NEWTON'S FIRST LAW OF MOTION: INERTIA
Introduction to Newton's First Law
Sir Isaac Newton (1642-1727), an English physicist and mathematician, formulated three
fundamental laws of motion that revolutionised our understanding of how objects move and
interact.
The Law of Inertia
Newton’s First Law of Motion is particularly fascinating, as it explains a fundamental property
of matter known as inertia. It states: An object at rest tends to stay at rest, and an object in
motion tends to stay in motion with the same speed and in the same direction, unless acted
upon by an unbalanced external force. This principle is essentially a description of inertia -
the natural tendency of objects to resist changes in their state of motion.
Inertia: The resistance an object has to changes in its motion
Balanced Forces: When the net force on an object is zero
Unbalanced Forces: When the net force is not zero, causing a change in motion
Forces are
Balanced
Objects at Objects in Motion
V = 0m/s
Rest V ≠ 0m/s
2 2
a=0 a=0
m/s m/s
Stays at Stays in Motion
V = 0m/s
Rest Same Velocity
Car Safety Features Demonstrating Inertia
1. Seatbelts When a car traveling at 60 km/h suddenly stops or crashes, your body
wants to continue moving forward at the original speed due to inertia. Seatbelts
provide the external force needed to stop your body from continuing its forward
motion, preventing potentially fatal injuries.
2. Airbags Working in conjunction with seatbelts, airbags deploy during a collision to
provide a cushioned stopping force. They gradually bring your body to rest, mitigating
the harmful effects of sudden deceleration caused by inertia.
3. Collapsible Steering Columns These innovative safety design features collapse in
a controlled manner during a collision. By absorbing energy and extending the
stopping time, they reduce injuries caused by the body's momentum.
4. Headrests Designed to prevent whiplash during rear-end collisions, headrests
provide a force to stop your head from snapping backward when the car is suddenly
moved forward, protecting your neck from injury.
Inertia is directly related to an object's mass. More massive objects have a greater tendency
to resist changes in their motion. This is why larger, heavier objects require more force to
start, stop, or change direction compared to smaller, lighter objects.
Questions:
1. State Newton's First Law of Motion in your own words.
2. What is inertia and how does it relate to moving objects?
3. Define balanced forces. What happens to an object when forces are balanced?
4. What are unbalanced forces? How do they affect an object's motion?
5. How do seatbelts protect passengers using the concept of inertia?
6. Why might an unsecured object on the dashboard become dangerous during sudden
braking?
7. How do airbags use the principle of inertia to protect passengers?
8. Explain the purpose of headrests in cars in relation to Newton's First Law.
9. Why do passengers feel like they're being pushed back into their seats when a car
accelerates quickly?
10. How does Newton's First Law explain why it's more difficult to start pushing a
shopping cart than to keep it moving?
The Coin Drop Experiment
Materials:
A small, smooth card
A coin
A cup or glass
Procedure:
1. Place the card flat on top of a cup or glass
2. Place the coin on top of the card, centred above the cup
3. Quickly flick the card horizontally (sideways, not downward)
4. Observe what happens to the coin
Questions:
1. What happened to the coin when you flicked the card away? Why did this occur?
2. How does this experiment demonstrate Newton's First Law of Motion?
3. If you flicked the card very slowly instead of quickly, how might the result differ?
Why?
4. How is this experiment similar to what happens to passengers in a car that stops
suddenly?
5. If you tried this experiment with a small piece of paper instead of a coin, how might
the result differ? Explain why using scientific principles.
Inertia in Everyday Life
For each of the following scenarios, explain how Newton's First Law of Motion (inertia) is
demonstrated:
1. A bus starts moving suddenly and standing passengers feel like they're falling
backward.
2. You need to tap the bottom of a ketchup bottle to get the ketchup to come out.
3. A tablecloth can be quickly pulled out from under dishes without disturbing them (if
done correctly).
4. It's harder to start running than to keep running.
5. Space probes can continue traveling through space even when their engines are
turned off.
A Real-Life Car Accident
In a recent crash, a driver who was not wearing a seatbelt was ejected through the
windshield, suffering severe injuries. Another passenger wearing a seatbelt remained inside
the vehicle with minor injuries.
Questions:
1. Using Newton’s First Law, explain why the unrestrained driver was ejected.
2. How did the seatbelt protect the second passenger?
Suggest additional safety features that could have reduced injuries
