# Homework Help: Transfer of momentum

1. Jun 26, 2017

### nilic1

1. The problem statement, all variables and given/known data
Does the speed of a ball increase, decrease or remain the same once it leaves the hand as it being thrown horizontally?

2. Relevant equations
Law of conservation of momentum

3. The attempt at a solution
According to my calculations, it increases as momentum is transferred from the arm/hand to the ball. I feel it is right though according to a friend of mine it is wrong as the speed should be unaltered according to Newton's 1st law. Who of us is right?

2. Jun 26, 2017

### Staff: Mentor

(Bold underline emphasis is mine)
Once the ball has left the hand, what influence can the hand possibly have on the ball afterwards? What forces, if any act on the ball once it has left the hand? Draw a FBD.

3. Jun 26, 2017

### nilic1

None. Except weight and drag. So who is right? :)

4. Jun 26, 2017

### Staff: Mentor

Does the force of its weight cause it to accelerate?

5. Jun 26, 2017

### Staff: Mentor

As it stands, based upon the logic used to deduce each answer, neither of you is right

6. Jun 26, 2017

### nilic1

Pardon me but how can it be that none of us is right. The speed of an object immediately after being thrown can either increase, decrease or remain the same. I don't see any other option. We are assuming no external forces are acting and using the law of conservation of momentum.

7. Jun 26, 2017

### Staff: Mentor

One of you can have the correct answer but for the wrong reasons (incorrect logic/working). You'd get few or no marks on a test or submitted homework for an answer based upon faulty logic.

8. Jun 26, 2017

### nilic1

I don't know whether you are understanding me. On the same principle, imagine 2 objects moving together as in the case of a person on a horse moving at 10 m/s. If the horse suddenly stops with zero velocity, the person will keep moving forward with a greater speed compared with the speed he was moving previously. Unless this is right my teachers are teaching me wrong stuff.

In my opinion Newton's 1st applies in conjunction with the principle of conservation of momentum.

When according to Newton's first law we say that the object continues to travel at constant speed, it does not mean the speed which both hand and ball have before they get detached from each other. The constant speed starts from the moment that the two bodies become detached. Am I right?

9. Jun 26, 2017

### Staff: Mentor

If you can neglect air drag, then, for the system you described, the horizontal component of velocity remains constant. What do you think happens to the vertical component of velocity? What is the speed of the ball expressed algebraically in terms of its horizontal and vertical velocity components?

10. Jun 26, 2017

### Staff: Mentor

You are either misunderstanding what your teacher is saying or your teacher is saying something incorrect.

The person would continue moving at 10 m/s if the horse suddenly stopped (presuming that the person and horse are not attached in some way). Momentum transfer, as in a collision, requires that forces act between the objects. If the man is simply perched on the horse and he can slide without friction off of it, then no momentum transfer will take place.
The hand and ball become entirely independent once contact is broken. They share the same speed up to and including the instant of release. It is not a collision (the hand is not batting the ball, it is throwing it).

11. Jun 26, 2017

### nilic1

So basically you are saying that momentum cannot be conserved in such cases. Just to be sure.. one last example on the same idea.. Having two kids riding the same bike and one of them jumps of landing with zero velocity does not imply that the velocity of the bike will increase. According to you as the boy jumps off he cannot transfer momentum and so the law of conservation of momentum does not apply. Sorry for insisting but these are all examples that we were given and in which the law of conservation was applied.

12. Jun 26, 2017

### Staff: Mentor

Momentum is conserved in all cases. Why are you not responding to my posts? I find this very rude.

13. Jun 26, 2017

### Staff: Mentor

It is not conserved since there are external forces acting on parts of the system (the horse suddenly stops... what makes it stop?). Any part of the system that is not influenced by those forces will be unaffected and will be effectively their own isolated systems and will retain their own momentum separately.
No! If he jumps off in such a way as to change his own velocity he must do so by imparting a force on the bike and/or other rider. That force interaction is the mechanism for momentum transfer between the elements of the system. In your horse example you did not have the rider jump or push off. You simply had the horse stopped while the rider slid off without friction, i.e., maintained his own momentum independently since there was no force acting between the horse and rider in the direction of motion.

Last edited: Jun 27, 2017
14. Jun 26, 2017

### nilic1

Got it finally. Your help is greatly appreciated!