1. Newton’s Third Law

2. Newton came up with one more law when he started thinking about the interaction of objects. ● He had already talked about what happens when there is no force (1st Law). ● He then talked about what happens when there is a force (2nd Law). ● But what happens when you have objects interacting, affecting each other?

3. Anytime something applies a force, there will be an equal force back in the opposite direction.
Push on the handle of a lawnmower to make it go forward, and it will push back against you in the opposite direction with just as much force.
This is the pressure that you feel of the handle against your hand.

4. Newton’s Third Law
Every force occurs as one member of a pair of interaction forces.
The two members of a pair of interaction forces act on two different objects.
The members of an interaction pair are equal in magnitude but opposite in direction.
That is, 𝐹 a on b = − 𝐹 b on a.

5. There is one important thing to remember when you are looking at action-reaction pairs.
The two forces that you are looking at are each acting on different objects!
If you are examining what you think are action- reaction forces, but the forces are both acting on the one object, it is not an action-reaction pair.
In the above example, you exert a certain force on the lawnmower. The lawnmower exerts an equal force on you. Two objects, two forces.

6. Predict Observe Explain
Action-reaction pairs can also happen without friction, or even with the objects not touching each other, known as "action at a distance" forces …
Time for a POE. We will do many of these, and the acronym stands for
Predict
Observe
Explain
Time for the fan car!

7. Some examples of action-reaction forces depend on the objects being in direct contact, meaning that the two objects involved are actually touching each other to exert forces on each other. These are called "contact forces." 1. Action: the tires on a car push on the road… Reaction: the road pushes on the tires. 2. Action: while swimming, you push the water backwards... Reaction: the water pushes you forward.

8. 1. Action: a rocket pushes out exhaust… Reaction: the exhaust pushes the rocket forward. What if we are in outer space, with no atmosphere? Does this change your answer? 2. Action: the earth pulls down on an apple… Reaction: the apple pulls up on the earth. How can this example be true?!?

9. Imagine an apple is falling towards the Earth from a height of 100 m.
What is the Force of the Earth on the apple?
~ 5 N.
How long does it take the apple to hit the Earth?
~4.5 s
How far does the Earth move in those ~4.5 s?
The mass of the Earth is ~ 6.0𝑥 𝑘𝑔.
The number is so small, we can’t even measure it! But nonetheless, it still moves the Earth!

10. Newton’s Third Law Continued….
Let’s first finish our conversation about the basketball.

11. Newton’s Third Law
Every force occurs as one member of a pair of interaction forces.
The two members of a pair of interaction forces act on two different objects.
The members of an interaction pair are equal in magnitude but opposite in direction.
That is, 𝐹 a on b = − 𝐹 b on a.

12. POE Time

15. Let’s write out a detailed explanation for our POE.
We are going to make a slice through the middle of the rope with one person pulling on it.

17. Now we will make a slice through the rope with people pulling on each end.

20. Identifying Action/Reaction Pairs
Get into a NEW group of 2 or 3.
We are going to work through one example together.
You will then try a few in your groups.

21. Mickey (40 kg) and Minnie (30 kg) are standing on a frictionless surface. Mickey gives Minnie a 0.20 seconds push of 120 N [east]. Determine the final velocity of each mouse after the push.

22. A passenger in a boat approaching a dock attempts to step onto the dock before the boat has been tied up. The passenger, whose mass is 57 kg, accelerates at a rate of 3.2 𝑚 𝑠 2 [forward] relative to the boat. If the mass of the boat is 97 kg, what is the boats acceleration relative to the passenger? Neglect the horizontal force of the water on the boat.

23. What is the net acceleration of the passenger relative to the dock
What is the net acceleration of the passenger relative to the dock? Is the passenger likely to fall into the water?

24. Blocks X and Y are attached to each other by a light rope, and can slide along a horizontal, frictionless surface. Block X has a mass of 5.0 kg. An applied force of 36 N [right] acts on block X.
Calculate the action/reaction forces the blocks exert on each other.

25. Three boxes are positioned next to each other on a horizontal, frictionless surface. An applied force acting on box A causes all of the boxes to accelerate at 1.5 𝑚 𝑠 2 [right] Calculate the force exerted by box C on box B.
Calculate the force that box B exerts on box A.