Question regarding momentum

[firebird69guy]

This is a question from Kaplan FL 6... If you haven't taken it yet, this is a warning to not read any further...



Two guns, separated by 5m and pointed at each other are tilted so that each gun points at an angle theta above the horizontal. Both guns are fired simultaneously, with each bullet leaving its gun at a speed of 250m/s. The bullets collide inelastically in midair, falling together to the ground.

Obviously KE is not conserved (since the collision is inelastic).

Why is total momentum not conserved? The way I understand it, momentum is conserved during a collision (both elastic and inelastic). And I'm sure this would be the case for say, two automobiles crashing head-on.

Question: How does gravity (acting on both bullets) change the conservation of momentum?

---

Members don't see this ad.

 

[Shrike]

Remember that momentum is the change in impulse, so as t (time the objects are falling) increases, so will impulse and, therefore, momentum.

Uh, wrong. But nice use of boldface.

Impulse is the change in momentum, not vice versa.

The rest of the above answer is correct. If the credited response said something about momentum's not being conserved, they're not considering the Earth, as the above post explains. We'd need to consider the other answer choices -- this is a process of elimination question.

The OP is correct that momentum is conserved in all collisions. Everything else, too, as it turns out.

 

[firebird69guy]

Answers:


A. Only the momentum of the system is conserved.

B. Only the KE of the system is conserved.

C. Both the total momentum and KE of the system are conserved.

D. Neither the total momentum nor the KE of the system are conserved.


The correct answer is D. So you mean that the earth gives momentum to the bullets? So if the bullets GAIN momentum, and the earth loses momentum, how is momentum is not conserved? I see what you mean, the earth must be included in the system in order for A to be correct.

BUT, how does a gain in momentum equal a loss in conservation of momentum?

 

[Prophecies]

Crap...I was trying to reply to my post and I accidently edited it...ANYWAY...if you realize that the momentum of a system is conserved, rather than always memorizing the "momentum of a collision," you can work it out logically and not have to rely on the process of elimination.

Yes, sorry about the mixup...I = change in p

 

[Prophecies]

Answers:


A. Only the momentum of the system is conserved.

B. Only the KE of the system is conserved.

C. Both the total momentum and KE of the system are conserved.

D. Neither the total momentum nor the KE of the system are conserved.


The correct answer is D. So you mean that the earth gives momentum to the system? So if the bullets GAIN momentum, momentum is not conserved?

Apparently the Kaplan test is not considering the Earth part of your system: only the bullets. Since the bullets are increasing in momentum then the momentum appears not to be conserved (since nothing is LOSING momentum). However (just to be safe), if the Earth were part of your system then the momentum WOULD be conserved because as the bullets gained momentum the Earth would be losing momentum. In real life, this is how it works and momentum is always conserved.

 

[firebird69guy]

Thanks for the clarification..


I guess the question needs to specifiy what the system consists of.




But I get the concept now..

 

[sdnstud]

This is a very common question on the MCAT.

momentum is not conserved when there is a force. in this case, force = gravity.

I had a similar question on the real mcat. 2 pendulums, one held at 90o to the left and one held at 90o to the right. they are let go and collide at the bottom (zero degree). is momentum conserved? the answer is no because gravity is acting on both pendulums.

Answers:


A. Only the momentum of the system is conserved.

B. Only the KE of the system is conserved.

C. Both the total momentum and KE of the system are conserved.

D. Neither the total momentum nor the KE of the system are conserved.


The correct answer is D. So you mean that the earth gives momentum to the bullets? So if the bullets GAIN momentum, and the earth loses momentum, how is momentum is not conserved? I see what you mean, the earth must be included in the system in order for A to be correct.

BUT, how does a gain in momentum equal a loss in conservation of momentum?

 

[tank you]

so is that momentum is not conserved because the force of gravity is acting upon the objects? or is it because the force of gravity causes an increase in internal energy when they collide??

 

[Shrike]

BUT, how does a gain in momentum equal a loss in conservation of momentum?

Conservation just means it stays the same. If it is increasing (or changing direction, at first), it isn't staying the same, i.e., it isn't conserved.