1. Newton's First Law of Motion
Law of Inertia
Mr. Morris

2. Newton's First Law of Motion
Every object in a state of uniform motion tends to remain in that state of motion unless an unbalanced force is applied to it.
An object at rest
An object in motion

3. An Object at Rest

4. An Object in Motion
Galileo's thought experiment I

5. An Object in Motion
Galileo's thought experiment II

6. Frictional force
Friction is the force that acts on a moving object to slow it down and bring it to stop.
Question 1: Evaluate the following assertion.
If there is no frictional force on the road, cars can move without consumption of energy by the Newton’s first law of motion. So we can save energy.

7. An Object in Motion A dynamic cart with a brick on it.
Get it moving and stopped.
Tape the brick to the cart.
Get it moving and stopped again.
Seat belts protect us from being hurt by keeping us tied to the vehicle.

8. An Object in Motion

9. Inertia
The resistance an object has to a change in its state of motion.
Question 2: What does the magnitude of inertial force depend on?
F = ma
m: inertial mass

10. Application to everyday life
Opening a bottle
Removing dusts from cloths
Seat belts
Seismometer

11. Seat Belt
Seat belts can exert forces to cause passengers to slow down at the same rate as the vehicle.
Question 3: How does a seat belt exert force on us?

12. The principle of retractors