1. Unit 4-2: Actions and Reactions

2. Identifying Action and Reaction Forces »Identifying action/reaction pairs is not always easy. »Try to identify the action/reaction in the following examples: »A rocket pushes out gas »A person pushes on a crate »A rock is freefalling through the air

3. Identifying Action and Reaction Forces »Lets look at that last one again: »A rock is freefalling through the air »Ask yourself “What is the source of the force?” »The action is the force of gravity from the earth on the rock. »What are we going to call the reactionary force? »It’s not weight, that’s just another term for the force of gravity. »What could it possibly be?

4. Identifying Action and Reaction Forces »The reaction is the boulder pulling up on the earth! »Action/Reaction pairs follow a very simple formula. Let’s say object A and object B have an interaction: »Action: Object A exerts a force on Object B »Reaction: Object B exerts a force on Object A

5. Identifying Action and Reaction Forces »It’s easy to remember. »Identify interactions objects A and B, »If the action is A on B, then the reaction is simply B on A. »Now for the million dollar question: »Why don’t all forces cause all objects to move?

6. Action and Reaction on Different Masses »When looking at a rock falling to the earth: »We say the earth is pulling down on the rock, »And the rock is pulling up on the earth. »But why would we describe it as the rock falling to the earth?

7. Action and Reaction Different Masses »We could also describe the earth as moving to meet the boulder, but the earth doesn’t move nearly as much. Why? »Because of the differences in their masses. »Remember Newton’s Second Law!

8. Action and Reaction on Different Masses »Force = Mass * Acceleration »So if the same magnitude force is applied (as it must be), then the objects will have different accelerations. The earth’s will be miniscule, while the rock’s will be very large.

9. Action and Reaction on Different Masses »A better example is that of a rifle being fired: »When the firing mechanism goes off, the gunpowder ignites and forces the bullet out of the rifle barrel. »This force also pushes back on the gun, which the uses feels in the form of kickback.

10. Action and Reaction on Different Masses »Why does a rifle kick? »The force that propels a bullet at high speeds gets applied to the gun in the opposite direction. »F=ma states that the acceleration of the object is inversely proportional to the mass of the object.

11. Action and Reaction on Different Masses »Since the bullet has relatively low mass, it accelerates a great deal. »But the rifle has significantly more mass, so it doesn’t accelerate as much.

12. Action and Reaction on Different Masses »Let’s look at this formulaically: »F bullet = -F gun »Since F=ma, then: »F bullet = m bullet a bullet »F gun = m gun a gun »m gun > m bullet : »We’ll make m gun visually bigger than m bullet

13. Action and Reaction on Different Masses »F bullet = m a bullet & F gun = m a gun »In order for the two forces to be equal, the accelerations must be different to accommodate: »F bullet = m a & F gun = m a

14. Do Action and Reaction Forces Cancel? »To answer this question, we first have to look at the system: »A system is composed of the object that we are looking at and the forces acting on it. »We ignore forces that it is imparting on other objects

15. Do Action and Reaction Forces Cancel? »Lets have an example where two people push off from each other »Our system is one of the two people, let’s focus on the left one.

16. Do Action and Reaction Forces Cancel? »As the person on the left (person A) pushes on the person on the right (person B), there is a pair of forces created. »A pushes B, B pushes A »If Person A is our system, then Person A does not feel the force that is applied to Person B. »Thus, they feel a net force that is not zero.

17. The Horse-Cart Problem »The Problem: »A horse is pulling on a cart, and the cart is pulling back on the horse. So these forces should cancel out, but the cart and horse accelerate forward. How is this possible?

18. The Horse-Cart Problem »There are multiple ways of looking at the situation (multiple systems): »There is the cart and horse together »The horse system »The cart system »By looking at the different systems, we can figure out the solution.

19. The Horse-Cart Problem »If all we are looking at is the cart, as a farmer would: »The pulling force from the horse applied to the cart accelerates it forward, simple as that.

20. The Horse-Cart Problem »If all we are looking at the horse, then: »The horse applies a pushing force onto the ground, and a pulling force on the cart. »The pulling force on the cart does not affect the horse. »The earth applies a force back on the horse and allows it to accelerate forward.

21. The Horse-Cart Problem »Looking at the horse & cart system: »The interior forces of the horse on the cart and the cart back on the horse do in fact cancel out, but they are not exterior forces. »It is the force of the horse pushing on the earth, and the earth back on the horse that results in the acceleration.