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Have you ever stopped to look at a
car accident?
When you look at a car accident you are
looking at the results
of the Forces of Motion around us.
Newton’s Laws of Motion
REVIEW
First we need to define the
word FORCE:
• The cause of motion (what
causes objects to move)
• Two types of forces
– Pushes
– Pulls
REVIEW
Forces are measured in
Newtons
• SI unit of force
• Symbol: N
• Measured by using a spring scale
REVIEW
Forces may be balanced or
unbalanced
• Balanced forces – all forces acting
on an object are equal
– There is NO MOTION
• Unbalanced forces – one or more
forces acting on an object are
stronger than others
– There is MOTION
• A NET FORCE
Newton’s Laws
• First Law – Inertia
• Second Law – Acceleration,
Force & Mass
• Third Law – Action-Reaction
Law # 1:
Every object at rest remains at rest and
every object in motion continues
moving in a straight line at a steady
rate unless a force acts it on.
This is the principle of inertia.
Can you tell me how this principle
applies to car accidents?
First Law
• Inertia & Mass
– Mass is the amount of matter in an
object.
– The more MASS an object has, the
more INERTIA the object has.
– Bigger objects are harder to start &
stop.
http://toons.artie.com
Law # 2
The amount of force needed to change
the speed of an object depends on the
mass of the object and the amount of
acceleration (or deceleration) needed.
This is the principle of accelerated
motion.
Can you tell me how this law applies to
car accidents?
Second Law
• Acceleration & Force
– The more force placed on an object, the
more it will accelerate [change its motion].
• Acceleration & Mass
– The more mass [or inertia] an object has, the
more force it takes to accelerate the object.
Second Law
Force = Mass x Acceleration
Example: A 25 g object with an acceleration
of 4 m/s2 will have a force of ______
Newtons.
Law # 3
For every action there is an equal and
opposite reaction. When an object is pushed
or pulled by a force in one direction, another
force pushes or pulls on it with equal strength
from the opposite direction.
Forces come in pairs, called action-reaction
pairs.
How does this law apply to car accidents?
Third Law
• Action – Reaction
– Forces are always produced in pairs with
opposite directions & equal strengths.
– For every force there is an equal and
opposite force.
Third Law
• Action – Reaction
– Action – Reaction Forces act on different
objects…
• When you kick a soccer ball, you exert a force on
the ball and the ball exerts a force on you. The
harder you kick the bigger the force on you (kicking
REALLY hard might hurt.)
Third Law
Which Law?
A swimmer pushes water back with
her arms, but her body moves
forward.
Which Law?
A frog leaping upward off his lily pad is
pulled downward by gravity and lands
on another lily pad instead of continuing
on in a straight line.
Which Law?
After you start up your motorcycle, as
you give it more gas, it goes faster.
Which Law?
A pitched baseball goes faster than one that
is gently thrown.
Which Law?
When you paddle a canoe, the canoe
goes forward.
Which Law?
As an ice skater pushes harder with his
leg muscles, he begins to move faster.
Which Law?
A little girl who has been pulling a sled
behind her in the snow is crying because
When she stopped to tie her hat on, the
sled kept moving and hit her in the back
of her legs.
All about Forces!
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