 50 m/s
Motion - 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 0.9 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 = 0.9 m/s
Motion - 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 0.9 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•m/s
LARGER sledgehammer = 4.0 kg X 0.9 m/s = 3.6 kg•m/s
Motion - 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 0.9 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.
Motion - 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.
Motion - 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)
Motion - Newton’s Third Law
Book M – Ch 2 Section 4 (pgs. 55-61)
Law of Conservation of Momentum
•states that, in the absence of outside forces,
the total momentum of objects that interact
does not change.
•Example of “outside force” – Friction
Motion - Newton’s Third Law
Conservation of Momentum
In the absence of friction, momentum is
conserved when two train cars collide.
Motion - Newton’s Third Law
Momentum Activity
Click the Active Art button to open a browser window and
access Active Art about momentum.