1. The First Two Laws of Motion
Unit 2: Physics
Chapter 4:
The First Two Laws of Motion

2. Chapter 4: The First Two Laws of Motion
UNIT 2: Physics
Chapter 4: The First Two Laws of Motion
I. Newton’s Laws of Motion
A. Developed by British scientist Isaac Newton (1687)
1. Describes how forces and motion are related
2. Called Newton’s Three Laws of Motion 

3. B. The First Law of Motion
1. Describes how an object moves when the net force acting on it is zero
2. Called Newton’s First Law of Motion
If the net force on an object is zero, the object remains at rest, or if the object is moving it continues to moving in a straight line with constant speed

4. C. Inertia and Mass
1. Inertia- tendency of an object to resist a change in its motion
a. Depends on objects mass
b. Greater mass of object, the greater the inertia
2. Newton’s First Law often called the Law of Inertia

5. D. What happens in a crash?
1. Explained by Newton’s First Law of Motion
2. Seat belts and air bags in cars protect in collisions

6. E. Newton’s Second Law of Motion
1. Describes how the net force on an object, its mass, and its acceleration are related
2. Larger forces create larger accelerations

7. 3. acceleration of an object depends on its mass, as well as the net force exerted on it.

8. F. The Second Law of Motion- states that the acceleration of an object is in the same direction as the net force on the object.
1. Equation
Acceleration =
net force (in newtons)
Mass (in kilograms)

9. Net force = mass x acceleration
2. Can calculate net force by rearranging equation
Net force = mass x acceleration

10. II. Gravity
A. What is gravity?
1. gravity- attractive force between any two objects that depends on the masses of the objects and distance between them

11. 2. Gravity is a week force. You need to have large mass to exert gravitational force
3. Gravity is one of the four basic forces (with electromagnetic force, the strong nuclear force, and the weak nuclear force)

12. B. Newton’s Law of Universal Gravitation
1. Newton used motion of planets to formulate law of universal gravitation
2. . Equation:
3. G is a constant, d is the distance between two masses, m1 and m2

13. 4. Gravitational force decreases rapidly with distance
5. Used law to discover planet Neptune in 1846

14. C. Earth’s Gravitational Acceleration
1. When all forces except gravity acting on an object can be ignored, the object is said to be in free fall

15. 2. Close to Earth’s surface, the acceleration of an object in free fall is 9.8 m/s2
3. Can calculate gravitational force with this equation
g = acceleration due to gravity)

16. 4. weight- the gravitational force exerted by Earth on the object
How much would you weigh on the moon?

17. D. Weightlessness and Free Fall
1. weightlessness- sensation of “floating”
2. The space shuttle in orbit is in free fall. Objects inside seem to be floating because they are all falling with the same acceleration

18. E. Projectile Motion
1. Projectile- anything that is thrown or shot through the air
2. Projectile motion- Earths gravity causes projectiles to follow a curved path

19. 3. Horizontal and vertical motions of a
3. Horizontal and vertical motions of a projectile are completely independent of each other

20. 4. Horizontal velocity of object remains constant
5. Gravity only acts on the vertical velocity

21. F. Centripetal Force
1. Centripetal acceleration- acceleration towards the center of a circle that keeps an object in circular motion
2. Centripetal Force- the net force exerted toward the center of a curved path.

22. 3. The tires on a car rounding a curve exert a centripetal force (frictional force, or traction of tires)

23. 4. Gravity is the centripetal force that keeps the moon in orbit around the Earth.

24.  III. The Third Law of Motion- describes action-reaction pairs
A. Newton’ Third Law
1. Action and Reaction- “to every action force there is an equal and opposite reaction force”

25. 2. The forces are equal and opposite in direction

26. B. Momentum-
1. Product of mass and velocity
2. The unit for momentum is kg·m/s
p = momentum, m = mass, v = velocity
Traveling at the same speed, which has the greatest momentum?

27. 3. Law of Conservation of Momentum- the momentum of an object doesn’t change unless its mass, velocity, or both change