Forces & Motion
ESSENTIAL QUESTIONS:
•W H A T A R E N E W T O N ’ S 3 L A W S O F M O T I O N ?
•H O W D O T H E S E L A W S A P P L Y T O R E A L - L I F E
SITUATIONS?
Calculating Speed
 If you know the distance an object travels in
a certain amount of time, you can calculate
the speed of the object.
 The speed of an object is the distance the object
travels per unit of time.
Calculating Speed
 Speed = Distance
Time
 The speed equation consists of a unit of distance
divided by the unit of time.
 Speed = 30 kilometers
1 hr
= 30 km/h
Average Speed
 To calculate average speed, divide the total distance
traveled by the total time
 Cyclist travels 32 km during the first 2 hrs and 13 km
in the next hr.
Distance
32 km + 13 km
45 km
Time
2 hrs + 1 h
3 hrs
Average Speed
45 km
3 hrs
15 km/h
What is Acceleration?
 Scientist define acceleration as the rate at which
velocity changes
 Recall velocity describes both the speed and
direction of an object.
 In science, acceleration refers to increasing
speed, decreasing speed, or changing
direction
http://www.khanacademy.org/science/physics/
mechanics/v/acceleration
Calculating Acceleration
 To determine the acceleration of an object
moving in a straight line, you must calculate
the change in speed per unit of time.
Acceleration = Final Speed – Initial Speed
Time
Calculating Acceleration
 Refer to figure 10
 Acceleration = 40 m/s – 0 m/s
5s
 Acceleration = 8 m/s²
Graphing Acceleration
 You can use both a speed-versus-time graph
and a distance-versus-time graph to analyze
the motion of an acceleration object.
Speed-Versus-Time Graph
 Line is slants upward= increasing acceleration
 Line is straight = constant acceleration
 You can find your acceleration by calculating the
slope of a line on a graph
 Slope = Rise
Run
 Refer to figure 11
 Slope = Rise
Run
 8 m/s – 4 m/s = 4 m/s
4 s -2 s
2s
 Slope = 2 m/s²
Distance-Versus-Time Graph
 Curved line = object is accelerating
 A simple distance vs. time graph
 Distance is on Y axis
 Time is on the X axis
 Plotted (X,Y)
Chapter Three
SECTION ONE
THE NATURE OF FORCE
What is 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.
What is Force?
 The strength of a force is measured in the SI unit
called the Newton
 The direction and strength of a force can be
represented by an arrow.
 The arrow points in the direction of a force. The
length of the arrow tells you the strength of a force.
Combining Forces
 Sometimes more than one force acts on an object at
one time.
 The combination of all forces acting on an object is
called net force .
Combining Forces
 Forces in the same direction = adding the forces
 Forces in opposite direction = subtracting the forces
5N
5N
5N
5N
10 N
10 N
5N
5N
0N
Unbalanced Force
 Whenever there is a net force acting on an object, the
forces are unbalanced
 Unbalanced Forces can cause an object to start
moving, stop moving, or change direction
 Unbalanced forces acting on an object result
in a net force and cause a change in the
object’s motion
Unbalanced Forces
 Example
 When two people push the box in opposite directions the net
forces on the box is different between their individual forces
 if the left side is pushed with greater force then the right side
the forces are unbalanced
 As a result the box moves to the right
Balanced Forces
 Equal forces acting on one object in opposite
directions are called balanced forces
 Balanced forces acting on an object do not
change the object’s motion
 When equal forces are exerted in opposite directions,
there is no net force
Friction and Gravity
SECTION TWO
Friction
 The force that two surfaces exert on each other when
they rub against each other is called friction
 Examples:
 Sled across snow
 Firefighters had against polished metal pole
The Causes of Friction
 In general, smooth surfaces produce less friction
than rough surfaces.
 The strength of friction depends on two
factors: how hard the surfaces push together
and the type of surfaces involved.
The Causes of Friction
 When the irregularities of one surface come into
contact with those of another surface, friction occurs.
 Friction acts in a direction opposite to the direction
of the object’s motion
 Without friction, a moving object might not
stop until it strikes another object.
Static Friction
 The friction that acts on objects that are not moving
is called static friction
 Because of static friction you must use extra force to
start the motion of stationary objects.
Static Friction
 Example: Desk
 If you push on a desk with a force less than the force of static
friction between the desk and the floor – the desk won’t move

If you push on a desk with a force more than the force of static
friction between the desk and the floor- the desk will move
Intended direction of
motion
Sliding Friction
 Sliding Friction – occurs when two solid surfaces
slide over each other
 Example:
 When you stop a bicycle with hand brakes, rubber pads slide
against the tire surfaces, causing the wheels to slow and
eventually stop.
Direction of Motion
Rolling Friction
 When an object rolls across a surface it is called
rolling friction
 Rolling friction is easier to overcome than sliding
friction for similar materials.
 Example:
 Skateboard rolling on blacktop
Fluid Friction
 Fluid Friction occurs when a solid objects moves
through a fluid.
 In this way, the solid parts move through the fluid
instead of sliding against each other.
Gravity
 A plane falls from the sky, an apple falls from the
tree these events take place because gravity.
 Gravity is a force that pulls objects toward each
other.
 Issac Newton concluded that a force acts to pull
objects straight toward the center of the earth.
Universal Gravitation
 Newton realized that gravity acts everywhere in the
universe, not just on Earth.
 It is the force that keeps all the planets in our solar
system orbiting around the sun.
Universal Gravitation
 The law of universal gravitation state that
the force of gravity acts between all objects
in the universe.
 This means that any two objects in the
universe, without exception attract each
other.
Factors Affecting Gravity
 Two factors affect the gravitational
attraction between objects: mass and
distance
 Mass is a measure of the amount of matter in an
object.

Mass is measured in kilograms
Factors Affecting Gravity
 The more mass an object has, the greater its
gravitational force.
 In addition to mass, gravitational force depends on
the distance between the objects. The farther apart
two objects are, the lesser the gravitational force.
Weight and Mass
 Mass is the measure of the amount of matter in an
object
 Weight is the amount of gravitational force exerted
on an object
DON’T GET THESE TWO CONFUSED!!!!!!!!!!!
Weight and Mass
 The force of gravity on a person or object at the
surface of a planet is known as weight
 Weight varies with the strength of the gravitational
force but mass does not.
Astronaut in Space
Weight on Moon
270 N
Weight on Earth
1,617 N
Mass on Moon
165 kg
Mass on Earth
165 kg
Gravity and Motion
 When you hold a book, you exert a force that
balances the force of gravity.
 When you let go of the book, gravity becomes an
unbalanced force and the book falls
Free Fall
 When the only force acting on an object is gravity,
the object is said to be in free fall.

Objects in free fall is accelerating
 In free fall the forces of gravity is an
unbalanced force, which causes an object to
accelerate.
Free Fall
Free Fall
 Near the surface of the earth the acceleration due to
gravity is 9.8 m/s²
 This means that for every second an object is falling,
its velocity increases by 9.8 m/s
 In the absence of air, two objects with different
masses fall at exactly the same rate.
Air Resistance
 Theoretically, all objects are supposed to fall at the
same rate but we know that this is not always the
case.
 Objects calling through air experiences a type of fluid
friction called air resistance
 Friction is the direct opposite to motion, so air
resistance is an upward force exerted on falling
objects
 Not all objects have the same air resistance
 Objects with more surface area has more air
resistance
Air Resistance
 Air resistance increases with velocity, or speed in a
given direction.
 As the object falling speeds up, the force of air
resistance increases
 At one point, an object falling will fall fast enough
that the upward force of air resistance becomes equal
to the downward force of gravity on the object.
Air Resistance
 The object continues to fall, but its velocity remains
constant.
 The greatest velocity a falling object reaches is called
its terminal velocity

Force of air resistance = weight of the object
Projectile Motion
 An object that is thrown is called a projectile
 When you throw a projectile at an upward angle, the
force of gravity reduces its vertical velocity
 Eventually the upward velocity of the projectile will
stop, and gravity will pull it back toward the ground.
 From this point, the projectile will fall at the same
rate as any dropped object.
Newton’s First and Second
Laws
SECTION THREE
The First Law of Motion
 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.
The First Law of Motion
 Gravity and friction are unbalanced forces that often
change an object’s motion
Inertia
 Inertia is the tendency of an object to resist a change
in motion
 Newton’s first law of motion is also called the law of
inertia.
 Example:
 When a car suddenly stops, the inertia keeps you moving
forward
Inertia Depends on Mass
 Some objects have more inertia than other objects.
 The greater the mass of an object is, the greater its
inertia, and the greater the force required to change
its motion.
The Second Law of Motion
 According to Newton’s second law of motion,
acceleration depends on the object’s mass
and on the net force on the object.
Determining Acceleration
 Acceleration is measured in meters per second per
second or m/s²
 Mass is measure in kilograms
 Force is measured in kilograms times meters per
second per second or kg•m/s². It is also called the
newton
Changes In Force and Mass
 One way to increase acceleration is by changing the
force.
 When you increase the force you increase the
acceleration
 When you decrease the force you decrease the
acceleration
Changes In Force and Mass
 Another way to increase acceleration is to change the
mass.
 If the force is constant, an increase in mass causes a
decrease in acceleration
 If the force is constant, a decrease in mass causes an
increase in acceleration
Newton’s Third Law
SECTION FOUR
Newton’s Third Law of Motion
 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”
Action-Reaction Pairs
 Example:
 Jumping, you push on the ground with your feet – action force
 The ground pushing back with and equal and opposite force –
reaction force
 What are other examples?
Detecting Motion
 You can not always detect when paired forces are in
action
 Example:
 Earth’s gravity pulling on an object – you can’t see detect
earth’s equal and opposite reaction
Do Action-Reaction Forces Cancel?
 The answer is no because action and reaction forces
do not cancel out because they are acting on different
objects.
 Example:
 Hitting a volleyball
Hitting the ball is an upward motion
 The ball hitting her wrists in a opposite and equal downward
motion

Momentum
 Momentum is a characteristic of a moving object that
is related to the mass and the velocity of the object.
 The momentum of a moving object can be
determined by multiplying the object’s mass
and velocity.
Momentum
 Momentum = Mass × Velocity
 Measured in kilogram-meter per second or kg•m/s
 The momentum is in the same direction as its
velocity.
Momentum
 The more momentum an object has the harder it is
to stop

Train is an example
 You may have two objects moving at the same speed
but if one has a higher mass it will have more
momentum
Conservation of Momentum
 In physical science, conservation refers to the
conditions before and after some event.
 The total amount of momentum objects have is
conserved when they collide.
 Momentum may be transferred from one object to
another but none is lost.
Conservation of Momentum
 The Law of Conservation of Momentum states that,
in the absence of outside forces, the total momentum
of objects that interact does not change.
 The total momentum of any group of objects
remains the same, or is conserved, unless
outside forces act on the objects.
Collisions With Two Moving Objects
 Before the collision, the blue car moves faster than
the yellow car, After to collision, the yellow car
moves faster than the blue car. The total momentum
stays the same.
Collisions With One Moving Object
•When the car on the right is at rest before the collision, all of the left car ‘s
momentum is transferred to it. The momentum is conserved
4 m/s
0 m/s
4 m/s
O m/s
Collisions With Connected Objects
 If the two cars coupled together, momentum is still
conserved. Together, the cars move slower than the
first car did before the collision.
2 m/s
2 m/s
What is Work?
The meaning of work
 Work is done on an object when the object
moves in the same direction in which the
force is exerted.
No Work Without Motion
 In order for work to occur:
 THE OBJECT YOU ARE EXERTING A FORCE ON
MUST MOVE!!!!
 If the object doesn’t move no work is being
done!!
Force in the Same Direction
 In order for work to take place, the force you exert
must be in the same direction as the objects motion
 Why don’t you do work when you hold books and
walking to school?




You exert a force on the books when you hold them
When you exert the force, it is an upward force (vertical)
When you walk, the motion is horizontal
Since force is vertical and the motion is horizontal they are not
in the same direction and no work is done
Calculating Work
 The amount of work you do depends on both the
amount of force you exert and the distance the object
moves .
 The amount of work done on an object can be
determined by multiplying work times
distance.
 Work = Force x Distance
Calculating Force
 When force is measured in Newton and distance in
meters, the SI unit of work is the Newton x meters
(N x m) also known as a Joule (J)
 One Joule is the amount of work you do when you
exert a force of 1 Newton to move an object a
distance of 1 meter
What is Energy?
CHAPTER FIVE SECTION ONE
Energy, work, and Power
 Work is done when a force moves an object through
a distance.
 The ability to do work or cause change is called
energy
Kinetic Energy
 Two basic kinds of energy kinetic energy
and potential energy.
 The energy an object has due to its motion is called
kinetic energy
Factors affecting Kinetic Energy
 Kinetic energy depends on mass and velocity
 Kinetic energy:
 Increase with an increase to mass
 Increase with an increase in velocity
 Kinetic Energy = ½ × Mass × Velocity²
 Do changes in velocity and mass have the same effect
on kinetic energy?

Changing the velocity of an object will have a greater affect on
its kinetic energy than changing its mass by the same factor.

Velocity is squared
 See figure 2
Potential Energy
 An object doesn’t have to be moving to have energy.
 Some objects have stored energy has a result of their
positions and shape is also known as potential
energy
Gravitational Potential Energy
 Potential energy related to an objects height is called
gravitational potential energy
 Gravitational potential energy = Weight × Height
 The force you use to life the object is equal to its
weight
 The distance you move the object is its height
Elastic Potential Energy
 An object gains a different type of potential energy
when it is stretched.
 The potential energy associated with objects that can
be stretched or compressed is called elastic potential
energy.