NEWTON’S LAWS
OF MOTION
Newton’s First Law
Law of Inertia
“An object at rest tends to stay at rest and an
object in motion tends to stay in motion with
the same speed and in the same direction
unless acted upon by an unbalanced force
.
”
Inertia
Galileo
Objects eventually stop due to a force called friction
Force is not needed to keep an object in motion
Law of Inertia
The
motion
of an object does not change
unless it is acted upon by a net force.
•
If v = 0, it remains 0
•
If v
is some value, it stays at that value
Another way to say the same thing:
•
No net force
•
velocity is constant
•
acceleration is zero
•
no change of direction of motion
Inertia
Inertia is the tendency of an object to remain at rest
or in motion with constant speed along a straight
line.
Mass is the quantitative measure of inertia.
Property of an object that measures how hard it is to
change its motion.
Loosely defined as the amount of matter in an object.
More mass means more inertia.
Mass
Gravitational mass
is the mass of an object
measured using the effect of a gravitational field
on the object.
Static measurement
Inertial mass
is the mass of an object measured
by its resistance to acceleration.
Dynamic measurement
Galileo’s Principle of Equivalence
2
nd
Law of Motion
The second law states that the acceleration of
an object is dependent upon two variables
–
the
net force
acting upon the object
Direct relationship
Increase in net force = increase in acceleration
the
mass
of the object.
Indirect relationship
Increase in mass = decrease in acceleration
Newton’s 2
nd
Law
Used for comparison of the same force exerted
on objects of different mass.
The force of the Earth pulls the apple down (“action”).
The force of the apple pulls the Earth up (“reaction”).
According to the 3rd law, the two forces are equal.
F
apple
= F
Earth
m
(apple)
*
a
(of apple to Earth)
=
m
(Earth)
*
a
(of Earth to apple)
F = m*a
Terminal Velocity
When Do We See These Laws In
Action?
What happens to the person when the car hits the
brick wall?
According to the 1
st
law, why does this happen?
What modern day technologies or safety features
have been created to limit the effects of
automobile accidents?
How do these technologies apply to Newton’s 2
nd
law?
Understanding the Second Law
The cause of acceleration is…
_________ resists acceleration
The greater the force, the ________ the
The greater the mass, the _________ the
acceleration.
Force.
Mass
Acceleration.
less
greater
Newton’s 3
rd
Law of Motion
For every action, there is an equal and
opposite reaction.
•
Finger pushes on box
•
F
finger
box
= force exerted on box by finger
F
finger
box
F
box
finger
•
Box pushes on finger
•
F
box
finger
= force exerted on finger by box
•
Third Law:
F
box
finger
=

F
finger
box
Identify the action

reaction pairs
Rocket Pushes Gas
Horse and Cart
–
Classic Problem
How does the horse move the cart?
Horse

Cart
Action

Reaction Pairs
3 points of view
–
the cart, the horse, the horse

cart system
The Horse

Cart Problem
3
rd
Law

A horse pulls a cart and the cart
pulls back on the horse with an equal and
opposite force!
Internal forces
Contribute nothing to the acceleration of the
system
Can be ignored
Internal forces (yellow horse

wagon) are irrelevant
Interaction between the horse

cart
system and the ground
(red ground, blue ground, blue
horse & red wagon)
Think of pushing a car

you cannot just push on the dashboard and expect it to move; you
must interact with the ground
The Horse

Cart Problem
When the horse pushes backward on
the ground, the ground pushes forward
on the horse.
If the horse pushes the ground with a
greater force than it pulls on the cart,
there will be a net force and
acceleration will occur.
Newton’s Laws in Action
A bug with a mass of
5 grams flies into the
windshield of a
moving 1000kg bus.
Which will have the
greater force, the bug
or the windshield?
Newton’s Laws in Action
The force exerted
by the car on the
bug is THE SAME
AS than the force
exerted by the bug
on the car.
For every action there is an
equal and opposite reaction,
but a bug can't withstand the
same amount of force as a
windshield, so it squishes.
During the collision between the car and
the bug, which one experiences the
greatest acceleration?
1. The car has a greater acceleration.
2. The bug has a greater acceleration.
3. The accelerations will be the same.
correct
F
= m
a
bug
car
bug
car
=
bug
car
x
Newton’s Laws in Action
The force would be the
same.
Accelerations are different.
Force
(bug)=
m
(bug)
x
A
(bug)
Force
(bus)
=
M
(bug)
x
a
(bug)
Newton’s Laws of Motion
Newton’s first law

an object with no net force acting
on it remains at rest or travels at a constant velocity.
Newton’s second law

When an unbalanced net force
acts on an object there is acceleration which is
summarized by F = ma
Newton’s third law

when one object exerts force on
a second object, the second exerts an equal and
opposite charge.
Understanding
Which law allows a spaceship to accelerate by burning
rocket fuel?
3
rd
What law relates force to acceleration?
2
nd
Which is the law of action/reaction?
3
rd
Which law governs when a tablecloth is pulled out from
under a setting of china without damaging it?
1
st
Understanding
Which is the law of inertia?
1
st
Which law governs when two skaters push apart in a
rink?
3
rd
Which law relates acceleration to mass?
2
nd
Which law is in control of a spaceship cruising through
space at constant speed without using any fuel?
1
st
What is
F
net
?
25 N
5 N
What is
F
net
?
25 N
35 N
What is
F
net
?
25 N
25 N
What is
a
?
25 N
25 N
5 kg
What is
a
?
25 N
5 N
5 kg
What is
a
?
30 N
25 N
5 kg
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