1. Your Free Body Diagram and Law #2 Problems are due – if they are finished turn them into the tray. If you still have your Forces Notes Question, turn them in finished or not. Read over Law #1 and Law #2 notes until we move on.

2. For every action there is an equal and opposite reaction.
Newton’s Third Law
For every action there is an equal and opposite reaction.

3. With the palm of your hand, push on the top of the table.
Push and be pushed!
With the palm of your hand, push on the top of the table.
You are exerting a force on the table.
At the same time, you can feel a force on your hand.
This is because there are two forces being exerted:
One is the force you are exerting on the table.
The other is the force the table is exerting on your hand!
ANSWER: THEY ARE EQUAL IN MAGNETUDE AND OPPOSITEDLY DIRECTED! This is true whether or not a big object is pushing a smaller object or whether the objects are stationary or moving. WHEN EVER TWO OBJECTS TOUCH THEY HAVE TO FOLLOW THIS SIMPLE RULE!

4. When two objects interact:
There are two forces that result – one on each object – because forces occur in pairs.
The forces are equal and opposite.
One force is called an action force, the other is called a reaction force.

5. Action Force: Object A exerts a force on object B in one direction.
Reaction Force: Object B exerts a force back on object A in the opposite direction.
Action Force
50 N A B
Reaction Force
50 N

6. A B The forces are acting on different objects.
The forces do not cancel each other out because they are acting on different objects.
We only cancel out forces acting on the same object.
Due to other forces on the balls, one could be at rest and one could be moving.
Net Force on object A = 0N, so it could be at rest.
Action Force =
50 N
Net Force on object B = 40N, so it would move to the right. A B
Applied force =
50 N
Friction Force =
10 N
Reaction Force =
50 N

7. Example #1:
45 N
If you were standing on a skateboard and pushed against the wall, the wall would have pushed back on you which would have caused you to accelerate (change your velocity – speed and/or direction).

8. Example #1:
45 N
Opposite: You pushed on the wall to the right (action), the wall pushed on you to the left (reaction) – opposite directions.

9. Example #1:
45 N
Equal: The force you exerted on the wall and force the wall exerted on you were equal. Remember, while the forces were equal, they did not cancel each other out because they were acting on different objects. The action force was exerted on the wall, the reaction force was exerted on you.

10. Example #1:
45 N
Movement: You: You moved because the forces on your body were unbalanced. (Law #1) The normal force the wall exerted on you to the left was greater than your inertia and friction, so you moved to the left. (The wheels on the skateboard reduced your friction. This made you easier to move.)

11. Example #1:
45 N
Wall: The wall did not move because it has a greater inertia due to its mass. The applied force you exerted was not large enough to make it
move.

12. Example #2:
When we walk, we push off the ground (action) and move forward because the ground pushes back on us (reaction) with the same force. The forces are in opposite directions – you push back on the ground, the ground pushes you forward.

13. Example #3:
A car driving down the road works much the same way as walking: The wheels spin and put a backward force on the road. (action force) The road exerts an equal and opposite forward force on the wheels (reaction force), pushing the car forward with the same amount of force as the wheels put on the road.

14. Example #4:
A fish is propelled through the water because of Newton’s Third Law: Action – Its fins put a backward force on the water. Reaction - The water puts a forward force on the fish, which propels the fish through the water.
The size of the force on the water equals the size of the force on the fish. The direction of the force on the water (backward) is opposite the direction of the force on the fish (forward).

15. Example #5:
Action-reaction force pairs make it possible for birds to fly:
Action - The wings of a bird put a downward force on the air. Reaction - The air puts an upward force on the bird’s wings.
The size of the force on the air equals the size of the force on the bird The direction of the force on the air (downward) is opposite the direction of the force on the bird (upward).

16. Same force, different effect: Example #1:
While forces in an action-reaction relationship are equal, their effects can be different:
A wrecking ball smashes through a wall. It seems like the ball puts a larger force on the wall than the wall puts on the wrecking ball, but the ball and the wall experience the exact same force.

17. Same force, different effect: Example #1:
The effect of the force on each object, however, results in a different acceleration.
Remember, according to the 2nd Law of Motion, when the force is the same, if mass increases, acceleration decreases and if mass decreases, acceleration increases.
You may think the wall has more mass than the ball, but look carefully. This wall is made of small bricks, with small mass, and therefore small inertia.

18. Same force, different effect: Example #1:
The smaller the mass, the larger the acceleration from the same force.
Since the bricks have smaller mass than the ball, they will accelerate, or move, more than the ball from the same force.
The wall and the ball experience the same force but they experienced different effects partly because of their different masses.

19. Same force, different effect: Example #2:
When you hit a baseball with a bat, the bat exerts a forward force on the baseball (the action force); the baseball exerts a backward force on the bat (the reaction force).

20. Same force, different effect: Example #2:
The baseball moves forward, but the bat does not move backward.
This is due to their different masses and other forces acting upon each object.

21. Same force, different effect: Example #2:
The baseball has less mass, so it will experience a greater acceleration from the same force.
The baseball has the additional force of air resistance, but that force is not enough to cancel out the applied action force from the bat, so the ball moves forward.
Air Resistance = 5 N
Action Force = 50 N
ball moves forward

22. Same force, different effect: Example #2:
The bat has more mass and has the additional contact force from your hands. Your contact force is greater than the ball’s reaction force, so the bat continues to move forward at a slightly slower speed.
Contact Force = 70 N
Reaction Force = 50 N
bat also moves forward

23. Can you think of other examples?

25. The hand hitting (pushing) the ball is the action force.
Example #1:
70 N
The hand hitting (pushing) the ball is the action force.
The ball pushing back on the hand is the reaction force.
The two forces are opposite directions.
The forces are equal in strength, or size.
The forces are acting on different objects.

26. Same force, different effect: Example #1:
Why does it hurt so much when you stub your toe? When your toe exerts a force on a rock, the rock exerts an equal force back on your toe. The harder you hit your toe against it, the more force the rock exerts back on your toe (and the more your toe hurts). The effect on your toe is worse than the effect on the rock, but the force is the same.

27. Review

28. The truck is in motion. What is the force that causes it to stop?
The push of the stopped car.
The car is at rest. What is the force that causes it to move?
The push of the truck.

29. What about the ladder on top of the truck?
The ladder is in motion because the truck is in motion.
When the truck stops, the ladder stays in motion.
The truck is stopped by the force of the car, but the ladder is not.
What force stops the ladder?
Gravity.

30. The truck is in motion, the car is at rest
The truck is in motion, the car is at rest. How do each of these vehicles accelerate?
The truck stops moving. The car starts moving.
Which one will be the hardest to accelerate?
The truck because it has the most mass.

31. Why does the car move [accelerate] when it is hit by the truck?
The heavy and moving truck has more force than the small, at rest car.
Why does the truck stop moving when it hits the car?
The force of the car pushing back on the truck, plus the force of friction between the massive truck and the road slow down, the stop the truck.

32. The truck hits the car. An action force stops the truck.
What is the equal and opposite reaction force?
The force that pushes the car forward.