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Newton’s
Laws
Explaining motion
Isaac Newton was the first
person to explain WHY objects
move the way they do.
He based his laws of motion
on two key concepts:
• mass
• force
What is mass?
All matter has mass and takes
up space.
A solid rock has mass. So do
gases and liquids.
With your hand out the window
of a moving car, you feel the
mass in the air pushing against
you.
What is force?
The concept of force:
A force is a push or a pull.
What is force?
The concept of force:
A force is a push or a pull.
Forces can cause an object
to change its motion.
Can you give some
examples of forces?
Types of forces
The concept of force:
A force is a push or a pull.
Forces can cause an object
to change its motion.
Can you give some
examples of forces?
Examples of forces:
•
•
•
•
•
weight
friction
tension from rope
force from a spring
electric force
Units of force
Quantitative:
Force is measured in newtons.
Units of force
Newton’s first
law
The first law has two parts. This first is obvious from experience.
Newton’s first law
The first law has two parts. The second part is not so obvious.
First law examples
Give an example of the first part of the first law.
Objects at rest
Give an example of the first part of the first law.
A book on the table, a chair, a seated person – all are at rest
and stay at rest. The net force is zero in each case.
Objects in motion
Give an example that illustrates the second part.
Reality
In real life all moving objects eventually slow down and stop
UNLESS you continually apply a force to keep them going.
Is the first law wrong?
In real life all moving objects eventually slow down and stop
UNLESS you continually apply a force to keep them going.
So, is the first law wrong? Was Newton hallucinating?
Is the first law wrong?
In real life all moving objects eventually slow down and stop
UNLESS you continually apply a force to keep them going.
So, is the first law wrong? Was Newton hallucinating?
What is the explanation?
Newton’s first law
The answer has to do with the word “net.”
Newton’s first law
In everyday life all moving objects eventually slow down
and stop because the net force is NOT zero.
What forces act on the ball?
In everyday life all moving objects eventually slow down
and stop because the net force is NOT zero.
What forces act on the ball?
What forces act on the ball?
The force we are looking for is friction.
Friction
The force we are looking for is friction.
Friction occurs whenever there is relative motion
between matter.
Friction creates forces that always opposes this motion.
Zero net force
To create motion at constant speed in the real world, a
constant force must be applied to make the net force zero.
Zero net force
To create motion at constant speed in the real world, a
constant force must be applied to make the net force zero.
The first law is correct (of course) and objects do move
at the same speed and in the same direction when the
net force is truly zero.
Looking deeper
Consider – sitting in your chair, are you at rest right now?
It depends on your frame of reference.
Looking deeper
Consider – sitting in your chair, are you at rest right now?
Again, it depends on your frame of reference.
Understanding the first law
These two parts of the first law are really
identical. They are both telling you this:
If Fnet = 0 then a = 0.
Understanding the first law
The first law goes both ways:
If Fnet = 0 then you know a = 0.
or
If a = 0 then you know Fnet = 0.
Understanding the first law
Motion only changes through the action of a net force.
If the net force is zero – there can be no changes in motion.
The law of inertia
Newton’s first law is also known as the law of inertia.
•Inertia is the tendency of an object to resist
changes in the speed or direction of its motion.
•The inertia of an object is related to its mass.
•The more massive an object is, the more inertia it
has, and the more it will resist having its motion
changed.
Newton’s second law
Net force (N)
Acceleration(m
/s2)
Mass (kg)
The acceleration of an object equals
the net force divided by the mass.
Newton’s second law
Net force (N)
Acceleration(m
/s2)
Mass (kg)
Velocity must change if a net force acts on an object.
How does the second law show that this statement is true?
The meaning of the second law
Net force (N)
Acceleration(m
/s2)
Mass (kg)
Velocity must change if a net force acts on an object.
According to the second law, a net force on an object causes it to
accelerate. If an object accelerates, its velocity must change.
The meaning of the second law
Net force (N)
Acceleration(m
/s2)
Mass (kg)
The net force is zero on an object with constant velocity.
How does the second law show that this statement is true?
The meaning of the second law
Net force (N)
Acceleration(m
/s2)
Mass (kg)
The net force is zero on an object with constant velocity.
If velocity stays constant, then the acceleration is
zero—so the net force must also be zero.
Direction of force and acceleration
Acceleration and force
are vectors.
Units
The second law can help you
remember the definition of a
newton.
Always use mass in kilograms
and acceleration in m/s2 when
applying Newton’s second law.
Applying Newton’s second law
If you know the force on an
object, you can predict changes
in its motion.
If you know the acceleration of an
object, you can determine the net
force on it.
Finding motion from forces
If you know the force on an object, you can predict changes in its motion.
A 0.25 kg ball is traveling 40 m/s to the right when it is hit with
a force of 3,000 N for 0.005 seconds. What is its final velocity?
Steps
A 0.25 kg ball is traveling 40 m/s to the right when it is hit with a
force of 3,000 N for 0.005 seconds. What is its final velocity?
1.
Use force and mass to find acceleration through the second law.
2.
Use the acceleration to find the change in velocity or position.
Solution
A 0.25 kg ball is traveling 40 m/s to the right when it is hit with a
force of 3,000 N for 0.005 seconds. What is its final velocity?
1.
Use force and mass to find acceleration through the second law.
Impacts can cause very large accelerations for short times!
Solution
A 0.25 kg ball is traveling 40 m/s to the right when it is hit with a
force of 3,000 N for 0.005 seconds. What is its final velocity?
1.
Use force and mass to find acceleration through the second law.
2.
Use the acceleration to find the change in velocity or position.
The ball reverses direction!
Applying Newton’s second law
If you know the force on an
object, you can predict changes
in its motion.
If you know the acceleration of an
object, you can determine the net
force on it.
Forces come in pairs
In your everyday life you can observe
that forces always occur in pairs.
Consider throwing a ball.
What force moves the ball?
Forces come in pairs
In your everyday life you can observe
that forces always occur in pairs.
Consider throwing a ball.
Your hand exerts a force on the ball –
that is the action force that causes the
ball to accelerate.
Forces come in pairs
In your everyday life you can observe
that forces always occur in pairs.
Consider throwing a ball.
Your hand exerts a force on the ball –
that is the action force that causes the
ball to accelerate.
The ball exerts an equal and opposite
reaction force back against your hand.
Forces come in pairs
How do you detect the presence of the
ball? Can you feel the reaction force?
If the ball was heavier, or covered with a
prickly material, it would make the
reaction force even more noticeable.
Describe a situation where you put a
force on something, and a force acted
back on you.
Action and reaction forces
Forces always come in pairs.
Action
If one object puts a force on a second object,
the second object always puts an equal and
opposite force back on the first object.
Reaction
Action and reaction forces
Forces always come in pairs.
Action
If one object puts a force on a second object,
the second object always puts an equal and
opposite force back on the first object.
Newton’s third law:
For every action (force) there is always
an equal and opposite reaction (force).
Reaction
Action and reaction forces
Forces always come in pairs.
Action
If one object puts a force on a second object,
the second object always puts an equal and
opposite force back on the first object.
Newton’s third law:
For every action (force) there is always
an equal and opposite reaction (force).
Newton’s third law is a law of
interactions between objects.
Reaction
An example
Action: racquet
pushes on ball.
Reaction: ball pushes
back on racquet.
There are always two objects
Action: racquet
pushes on ball.
Reaction: ball pushes
back on racquet.
Action-reaction forces always
act on different objects:
One force acts on the racquet.
Its partner force acts on the ball.
Free-body diagrams
Action: racquet
pushes on ball.
Reaction: ball pushes
back on racquet.
Only one of these forces appears
on an object’s free-body diagram:
the force that acts ON the object.
Force pairs don’t cancel out
Action-reaction pairs
don’t cancel out because
they always act on
different objects:
• One force acts
on the racquet.
• Its partner force
acts on the ball.
Only forces that act on the same
object can cancel each other out.