AQA (New spec) mechanics question help

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jessyjellytot14
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#1
Report Thread starter 6 years ago
#1
(a) Show that the change in momentum of the golf ball during the collision isabout 0.5 N s.
(Don't worry about this, I got 0.495 Ns )

(b) The ball strikes the plate with a speed of 7.1 m s–1 and has a mass of 45 g. It leaves the plate with a speed of 3.9 m s–1.Show that this is consistent with a change in momentum of about 0.5 N s.
Can't show this :\ I keep getting 0.144

(c) The ball continues to bounce, each time losing the same fraction of its energy when it strikes the plate. Air resistance is negligible.Determine the percentage of the original gravitational potential energy of the ball that remains when it reaches its maximum height after bouncing three times.

Wtf???

Basically can you help me with parts (b) and (c) please, they're making me want to rip my hair out.

Here's the link to the paper if you want to see the actual question, it is question 3 in section B of the paper.
http://physicsloreto.weebly.com/uplo..._2_qp_v1.1.pdf
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Teenie2
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#2
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#2
(b) Change in momentum = (mass * final velocity) - (mass * initial velocity) = mass ( final velocity - initial velocity) = 0.045 * (3.9 - ( -7.1)) = 0.045 * 11 = 0.495 Ns

The initial velocity is in the opposite direction from the final velocity so the sign is negative.
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Someboady
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#3
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#3
(Original post by jessyjellytot14)
(a) Show that the change in momentum of the golf ball during the collision isabout 0.5 N s.
(Don't worry about this, I got 0.495 Ns )

(b) The ball strikes the plate with a speed of 7.1 m s–1 and has a mass of 45 g. It leaves the plate with a speed of 3.9 m s–1.Show that this is consistent with a change in momentum of about 0.5 N s.
Can't show this :\ I keep getting 0.144

(c) The ball continues to bounce, each time losing the same fraction of its energy when it strikes the plate. Air resistance is negligible.Determine the percentage of the original gravitational potential energy of the ball that remains when it reaches its maximum height after bouncing three times.

Wtf???

Basically can you help me with parts (b) and (c) please, they're making me want to rip my hair out.

Here's the link to the paper if you want to see the actual question, it is question 3 in section B of the paper.
http://physicsloreto.weebly.com/uplo..._2_qp_v1.1.pdf

Lol rest assured it was a tricky question.
For part a, its important you use the graph
For part b I = m (v - u). Convert the mass into kg, decide on a positive direction and substitute apt values.
For part c (i actually wrote wtf on this question ) xD but basically, the clue is "conservation of momentum". Momentum must be conserved. Thus as the ball approaches earth, the eart approaches the ball. Make sense? It sounds ridiculous I know but it approaches the earth very very slowly due to its large mass. If you think about this along with conservation and give 3 points, you should get the marks
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Teenie2
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#4
Report 6 years ago
#4
The PE at the start would be the same as the KE just before impact = 0.5 mv^2 = 0.5 * 0.045 * 7.1 * 7.1 = 1.134 J

After the first bounce, KE = 0.5 * 0.045 * 3.9 * 3.9 = 0.342 J. This will convert back into PE at the top of the bounce meaning that the PE has changed from 1.134 to 0.342 i.e. 30% of the original value. It bounces two more times - each time losing 30% of its value so you can work it out from there.
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jessyjellytot14
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#5
Report Thread starter 6 years ago
#5
(Original post by Someboady)
Lol rest assured it was a tricky question.
For part a, its important you use the graph
For part b I = m (v - u). Convert the mass into kg, decide on a positive direction and substitute apt values.
For part c (i actually wrote wtf on this question ) xD but basically, the clue is "conservation of momentum". Momentum must be conserved. Thus as the ball approaches earth, the eart approaches the ball. Make sense? It sounds ridiculous I know but it approaches the earth very very slowly due to its large mass. If you think about this along with conservation and give 3 points, you should get the marks
Thank you, this makes a bit more sense. I would never get that in an exam though :p:S
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