Newton’s Laws of
Motion
Newton’s First Law of Motion

An object at rest will remain at rest and an
object in motion will remain in motion at
constant speed in the same direction
unless an unbalanced force acts on it

In other words:


If something is just sitting there, it is not going to
move on its own without a new force being
introduced
Likewise, if an object is moving, it will not stop
moving without a new force being introduced
Balanced or unbalanced?

According to Newton’s first law, will an
object’s motion change if an unbalanced
force is not introduced?


No!
So, what is the net force on the object’s
described by the first law?

Zero Newtons!
Resting? Are they asleep?

Why are a chair on the floor and a
golf ball sitting on a tee said to be “at
rest”?


They are not moving
When will they no longer be at rest?

When an unbalanced force is applied to
them (golf club hits the ball or hand
pushes the chair) and they begin to move
It keeps going and going and going

According to Newton’s First law, an object
will continue to move forever with the same
velocity unless an unbalanced force acts on
it – what does this mean?

Any object will continue to move in the same
direction and at the same speed forever if left
untouched by other forces

What happens to a ball when it is left to roll on a
field?
 It will eventually stop because friction slows it down

What do you think would happen to a ball if left to
“roll” in outer space?
 It will continue to “roll” because there is no friction in
space to sow it down
Bump in the night (or in the car)

What happens to you when the bumper car
you are riding in hits another bumper car?

You keep moving forward even though the
bumper car stops

Hitting the other bumper car does not affect your
motion, only the motion of your car
 That is why there are seat belts in bumper cars!!
What about Friction?

According to Newton’s first law, when you
push your desk, it should continue to move
forever, but it doesn’t – it stops quickly.
Why?

Friction between the desk and floor works
against the motion of the desk

Friction causes motion to slow down and
eventually stop
Inertia

Newton’s First Law is also
known as the Law of Inertia

Inertia is the tendency of all objects
to resist any change in their motion


All objects either want to remain at
rest, or keep moving
This is why you are pushed to the
door of the car when it turns

Your body wants to continue to
move forward, but the car begins
to turn
Increasing Inertia

How are mass and inertia related?

Mass is a measure of inertia



The more mass an object has, the more inertia
Similarly, the less mass, the less inertia
This is why its easier to pitch a softball than a
bowling ball

It is easier to change the motion (throw) the
softball because it has less mass – you can get it
moving easier than the bowling ball
Newton’s Second Law of Motion

The acceleration of an object depends on
the mass of the object and the amount of
force applied to the object

If you apply the same force when kicking a wall
and kicking a soccer ball, which will move
faster/farther?


The soccer ball because it has less mass
The wall will accelerate, but because it is so
massive, you will not be able to see it
Mass and Acceleration

In other words:


If you increase mass, but use the same force,
acceleration will decrease
If you decrease mass, but use the same force,
acceleration will increase



Ex: a small force applied to an empty shopping cart vs a full
shopping cart will cause the empty cart to move farther away
Acceleration increases as the force on an object
increases
Acceleration decreases as the force on an object
decreases
Newton and Math

To find acceleration, divide the force
applied by the mass of the object:


A=F
M
To find the force applied, multiply the mass
of the object times its acceleration:

F=MxA
Hit the floor

If you were to drop a watermelon
and an apple at the same time,
from the same height, they would
hit the floor at the same time

HOW????

It takes more force to accelerate the
watermelon because of its greater mass
 The larger force needed, slows down the
watermelon’s acceleration
 The apple, which has a smaller mass,
requires a smaller force by gravity, so its
acceleration is faster
Affects of changing force or mass

You are pushing a 52 kg object with a force of 54N – what is
the object’s acceleration?


What happens to the acceleration if the force you apply
doubles to 108N, but the mass of the object stays the same?
(will it increase or decrease?)


Acceleration will increase
What kind of force are you applying if the mass stays the
same, but acceleration decreases to 1 m/s2? (larger or smaller
force?)


Acceleration will increase
What happens to acceleration if the force you apply stays at
54N, but the mass of the object is reduced by half to 26kg?
(will it increase or decrease?)


A=F/M  A=54N/52Kg  A= 1.04 m/s2
Smaller force
What happens to the mass of the object if its acceleration
stays the same, but force has been increased to 208N? (mass
increases or decreases?)

Mass would have increased too
Newton’s Third Law of Motion


When an object exerts a force on a second
object, the second object exerts an equal
and opposite force on the first object
Forces act in pairs


If a force is exerted, another force occurs that is
equal in size and opposite in direction
**For every action, there is an equal and opposite
reaction


EX: your action is pushing down on the chair, the
reaction is the chair pushing back up on you
You do not move because the force pushing up is
equal to your weight
 The net force is 0N
Keep it moving

Action and reaction forces do not work on
the same objects


If they did, nothing would ever move because net
force would always be 0!
Think about swimming


When you push your hands through the water, you
are exerting a force on the water – the action force
The reaction is the water pushing back on your
hand
 You move forward in the water because your action
force is stronger than the water’s reaction force
You can’t see me

Its not always easy to see the
reaction force

Take for example a falling ball:


The action force is gravity pulling the
ball down
Every object exerts a gravitational
force (creates gravity)
 That means that the ball is also creating a
gravitational force on the Earth
 Why can’t you see the effect of the ball’s
gravity on Earth?
 Earth is so massive, that you can not
see its acceleration
 Thus, it is hard to detect the reaction
force of the ball