Since you have a link to the question, do you mind answering my question then?

@TheMan ok, well if I understand correctly, you can use momentum conservation to justify both parts. It seems like you didn't in the first part. So try thinking about that first question in terms of conservation of total momentum.

What you said for the second part makes sense

@DavidZ It seems that I don't quite understand how to answer the 1st question. Do you mind explaining it to me?

@TheMan walk me through your thought process: what don't you understand?

@DavidZ Momentum change = Mass x Velocity, right? Therefore since the question is asking about the momentum change of the two skaters, it is possible that the heavier skater is moving at a slower speed and the lighter skater is moving at a higher speed, therefore they would both have the same momentum change. However, it is also possible that they don't, therefore having a different momentum change, which is why I think the answer is D. This is my thought process.

However, since you said that momentum conservation can be used to justify this question, how do you justify it?

@TheMan momentum change is avg. force x time

yeah, that ^ so think about what forces are exerted on the pair during the time you think their momentum might be changing

@DavidZ @0celo7 Ermmm.. How does force relate to the question?

@TheMan because momentum is not conserved in the presence of unbalanced external forces

@DavidZ What do you mean by that statement?

As far as I can tell, there are no unbalanced external forces like friction acting, right?

I would guess that's why they made the problem about ice skaters rather than ballet dancers. I guess you can't be absolutely sure what the intended model was, but it seems like a reasonable assumption.

@DavidZ Alright so since there are no unbalanced external forces acting on it as far as we can tell, momentum would be conserved, right?

@TheMan yes (or do you have some reason to think otherwise? :-P)

@DavidZ I'm getting a little confused here. So since momentum is conserved, how does it answer the question?

@TheMan if momentum is conserved, it means initial momentum equals final momentum. What do you know about either the initial or final momentum?

@DavidZ I suppose we won't be really certain on the momentum since the velocity is not given?

@TheMan look carefully

@DavidZ Would they have the same velocity if they are skating towards each other?

@TheMan what do you think - is there a reason they should have the same velocity?

@DavidZ I don't think so

@TheMan OK, so maybe try it like this: what information are you given in the problem?

@DavidZ One skater is heavier than the other, and they both have a final velocity of 0

@TheMan yep, so as I said before, what do you know about either the initial or final velocity?

@DavidZ They both have the same final velocity! However, I don't think that we know anything about their initial velocity

@TheMan that's not all you know about the final velocity

@DavidZ huh?

@TheMan the fact that the skaters' final velocities are the same is not all you know about them

@DavidZ We know that the heavier skater lifts the lighter skater?

@TheMan about the skaters' final velocities

@DavidZ I've ran out of things that I can find from the question

@TheMan I'm getting at something you already mentioned a few messages back

@DavidZ The fact that it is possible that the lighter skater is travelling at a higher speed?

Actually, let's move this to another chat room so the conversations aren't so garbled. Give me a moment....

← 36 messages moved from The h Bar

@TheMan not that

they both have a final velocity of 0?

yep

you also need to think about this: what is the system under consideration?

When you talk about momentum conservation, you always need to have a system in mind, for which momentum will be conserved (or not). What's the system in this case?

The 2 skaters?

so momentum of lighter skater = momentum of heavier skater?

No, the momentum you need to work with is the total momentum of the system

Or, rather, yes, but you skipped about half a dozen steps there

wait what did i skipped?

OK let's back up to the point where you identified the system as the two skaters.

I was asking what you know about the final momentum of the system.

okay

so what do you know about the final momentum of the system?

they are equal with the initial momentum of the system?

That is true, but that's just the general law of conservation of momentum (when the net external force is zero). What do you know about the final momentum of this system specifically?

0

So then what is the initial momentum of the system?

I mean like, the final momentum is zero? So therefore the initial momentum is 0 as well?

Yep

Why, did you think it would be something else?

I didn't think about it like that until you put it in this persperctive

Ah, well all part of the learning process then

Haha So how does it relate back to the question?

OK, so you know the initial momentum of the system is zero. What does that tell you about the momenta of the two skaters?

They are equal to each other?

Yes (equal in magnitude at least)

So the lighter female skater was indeed travelling at a higher speed! Right?

Yep, but remember what the problem is asking about

it's asking about the momentum change of the two skaters.

So the answer is (c)?

Well, that's my reasoning for why (c) seems like a reasonable answer

OHHHHHHHHH!!!! Awesome.

Thank you SO MUCH for your help!

No problem

Got to go now. Have a good day!

You too