FORCE & MOTION (Inertia) FORM 4

1. INERTIA

2. OBJECTIVES
• To analyse the relationship between
inertia and mass.
• To understand the effects of inertia in
daily life experience.

3. What is Inertia?
• The inertia of an object is the
tendency of the object to
remain at rest or, if moving, to
continue its motion

4. If the object is moving….
INERTIA makes the moving object
continue to move at a constant
speed in the same direction unless
some external force changes the
object’s motion

5. If an object is at rest…
It tends to stay in that position
unless some force puts that
object into motion

6. Inertia is not physical quantity and
thus it does not have units
The property of inertia is found from
the Newton’s first law of motion

7. Newton’s First Law of Motion
state that:
"Every object continues in its
state of rest or of uniform
motion unless it is acted upon
by an external force"

9. RELATIONSHIP
BETWEEN INERTIA AND
MASS

11. • Constant wherever it is measured
• Base quantity
• Scalar quantity
• The SI unit of mass is kilogram
(kg)

12. Mass can influence
the inertia

13. An object with a larger
mass has a larger
inertia.
MASS INCREASES
INERTIA INCREASES

14. SITUATIONS
INVOLVING INERTIA

15. • When the cardboard
is pulled away
quickly, the coin
drops straight into
the glass.
• The inertia of the coin
maintains its state at
rest. The coin falls
into the glass due to
gravity.

16. Chilli sauce in the bottle
can be easily poured out
if the bottle is moved
down fast with a sudden
stop. The sauce inside
the bottle moves
together with the bottle.
When the bottle stops
suddenly, the sauce
continue in its state of
motion due to the effect
of its inertia.

17. • Body moves forward when the car stops
suddenly
• The passengers were in a state of
motion when the car was moving. When
the car stopped suddenly, the inertia in
the passengers made them maintain
their state of motion. Thus when the car
stop, the passengers moved forward.

18. Ways to reduce
the negative
effects of inertia

20. Safety in a car:
•(a) Safety belt secure the driver to their seats.
When the car stops suddenly, the seat belt
provides the external force that prevents the
driver from being thrown forward.
•(b) Headrest to prevent injuries to the neck
during rear-end collisions. The inertia of the
head tends to keep in its state of rest when the
body is moved suddenly.
•(c) An air bag is fitted inside the steering
wheel. It provides a cushion to prevent the
driver from hitting the steering wheel or
dashboard during a collision.

22. • Furniture carried by a lorry normally
are tied up together by string. When the
lorry starts to move suddenly, the
furniture are more difficult to fall off
due to their inertia because their
combined mass has increased.