Year 12s: what will be the first way you research your future options? >>

M1 help

G.Y

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I don’t really understand part b at all

Reply 1

Prasiortle

Original post by G.Y

I don’t really understand part b at all

Use conservation of momentum to write an equation. Note also that there is a slight ambiguity in the question, as the relative velocity could be interpreted as starting to act just before or just after the sack is thrown. The mark scheme allowed either interpretation to gain full marks.

Reply 2

G.Y

Original post by Prasiortle

I got an equation of 40 - 80v = 40 but obviously that’s wrong but I can’t see how

Reply 3

Prasiortle

Original post by G.Y

I got an equation of 40 - 80v = 40 but obviously that’s wrong but I can’t see how

It should be 40 - 80v = 20*(0.4-2), or 40 - 80v = 20*(v-2), depending on which interpretation you take. You have to read carefully and see that the 2 is a relative velocity.

(edited 5 years ago)

Reply 4

G.Y

Original post by Prasiortle

It should be 40 - 80v = 20*(1.4-2), or 40 - 80v = 20*(v-2), depending on which interpretation you take. You have to read carefully and see that the 2 is a relative velocity.

So I need to work out the actual velocity?

Reply 5

Prasiortle

Original post by G.Y

So I need to work out the actual velocity?

Yes, momentum requires the actual velocity.

Reply 6

G.Y

Original post by Prasiortle

Yes, momentum requires the actual velocity.

So the trolley has a velocity of 0.4 and if the velocity of the bag relative to the trolley is 2 so the actual velocity is 1.6?

Reply 7

Prasiortle

Original post by G.Y

So the trolley has a velocity of 0.4 and if the velocity of the bag relative to the trolley is 2 so the actual velocity is 1.6?

Well, it's actually -1.6 according to the mark scheme. I think in (a) the velocity is -0.4, but you give the answer as 0.4 as it asks for the speed. But here you need to use the velocity, not the speed.

Reply 8

Prasiortle

Original post by G.Y

So the trolley has a velocity of 0.4 and if the velocity of the bag relative to the trolley is 2 so the actual velocity is 1.6?

EDIT: To clarify some confusion, I've looked at this a bit further and it appears that the mark scheme is taking backwards (the "negative" direction) as the positive direction in part (b), so they get 40 - 80v = -20(2-v). If we use the positive direction is positive, i.e. keep the negative sign of -0.4 from (a), we get 100*-0.4 = 80v + 20(v+2), since if A has a velocity of k relative to B, and B's actual velocity is m, then A's actual velocity is k + m.