Newtons Laws of Motion

Newton’s
Laws of
Motion
I. Law of Inertia
II. Law of Acceleration
III. Law of Interaction
While most people know
what Newton's laws say,
many people do not know
what they mean (or simply do
not believe what they mean).
Newton’s Laws of Motion
 1st Law – An object at rest will stay at rest,
and an object in motion will stay in
motion at constant velocity, unless acted
upon by an unbalanced force.
 2nd Law – Force equals mass times
acceleration.
 3rd Law – For every action there is an
equal and opposite reaction.
1st Law of Motion
(Law of Inertia)
An object at rest will stay at

rest, and an object in motion
will stay in motion at
constant velocity, unless acted
upon by an unbalanced force.
FORCES
1 Law
st
 Inertia is the
tendency of an
object to resist
changes in its
velocity:
whether in
motion or
These pumpkins will not move unless acted on
motionless. by an unbalanced force.
Why then, do we observe every
day objects in motion slowing
down and becoming motionless
seemingly without an outside
force?
It’s a force we sometimes cannot see –
friction.
Objects on earth, unlike the
frictionless space the moon
travels through, are under the
influence of friction.
What is this unbalanced force that acts on an object in motion?
 There are four main types of friction:

 Sliding friction: ice skating
 Rolling friction: bowling
 Fluid friction (air or liquid): air or water resistance
 Static friction: initial friction when moving an object
Slide a book
across a table and
watch it slide to a rest
position. The book
comes to a rest
because of the
presence of a force -
that force being the
force of friction -
which brings the book
to a rest position.
 In the absence of a force of friction, the
book would continue in motion with the
same speed and direction - forever! (Or at
least to the end of the table top.)
2nd Law
(Law of
Acceleration)
Two statements in Newton’s
Second Law of Motion
 Force and acceleration
 States that the greater the unbalanced force, the
greater the acceleration of the body being acted upon.
 Mass and acceleration
 states that the greater the mass of an object, the
greater the amount of matter that has to be
accelerated. Hence, the greater the amount of force
should be applied.
2 Law
nd
The net force of an object is

equal to the product of its mass
and acceleration, or F=ma.
2 Law
nd
 When mass is in kilograms and acceleration is

in m/s/s, the unit of force is in newton (N).
 One newton is equal to the force required to
accelerate one kilogram of mass at one
meter/second/second.
Formulas:
F=ma
m=F/a
a=F/m
2 Law (F = m x a)
nd
How much force is needed to accelerate a 1400

kilogram car 2 meters per second/per second?
Write the formula
 F=mxa
Fill in given numbers and units
 F = 1400 kg x 2 meters per second/second
Solve for the unknown

2800 kg-meters/second/second or 2800 N
4
10
If mass remains constant, doubling the acceleration, doubles the force. If force remains
constant, doubling the mass, halves the acceleration.
Newton’s 2nd Law proves that different masses
accelerate to the earth at the same rate, but with
different forces.
• We know that objects

with different masses
accelerate to the
ground at the same
rate.
• However, because of
the 2nd Law we know
that they don’t hit the
ground with the same
force.
F = ma F = ma
98 N = 10 kg x 9.8 m/s/s 9.8 N = 1 kg x 9.8
m/s/s
Check Your Understanding
1. What acceleration will result when a 12 N net
force applied to a 3 kg object? A 6 kg object?
2. A net force of 16 N causes a mass to accelerate at
a rate of 5 m/s2. Determine the mass.
3. What is the force on a 1000 kg elevator that is
falling freely at 9.8 m/sec/sec?
4. A player kicked a 0.40 kg football with a force of
25 N. What is the acceleration of the football?
3 Law
rd
(Law of Interaction)
 For every action, there is an

equal and opposite reaction.
3 Law
rd
According to Newton,
whenever objects A and
B interact with each
other, they exert forces
upon each other. When
you sit in your chair,
your body exerts a
downward force on the
chair and the chair
exerts an upward force
on your body.
3 Law
rd
There are two forces

resulting from this
interaction - a force on
the chair and a force on
your body. These two
forces are called action
and reaction forces.
Newton’s 3rd Law in Nature
 Consider the propulsion of a
fish through the water. A fish
uses its fins to push water
backwards. In turn, the water
reacts by pushing the fish
forwards, propelling the fish
through the water.
 The size of the force on the
water equals the size of the
force on the fish; the direction
of the force on the water
(backwards) is opposite the
direction of the force on the
fish (forwards).
3 Law
rd
Flying gracefully
through the air, birds
depend on Newton’s
third law of motion. As
the birds push down on
the air with their wings,
the air pushes their
wings up and gives
them lift.
 Consider the flying motion of birds. A bird flies by
use of its wings. The wings of a bird push air
downwards. In turn, the air reacts by pushing the bird
upwards.
 The size of the force on the air equals the size of the
force on the bird; the direction of the force on the air
(downwards) is opposite the direction of the force on
the bird (upwards).
 Action-reaction force pairs make it possible for birds
to fly.
Other examples of Newton’s
Third Law
 The baseball forces the

bat to the left (an
action); the bat forces
the ball to the right (the
reaction).
3 Law
rd
 Consider the motion of

a car on the way to
school. A car is
equipped with wheels
which spin backwards.
As the wheels spin
backwards, they grip the
road and push the road
backwards.
3 Law
rd
The reaction of a rocket is an

application of the third law of
motion. Various fuels are
burned in the engine, producing
hot gases.
The hot gases push against the
inside tube of the rocket and
escape out the bottom of the
tube. As the gases move
downward, the rocket moves in
the opposite direction.

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Newtons Laws of Motion

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Newton’s

An object at rest will stay at

 There are four main types of friction:

The net force of an object is

 When mass is in kilograms and acceleration is

How much force is needed to accelerate a 1400

• We know that objects

 For every action, there is an

There are two forces

 The baseball forces the

 Consider the motion of

The reaction of a rocket is an

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