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# M1 mechiancs help :) Watch

1. A ball of 0.2kg falls vertically and strikes the horizontal ground with a speed of 8m/s. The ball rebounds vertically with a speed of 6m/s

i) find the impulse on the ball.

okay so I drew a diagram and assigned a positive direction.

I then did I= (0.2)(6)-(0.2)(-8) = 2.8Ns

according to my mark scheme they did I=(0.2)(-6)-(0.2)(8) = -2.8Ns.

Surely i can take either as positive? Why is this? Thanks!
2. The impulse on the ball with be equal to the impulse on the wall... But because you've taken downwards as positive (i assume) the impulse on the ball will be upwards and therefore negative.

Hope this helped

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3. Also if you're interested the -6 came because the velocity was upwards which is negative if you've taken downwards to be positive

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4. (Original post by Toysith)
The impulse on the ball with be equal to the impulse on the wall... But because you've taken downwards as positive (i assume) the impulse on the ball will be upwards and therefore negative.

Hope this helped

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But what is the correct answer? The mark scheme says -2.8 ...
5. The correct answer is -2.8, you forgot to take the final velocity of the ball as negative... because you defined positive as downwards then the final velocity of the ball is -6, because the ball is travelling upwards...
6. Actually sorry... ignore everything I've just said... You've taken positive to be upwards, so what you said is correct because the impulse acts in that direction

Sorry I didn't read what you'd put properly...

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