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Forces - Friction and Gravity
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.
Forces - Friction and Gravity
Predicting:
What will the speed of the
object be at 6 seconds?
58.8 m/s
Forces - The Nature of Force
What is a Force?
oA force is a push or pull
oEvery force has two components:
 How strong the force is (in N)
 The direction in which the force acts
oThe strength of a force is measured in Newtons (N)
Forces - The Nature of Force
Combining Forces
The combination of all forces acting on an object is called the
net force.
Forces - The Nature of Force
Balanced Forces
If the net force on an object is zero, the forces are said to be balanced.
Balanced forces acting on an object do not change the object’s motion.
Forces - The Nature of Force
Unbalanced Forces
If there is a non-zero net force acting on an object, the forces are said
to be unbalanced. Unbalanced forces acting on an object will cause
the object to accelerate (speed up, slow down, or change direction).
Forces - Friction and Gravity
Friction
• Friction is the force that two surfaces exert on each other when they rub
against each other.
• Friction always acts in the direction opposite an object’s motion.
• The two factors that affect friction are the types of surfaces involved
and how hard they are pushed together.
Not quite enough friction!
Forces - Friction and Gravity
Gravity
Gravity is a force that pulls all objects toward each other. Two factors
affect the gravitational attraction between objects: the mass of the
objects and the distance between them.
Forces - Friction and Gravity
Gravity
Strange but True!
The force of gravity on Earth
causes all objects to fall at the
same rate. As long as there is no
air resistance, an apple and a
feather dropped from the same
height at the same time will hit
the ground at the same time.
Forces - Friction and Gravity
Gravity
Strange but True!
The force of gravity on Earth causes all objects to fall at the
same rate. If there were no air resistance, a bullet fired exactly
horizontally and a bullet dropped from the same height as the
gun will hit the ground at the same time.
Forces - Newton’s First Law
“An object in motion will remain in motion and an object at rest
will remain at rest unless acted on by an unbalanced force.”
Newton’s First Law
In other words:
“If an object is not moving, it will not start moving unless a
net force acts on it. If an object is moving, it will continue at a
constant velocity until a net force acts to change its speed or
direction.”
Forces - Newton’s First Law
“An object in motion will remain in motion and an object at rest
will remain at rest unless acted on by an unbalanced force.”
Newton’s First Law
• Newton’s First Law tells us what an object will do when it has
BALANCED forces acting on it.
• Newton’s First Law is also called the Law of Inertia.
• Inertia is the tendency of an object to resist a change in its motion.
• Inertia is dependent on mass. The greater an object’s mass, the
more the object resists a change in its motion. (This is why it is
harder to move a heavy object than a light one!)
Forces - Newton’s First Law
“An object in motion will remain in motion and an object at rest
will remain at rest unless acted on by an unbalanced force.”
Newton’s First Law
What are some examples of Newton’s First Law?
•
•
•
A hockey puck glides across ice after being hit with the
hockey stick
A baseball flies after being hit with bat
A rock does not begin to roll across the ground on its own
Forces - Newton’s Second Law
“The acceleration of an object depends on the net force
acting on it and its mass .”
Newton’s Second Law
•
•
•
•
Newton’s Second Law tells us what an object will do
when it has UNBALANCED forces acting on it.
A net force acting on an object will cause the object to
accelerate (speed up, slow down, or change direction).
The greater the net force acting on an object, the larger the
acceleration of the object will be.
The larger the object’s mass, the greater the force required
to accelerate the object.
Forces - Newton’s Second Law
“The acceleration of an object depends on the net force
acting on it and its mass .”
Newton’s Second Law
What are some examples of Newton’s Second Law?
•
•
It takes more force to speed up a full shopping cart than an
empty one
When hit with the same amount of force, a lighter baseball
will accelerate more than a heavier softball
Forces - Newton’s Second Law
“The acceleration of an object depends on the net force
acting on it and its mass .”
Newton’s Second Law
Newton’s Second Law is also written as a math equation:
FORCE = MASS X ACCELERATION
Forces - Newton’s Second Law
“The acceleration of an object depends on the net force
acting on it and its mass .”
Newton’s Second Law
How much
force does it
take to give
each car an
acceleration
of 0.05 m/ss?
F = ma
Forces - Newton’s Second Law
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.
What information have you been given?
Mass of the water-skier (m) = 55 kg
Acceleration of the water-skier (a) = 2.0 m/s2
What quantity are you trying to calculate?
The net force F = ?
What formula will you use?
F=mXa
Perform the calculation.
F = m X a = 55 kg X 2.0 m/s2
F = 110 kg • m/s2
F = 110 N
Forces - Newton’s Second Law
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 Second Law
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)
Forces - Newton’s Third Law
“For every action there is an equal and opposite
reaction.”
Newton’s Third Law
In other words:
“If one object exerts a force on a second object, the second
object always exerts a force of EQUAL strength in the OPPOSITE
direction on the first object.”
Forces - Newton’s Third Law
“For every action there is an equal and opposite
reaction.”
Newton’s Third Law
EXAMPLE:
A bird flies by flapping its wings.
Action:
The wings of a bird push the air
downward.
Reaction: The air pushes upward on the
bird. This is what keeps the bird
in the air.
For every action, there is an equal (in size)
and opposite (in direction) reaction.
Forces - Newton’s Third Law
“For every action there is an equal and opposite
reaction.”
Newton’s Third Law
EXAMPLE:
A car runs into a brick wall, causing the
front of the car to be smashed.
Action:
The car exerts a force on
the wall.
Reaction: The wall exerts a force
on the car. This is what
causes the car to be smashed.
For every action, there is an equal (in
size) and opposite (in direction) reaction.
Forces - Newton’s Third Law
“For every action there is an equal and opposite
reaction.”
Newton’s Third Law
EXAMPLE:
A person on a skateboard pushes off from
a wall.
Action:
The person pushes on the wall.
Reaction: The wall pushes on the person,
causing the person to roll.
For every action, there is an equal (in size)
and opposite (in direction) reaction.
Forces - Newton’s Third Law
“For every action there is an equal and opposite
reaction.”
Newton’s Third Law
EXAMPLE:
A balloon flies away when the air is released.
Action:
The balloon pushes on the air,
forcing the air out of the balloon.
Reaction: The air pushes on the balloon.
This is what causes the balloon to
fly away.
For every action, there is an equal (in size)
and opposite (in direction) reaction.
Forces - Newton’s Third Law
“For every action there is an equal and opposite
reaction.”
Newton’s Third Law
Forces
Click here to see Newton's Laws in action!!