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NEWTON’S 1ST LAW OF
MOTION
LAW OF INERTIA
NEWTON’S FIRST LAW OF MOTION
• An object at rest will stay at rest
and an object in motion will
continue in motion with the same
speed and direction unless acted
on by a force.
• aka – Law of Inertia
WHAT DOES THIS MEAN?
Basically, an object will “keep doing what it
was doing” unless acted on by an
unbalanced force.
If the object was sitting still, it will remain
stationary. If it was moving at a constant
velocity, it will keep moving.
It takes force to change the motion of an
object.
REAL-LIFE EXAMPLES
1. Explain three real-life examples of
Newton’s First Law of Motion.
SOME EXAMPLES FROM REAL LIFE
• A soccer ball is
sitting at rest. It
takes an
unbalanced force of
a kick to change its
motion.
Two teams are
playing tug of war.
They are both
exerting equal force
on the rope in
opposite directions.
This balanced force
results in no
change of motion.
FORCE
• Force
• A push or pull
• Any action that has
the ability to change
motion.
• Units = newtons (N)
• There can be no
change in motion
without the presence
of a force.
INERTIA
• Inertia:
• the tendency of an
object to resist
changes in its state
of motion
• Objects with more
mass have more
inertia = more
resistant to changes
in motion
LEARNING CHECK
2. What is a force?
3. What are the units
for force?
4. What has more
inertia a bowling ball
or a golf ball? Why?
MASS VS. WEIGHT
• Mass:
• The amount of
matter contained
in an object
• Units: kilograms
(kg)
• Weight:
• The gravitational
force of an object
• Units = newtons
(N)
The weight of an object is
proportional to its mass!
CALCULATING WEIGHT
W = m·g
W = weight (N)
m = mass (kg)
g = acceleration due to gravity =
9.8m/s2
EXAMPLES OF WEIGHT PROBLEMS
5. What is the weight of
each of the following
objects?
a. 0.113 kg hockey puck
b. 108 kg football player
c. 871 kg automobile
Answers:
a. 1.11 N
b. 1060 N
c. 8540 N
MORE EXAMPLES
6. Find the mass, in
kilograms, of each of
these weights.
a. 95 N
b. 82 N
c. 0.98 N
Answers:
a. 9.7 kg
b. 8.4 kg
c. 0.10 kg
IF OBJECTS IN MOTION TEND TO
STAY IN MOTION, WHY DON’T MOVING
OBJECTS KEEP MOVING FOREVER?
• Things don’t keep moving forever because there’s
almost always an unbalanced force acting upon it.
7. A book sliding across a table slows down and stops
because of the force of ___________.
8. If you throw a ball upwards it will eventually slow
down and fall because of the force of _____________.
In outer space, away from gravity and
any sources of friction, a rocket ship
launched with a certain speed and
direction would keep going in that
same direction and at that same speed
forever.
WHAT IS FRICTION?
• The force that results
from the relative
motion between
objects that are in
contact.
• Example:
• The tire and the road
• Ice skate and ice
surface
• Air resistance
Friction between sled
and snow surface
LEARNING CHECK
9. Explain the
difference between
mass and weight.
10. Give three
examples of friction
affecting motion.