Physics help!

Question will be in the next post :smile:

Reply 1

5hyl33n

OP

Volunteer Team, Clearing and Applications Advisor

21

For question 6, I should've done:
Momentum = mass x velocity
= 5 x 10^26 x 500
= 2.5 x 10-23 kg ms^-1

Is that correct?

Also, I have no clue how to do question 7 and 8, but perhaps I'm just being stupid. :dontknow:

Physics.jpg185.7KB

Reply 2

5hyl33n

OP

Volunteer Team, Clearing and Applications Advisor

Q6 seems correct to me, I'm just having a look at the other two for you...

Reply 4

jamiet0185

16

I don't think Q7 is even a correct English sentence, is it? Doesn't make any sense to me I'm afraid and I get the feeling you need the answer for that in Q8. Hopefully somebody might be able to come along and be of more help. Sorry 😬

Reply 5

5hyl33n

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Volunteer Team, Clearing and Applications Advisor

21

Original post by jamiet0185

I don't think Q7 is even a correct English sentence, is it? Doesn't make any sense to me I'm afraid and I get the feeling you need the answer for that in Q8. Hopefully somebody might be able to come along and be of more help. Sorry 😬

That's okay! It didn't make sense when I read it too. I believe they are asking what the momentum is from rebounding from the end of the box to the front of the box. Does that makes sense?

Reply 6

jamiet0185

16

I see what you mean yeah...
I'm not sure but there's no indication that the particles lose any velocity after hitting the walls. I guess if they did they'd all end up stationary eventually? So maybe the velocity is the same as before just in the opposite direction so the negative of the original momentum. I don't know whether that's right, it's just the best I can offer I'm afraid 😄👍

Reply 7

Stonebridge

13

Original post by 5hyl33n

For question 6, I should've done:
Momentum = mass x velocity
= 5 x 10^26 x 500
= 2.5 x 10-23 kg ms^-1

Is that correct?

Also, I have no clue how to do question 7 and 8, but perhaps I'm just being stupid. :dontknow:

Yes that's correct.
This question is all about what is known as the Kinetic Theory of Gasses. This idea of air molecules pounding against the inside of the container explains to the pressure of the gas inside.
AS the molecule hits the wall at right angles, it rebounds straight back.
Such a collision must be elastic, meaning there is no energy loss, and it bounces back with the same speed it hit the wall.
So if the momentum before was, as you calculated, plus 2.5 x 10^-23, then after the collision, the momentum is minus that, because it is in the opposite direction. (momentum is a vector)
So the change in momentum is 2 x that.
Consider momentum before = mu and momentum after = - mu
change in momentum is momentum after minus momentum before = (-mu) - mu = -2mu

(edited 3 years ago)

Reply 8

5hyl33n

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Volunteer Team, Clearing and Applications Advisor

21

Thanks @Stonebridge! I figured it out before but this was a good way to check my answer!