Download Motion and Forces

Forces
Chapters 2.3-3
Physical Science
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Forces Objectives
1. Explain how force and motion are related.
2. Distinguish between balanced and unbalanced forces.
3. Describe the different types of forces, including frictional
force, gravitational force, and centripetal force.
4. Describe the four types of frictional force.
5. Describe inertia and how it is related to Newton’s 1st law of
motion.
6. Define and apply Newton’s 2nd law of motion.
7. Calculate force, mass, and acceleration with Newton’s
second law.
8. Distinguish between mass and weight.
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9. State Newton’s 3rd law of motion and use it to describe
and
calculate momentum.
10. Identify action/reaction forces.
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Force
force: cause of acceleration, or change in an object’s velocity
-push or pull that gives energy to an object.
-a force can cause an object to stop moving, start moving,
or change its direction
-net force: combination of all forces acting on an object
♦forces in the same direction add together, while
forces in the opposite direction subtract from one
another
-measured in newtons (N), kg·m/s2
The description of force includes both a size and direction,
making it a vector.
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Forces
Forces will do one of two things:
1. Cancel each other out completely (balanced forces)
-net force = zero
-no change in object’s motion
2. Do not cancel each other out completely (unbalanced
forces)
-net force ≠ zero
-change in object’s motion
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Forces
=
0
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Forces
A. Forces add together when
they are acting in the same
direction
=
0
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Forces
A. Forces add together when
they are acting in the same
direction
=
0
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Forces
A. Forces add together when
they are acting in the same
direction
B. Forces subtract from each
other when they are acting
in opposite directions
=
0
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Forces
A. Forces add together when
they are acting in the same
direction
B. Forces subtract from each
other when they are acting
in opposite directions
=
0
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Forces
A. Forces add together when
they are acting in the same
direction
B. Forces subtract from each
other when they are acting
in opposite directions
=
0
C. Forces that are opposite in
direction and equal in size
cancel each other out
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Forces
Here is an example of adding and subtracting
forces to get a net force.
Is this a balanced or unbalanced net force?
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Forces Review 1
1. The overall force acting on an object after all forces
are combined is the ______________.
2. How do balanced and unbalanced forces affect the
motion of an object?
3. What are the two ways to write the units for force?
4. How is force a vector?
5. What is the net force of the following:
5N
2N
6.
a. What are the balanced forces?
b. Will the refrigerator move? If so, in
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which direction?
Forces Quiz 1
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Types of Forces
There are three types of forces:
1. frictional force (friction)
2. gravitational force (gravity)
3. centrepetal force
We will discuss frictional force first.
friction: force between two objects in contact that opposes
the motion of either object
-type of unbalanced force
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Friction
Q: What does frictional force depending upon?
A: the surfaces in contact
-the rougher the surface, the greater the frictional force
Example: Imagine you are wearing a pair of dress shoes.
What would happen if you were to walk across ice in winter
and grass in summer?
Which is easier?
Why?
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Frictional Force (Friction)
Q: How do we overcome friction?
A: a constant force must be applied to overcome the
friction
vehicle tires are designed to use friction
to increase grip
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Types of Friction
1. static: friction that acts on objects that are not in motion
-acts in the direction opposite of the applied force
-ex: trying to push a box across the floor and it stays put
2. sliding: force that opposes the direction of motion of an
object as it slides over a surface
-ex: actually pushing the plant across the floor
(static friction not longer acts on the plant)
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3. rolling: force that acts on rolling objects
-friction is 100-1000 times less than static or sliding
-exs: using a dolly to move large objects;
ball bearings in skates, skateboards,
and automobiles
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4. fluid: force that opposes the motion of an object as it moves
through a fluid (liquid or gas)
-air resistance: fluid friction acting on an object moving
through air
-ex: airplane flying through
the air; submarine
moving through the
water
-also called viscous friction
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Air Resistance
Air resistance occurs between the surface of a moving object
and the air molecules.
Q: How can objects experience greater air resistance?
A: 1. increase surface area
2. increase the object’s speed
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Air Resistance
Since friction is a force, you can add and subtract
the vectors to get a net force:
Is this a balanced or unbalanced net force?
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Forces Review 2
1. A box is in the back of a truck. If the truck accelerates
quickly, what happens to the box?
2. Can an object continue moving and have a net force of
zero?
3. What is the net force equal to if unbalanced forces exist?
4. Is the following statement true or false? Why or why not?
“Friction is a force that helps objects that are touching
move past each other mores slowly.”
5. What can we do to the surfaces of objects to increase
friction?
6. What are the two ways to write the units for force?
7. Define what we mean by a fluid.
8. How do we increase air resistance?
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Gravitational Force (Gravity)
Introduction:
Isaac Newton made a discovery about gravity:
Legend has it that he sat under an apple tree one
evening and saw an apple fall. After giving it some
thought, Newton realized that a force pulled the
apple to the ground.
-called it the Universal Law of Gravitation
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Gravity
gravity: force of attraction between objects/particles of
matter due to their mass.
-all objects exert gravitational forces on all other objects,
even when they do not touch
-can act over large distances
Acts on:
1. falling objects
-causes objects to accelerate downward, where air
resistance reduces acceleration
♦terminal velocity: constant velocity of a falling object
when the force of air resistance = force of gravity
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2. projectile motion: motion of a falling object after it is
given an initial forward velocity
-follows a curved path
-gravity and air resistance act on the object
Objects fall and undergo projectile motion due to the
attraction between objects.
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Gravity
The attraction between objects depend on two things
1. the mass of the objects
-greater the mass, the greater the attraction
2. the distance between the objects
-shorter the distance, the greater the attraction
The moon is closer to the earth than the
sun, therefore the earth has a greater
pull on it, keeping it in orbit.
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Centripetal Force
centripetal force: center directed
force that continuously changes
the direction of an object to
make it move in a circle
-ex: Earth’s gravity pulls the
moon into a nearly circular
path around the Earth;
satellites in orbit
-reason for the tide changes at
the ocean
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Forces Review 3
1.What do we call an y force that keeps an object in a
circle?
2. How do we overcome friction?
3. List and describe the types of friction?
4. How does gravity and air resistance affect the motion of
a falling object?
5. Define what we mean by a fluid.
6. How do we increase air resistance?
7. Define gravity. What is gravity dependent on?
8. What is meant by terminal velocity?
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Forces Quiz 2
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Newton’s Laws of Motion
Isaac Newton summarized his study of motion
and force into three laws that we now call
Newton’s Laws of Motion.
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Newton’s First Law
Newton’s 1st Law of motion: an object at rest will stay
at rest and an object in motion will stay in motion until
acted upon by an outside unbalanced force.
-net force = 0; keeps the object in unchanging motion
-A car will continue to stay in motion until a force, such as
friction, slows it down.
-Ex: You keep moving forward, even when the car
you are riding in comes to a stop.
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Law of Inertia
Also called the “Law of
Inertia”, where inertia is the
tendency of an object to remain
at rest or in motion with a
constant velocity
An object’s velocity will change
if an unbalanced force acts on
it, causing the object to
accelerate.
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Newton’s 2nd Law of Motion
Newton’s 2nd Law of Motion states that the force
is proportional to the mass and is also proportional
to the acceleration of an object.
-can be stated as the equation:
F= ma
(measured in newtons, N = kgm/s2)
-more mass requires more force to accelerate the object
-more force causes more acceleration
This law means:
Forces cause acceleration and mass resists acceleration.
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Newton’s Second Law: Example 1
Zookeepers lift a stretcher that holds a sedated lion. The
total mass of the lion and stretcher is 175 kg and the
lion’s upward acceleration is 0.657 m/s2. What is the
unbalanced force necessary to produce this
acceleration?
Known variables: F = ?
m = 175 kg
Unknown variable:a = 0.657 m/s2
Formula: F = ma
= (175 kg)(0.657 m/s2)
= 115 kgm/s2
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Newton’s Second Law: Example 2
If a 50 N force pulls on a 10 kg object, how much
acceleration will occur?
Known variables: F = 50 N
m = 10 kg
Unknown variable:a = ? m/s2
Formula:
F = ma
50 N = (10 kg)a
50 kgm/s2 = (10 kg)a
50 kgm/s2 = (10 kg)a
10 kg
10 kg
= 5 m/s2
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Newton’s Second Law Practice 1
1. What is the net force on a dragster with a
mass of 900 kg if is acceleration is 32.0m/s2?
2. What is the force on a helicopter that has a
mass of 4500 kg if it’s acceleration is 4m/s2?
3. If a car with a mass of 1500 kg has an
acceleration of 2.0m/s2, what is the force
needed to accelerate the car?
4. You push a friend on a sled. Your friend and
the sled together has a mass of 70 kg. If the
net force on the sled is 35 N, what is it’s
acceleration?
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Forces Homework 1
1. How much force is needed to accelerate a 1000
kg car at a rate of 3 m/s2?
2. If a 70 kg swimmer pushes off a pool wall with a
force of 250 N, what is her acceleration?
3. If a person is pushing a cart with a force of 40 N
and it accelerates at 0.5 m/s2, what is the mass
of the object?
4. What is the acceleration of a 3 kg rock that is
thrown with a force of 18N?
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Forces Review 4
1. Describe the two factors that affect the acceleration of an
object?
2. Are these statements true or false? Why or why not?
a. “According to Newton’s 1st law of motion, an object’s
motion does not change as long as the net force acting
on it is zero.”
b. “The law of inertia states that an object in motion will
eventually slow down and stop if it travels far enough.”
3. Describe Newton’s 2nd law of motion.
4. A 70N force pushes on 3 objects: 2kg, 4kg and 6 kg
a. Which will have the greatest acceleration?
b. Which will push back with the greatest force?
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Forces Quiz 3
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Free Fall
free fall: the motion of a body when only the force of gravity
is acting on it
-its directed toward the center of the earth
-often abbreviated as the letter ‘g’ due to the acceleration
due to gravity
Free-fall acceleration near Earth’s surface is constant.
-in the absence of air resistance, all objects near Earth’s
surface accelerate at the same rate, 9.8 m/s2
-in our calculations, we will disregard air resistance
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Weight
weight: force of gravity on a mass
-weight changes as gravity changes
-unit is the N, since it is a force
-formula is
W = mg
~‘g’ is different on different planets and in space
(it is 0 in space)
~use 9.8 m/s2 unless stated differently
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Newton’s Third Law
Newton’s 3rd law of motion: for every
action, there is an equal and opposite
reaction.
-forces always occur in pairs, though
they may act on different objects
~Ex: kicking a soccer ball – the action
force acts on the ball, but the reaction
force acts on the foot
-action/reaction forces always occur at
the same time
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Momentum
If a tractor trailer truck and a car are moving in the same
direction at the same speed, why is it more difficult for the
tractor trailer to stop?
-it has more mass, therefore a greater momentum.
momentum (p): quantity defined as the product of an objects
mass and velocity
momentum = mass x speed
p = mv
-an object must be moving to have momentum
-the more momentum, the harder it is to stop
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Momentum
Law of conservation of momentum: the total amount of
momentum in a system does not change.
-can use it to describe what happens between pairs of
forces, as mentioned in Newton’s 3rd Law
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Momentum: Example 1
Ex. Calculate the momentum of a 6.00 kg bowling ball
moving at 10.0 m/s down the alley.
p=?
m = 6.00kg
You MUST show
the units in both the
v = 10.0m/s
problem and the
p = mv
answer!!
p = (6.00 kg)(10.0 m/s)
p = 60.0 kgm/s
REMEMBER- law of conservation of momentum: the
total amount of momentum in a system is conserved 50
Momentum: Example 2
Ex. What is the mass of an object going 3 m/s with a
momentum of 36 kg•m/s
p = 36 kg•m/s
m = ? kg
You MUST show
v = 3 m/s
the units in both the
problem and the
36 kg•m/s = mv
answer!!
36 kg•m/s = m(3 m/s)
36 kg•m/s = m(3 m/s)
3 m/s
3 m/s
12 kg = m
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Momentum (and Force) Practice
1. What is the momentum of a 2kg toy truck that moves at
10m/s?
2. What is the speed of a 25kg object if the momentum of
that object is 300kg•m/s?
3. What is the mass of an object that goes 25m/s and has a
momentum of 500kg•m/s?
4. If a 500kg thoroughbred at the Kentucky Derby is
galloping with a momentum of 8220kg•m/s, what is its
speed?
5. An automobile with a mass of 1000 kg accelerates when
the traffic light turns green. If the net force on the car is
4000N, what is the car’s acceleration?
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Forces Homework 2
1. How much momentum does a 30 kg object
going 4 m/s have?
2. How fast is a 15 kg object going if it has a
momentum of 45 kg•m/s?
3. A 2N and 6N force pull on an object to the right
and a 4N force pulls to the left. What is the net
force of the object? Draw a diagram.
4. A 20 kg bike accelerates at 10 m /s. With what
force was the person pedaling?
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Forces Review 5
1. Are these statements true or false? Why or why not?
a. “The loss of momentum of one object equal the gain in
momentum of the other.”
b. “Action reaction forces do not cancel each other out
because the action force is always greater than the
reaction force.”
2. How are mass and weight different?
3. What is the unit for momentum?
4. Describe the two factors that affect the momentum of an
object?
5. Which of Newton’s law’s applies?
a. How a rocket flies in space.
b. Braking hard wears out your breaks faster.
c. A car turns & objects in the car keep going straight.
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Forces Quiz 4
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Forces Practice 2
1. An automobile with a mass of 1200kg accelerates at a rate
of 3.0m/s2 in the forward direction. What is the net force
acting on the automobile?
2. How much force was exerted on a sled that has a mass of
70kg and accelerated at a rate of 0.5m/s2?
3. What is the force on a dragster with a mass of 900kg if its
acceleration is 32.0m/s2?
4. During a crash test, an airbag inflates to stop a 75 kg
dummy’s forward motion. If the net force on the dummy is
825 N, what is the dummy’s acceleration?
5. A car is being pulled by a tow truck. What is the car’s mass
if the net force on the car is 3000N and has an acceleration
of 2.0m/s2?
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Forces Homework 3
Forces Homework Worksheet
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