1. Force is part of an interaction
Force is push or pull
It is an interaction between one thing and another.
Example;
An apple exacts a force on someone’s palm (force of gravity).
The person’s palm exacts an upward force of reaction.
There are therefore two forces acting in an interaction.

2. Newton’s 3rd law of motion
Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object.
One force is called Action Force
The other is called Reaction Force
Neither force exist without the other.
Newton’s 3rd law can be summarized;
To every action there is an equal opposite reaction.

3. Action = reaction
Which of these two people is pulling with a bigger force?_______
Can the big guy pull more than the little guy could?_______

4. Pairs of Action and Reaction
Event
Pairs of objects in an interaction
Action
Reaction
Moving car
Car tires and the Road
Tires push road backwards
Road push tires forward
Firing a rocket
Rocket and Gas
Falling boulder
Earth and the Boulder
Firing a cannon
Cannon and cannon ball
Hitting a nail with a hammer
Nail and hammer
Hammer on nail
Nail on hammer

5. Action –reaction pairs do cause motion
If action equal reaction, how can motion be caused by such pairs of forces?

6. Action and reaction Vs acceleration?
How do equal forces (Action and reaction) produce unequal acceleration?
Because force is applied on objects with unequal masses.
The cannon has a large mass, than the cannon ball.
The action force F is equal to reaction force F. Therefore; acceleration of cannon is smaller than that of the cannon ball.

7. Action and reaction Vs acceleration?
Equal force, Unequal mass—Unequal acceleration

8. Elevator problem
Dave stands on a bathroom scale in an elevator . What is Dave's mass if the scale reads 840 N while the elevator accelerates upward at 2.2 m/s2
Use the formula
Net force = apparent weight – actual weight

9. answer Normal force = apparent weight Action = Reaction
840N up = 840N down
apparent weight= Net force + actual weight
840 = ma + mg
Ma +mg = 840N
m (a +g) = 840N
m ( ) = 840N
m (12) = 840
M = 840/12 = 70 kg
Normal force
(Action)
Net Force (ma)
Actual weight (mg)
Apparent Weight
(reaction)

10. class work and Homework
Class work: Textbook page 84, #