1. MOTION & ITS APPLICATIONS Newton’s Third Law of Motion (P .46-51)
SPH4C COLLEGE PHYSICS
MOTION & ITS APPLICATIONS
Newton’s Third Law of Motion (P )

2. Newton’s Third Law of Motion
Newton’s first law of motion is descriptive and his second law is mathematical. In both cases, we consider the forces acting on only one object. However, when your hand pushes on a desk in one direction, you feel a force of the desk pushing back on your hand in the opposite direction. This brings us to the third law, which considers forces acting in pairs on two objects.

3. Newton’s Third Law of Motion
To illustrate Newton’s third law of motion, imagine a ball shot horizontally out of the tube of a toy cart on wheels. When the ball is pushed into the cart, a spring becomes compressed. Then, when the spring is released, the spring (and thus the cart) pushes forward on the ball. We call this force the action force.

4. Newton’s Third Law of Motion
At the same instant, the ball pushes backward on the spring (and thus the cart). We call this the reaction force of the ball on the cart. The action and reaction forces are equal in size but opposite in direction, and act on different objects.

5. Newton’s Third Law of Motion
Newton’s third law of motion deals with these action and reaction forces. For this reason, it is sometimes called the action-reaction law.

6. Newton’s Third Law of Motion

7. Newton’s Third Law of Motion
PRACTICE
Describe the reaction force for each of the following forces.
(a) the southward force of a field goal kicker’s toe on a football
(a) the northward force of the football on the kicker’s toe

8. Newton’s Third Law of Motion
PRACTICE
Describe the reaction force for each of the following forces.
(b) the downward force of a book on a desk
(b) the upward force of the desk on the book

9. Newton’s Third Law of Motion
PRACTICE
2. Two skaters are standing on ice facing each other. Skater 1 pushes on skater 2 with a force of 70 N[E]. No friction acts on either skater. The mass of skater 1 is 50 kg and the mass of skater 2 is 70 kg. (a) State the action and reaction forces.
F1on2 = 70 N[E]  action
F2on1 = 70 N[W]  reaction
(b) State what will happen to each skater.
(b) skater 1 will accelerate west
skater 2 will accelerate east

10. Newton’s Third Law of Motion

11. Newton’s Third Law of Motion

12. Check Your Learning
TEXTBOOK
P .48 Q.1,4
P .50 Q.5,6