# Homework Help: Corresponding Reaction Forces

1. Oct 5, 2013

### physicsnobrain

1. The problem statement, all variables and given/known data
A bowling ball is suspended from a ceiling hook using a thin piece of rope. If the rope is
exerting a 65 N force upward on the ball, describe the corresponding reaction force in
terms of:
(a) its magnitude
(b) its direction
(c) on what object it is exerted
(d) by which object it is exerted

3. The attempt at a solution

A) The magnitude corresponds to the force of tension in the rope, and the ball isn't moving anywhere.
B) direction corresponds to the force of gravity because this is where the ball will go if it is released from the rope.
C) Force is exerted on the rope
D) Force is exerted by bowling ball because of F = ma

Am I right?

thanks

2. Oct 5, 2013

### physicsnobrain

Or is A) the normal force

3. Oct 5, 2013

### physicsnobrain

Or it is

A) The magnitude is upwards to the cieling hook
B) The direction of the bowling ball is south
C) The force is exerted onto the hook
D) The force is exerted by the bowling ball

4. Oct 5, 2013

### MuIotaTau

D) is certainly correct.

I'm not quite sure that C) is right. It's true the hook feels a force, but the rope is what pulls down on the hook. What pulls on the rope? (Hint: reaction forces occur between two objects only. So if object X exerts a force on Y, the reaction force must be exerted by Y onto X)

For B), I would simply say down because the problem doesn't mention north, south, east, or west. You certainly have the right idea, though. Why is it that the answer to B) is down, though? Can you perhaps think of a physical law that might explain why that's true?

And as for A), magnitude refers to a number. So, for instance, the magnitude of the force the rope is exerting on the ball is 65 N. So, do you know what the magnitude of the force the rope is exerting on the ball is?

5. Oct 5, 2013

### physicsnobrain

I think that that C and D are now:

C) The force is exerted onto the bowling ball
D) The force is exerted by the rope

6. Oct 5, 2013

### physicsnobrain

You said the magnitude of the force the rope is exerting on the ball is 65N, then you asked what is the magnitude of the force the rope is exerting on the ball.

What?

7. Oct 5, 2013

### MuIotaTau

You're right, I'm sorry, that was a typo. You know the magnitude of the force the rope exerts on the ball is 65 N. What is the magnitude of the force the ball exerts on the rope?

8. Oct 5, 2013

also 65N

9. Oct 5, 2013

### MuIotaTau

Exactly! That comes from Newton's Third Law of Motion, which says that the force that X exerts on Y is equal and magnitude and opposite in direction of the force Y exerts on X. So that's A), now what about B)?

10. Oct 5, 2013

### physicsnobrain

The direction is then upwards, in the same direction as the tension force.

11. Oct 5, 2013

### MuIotaTau

Not quite. The reaction force is equal in magnitude, but opposite in direction. So if the rope (the tension force) is pulling the ball up, then the ball should be pulling the rope in which direction?

12. Oct 5, 2013

### physicsnobrain

downward

13. Oct 5, 2013

### physicsnobrain

And C is the force is exerted onto the bowling ball

and D it is exerted by the rope