# Conservation of Momentum & Newton's 3rd Law

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#1
Hey all,

Just revising for my OCR Gateway physics exam tomorrow and I got a little confused. Could somebody please answer these questions and explain how?!

Q1. Claire (mass 55kg) is standing on a skateboard (mass 5kg) at rest. She then jumps forwards off the skateboard at 1.8 m/s.

a) What was the total momentum of Claire and the skateboard before jumping?
b) what was Claire's momentum after she jumped?
c) Calculate the final momentum of the skateboard?

Also this question:

A 90g apple is fired at 10m/s from a stationary toy cannon which has a mass of 2kg. The cannon recoils as the apple is fired. Calculate how fast it will move backwards.

Ellie
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6 years ago
#2
(Original post by staywithme)
Hey all,

Just revising for my OCR Gateway physics exam tomorrow and I got a little confused. Could somebody please answer these questions and explain how?!

Q1. Claire (mass 55kg) is standing on a skateboard (mass 5kg) at rest. She then jumps forwards off the skateboard at 1.8 m/s.

a) What was the total momentum of Claire and the skateboard before jumping?
b) what was Claire's momentum after she jumped?
c) Calculate the final momentum of the skateboard?

Also this question:

A 90g apple is fired at 10m/s from a stationary toy cannon which has a mass of 2kg. The cannon recoils as the apple is fired. Calculate how fast it will move backwards.

Ellie
Hi. The principle of the conservation of momentum is that the momentum before an event (collision/impact/firing e.g. of a gun/cannon, etc) is the same as the momentum afterwards, i.e. no momentum is gained or lost.

Momentum = mass x velocity and is a vector, so momentum in the opposite direction is treated as negative.

Taking the first question.
a) As Claire and the skateboard were at rest, the momentum before is zero.
b) The momentum afterwards has to be zero (applying the conservation of momentum), so the forwards momentum of Claire is compensated by the skateboard moving backwards.
Claire's forward momentum = 55 x 1.8 = 99Nm (or kgms-1)
c) Hence the momentum of the skateboard has to be 99Nm backwards (in the opposite direction)

From this, see how you get on in the second question. It may be worth ensuring first if you understand why the toy cannon recoils after firing, before attempting the calculation. If you're not sure, go back to your first question and see if you can work out the speed of the skateboard (not asked for, but good practice for revision), then come back. Otherwise I'll put up my answers, and working, if needed.

p.s. note that the mass of the apple is given in grams, not kilograms!
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#3
Thankyou- you saved me before my exam!
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