Toast210
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#1
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#1
I'm stuck on one part of the whole question which is really annoying.
A pile-driver has a block of mass 2 tonnes (2000kg) which is dropped from a height of 5m on to a pile of mass 600kg which it is driving vertically into the ground. The block rebounds with a speed of 2m/s immediately after the impact. Taking g to be 10m/s^2, find

I) the speed of the block before impact (Answer is 10 m/s)

PE goes into KE
MGH = 2000 * 10 * 5 = 100000 J
Square root(100000/0.5*2000) = 10m/s

ii) the impulse acting on the block (Answer is 24000Ns)

Impulse = Change in momentum
Impulse = m(v-u)
Impulse = 2000(10--2)
Impulse = 24000 Ns

iii) the impulse acting on the pile (Same answer as above)

Due to Conservation of Momentum, impulse must be equal but in the opposite direction. So 24000 Ns but in opposite direction.

From the moment of impact the pile takes 0.025s to come to rest.
iv) Calculate the force of resistance on the pile, assuming it to be constant.
I am unable to do iv. I'm sure I'm forgetting something in the question which I have to factor in as I got 960000 N while the correct answer is 952000N.
Any help would be appreciated
Last edited by Toast210; 1 year ago
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Gent2324
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where is this question from?
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Dancer2001
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For part 1 you should just be able to use suvats, and for part 2, have you tried writing an equation for the change in momentum using the information given in the question?
Could you post your working out as well?
Last edited by Dancer2001; 1 year ago
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Toast210
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(Original post by Gent2324)
where is this question from?
It's from MEI OCR Further Maths Mechanics. It is in the Momentum and Impulse chapter.
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Toast210
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(Original post by Dancer2001)
For part 1 you should just be able to use suvats, and for part 2, have you tried writing an equation for the change in momentum using the information given in the question?
Could you post your working out as well?
For the first parts, I actually used Work and Energy.
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Dancer2001
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(Original post by Toast210)
For the first parts, I actually used Work and Energy.
That’s probably a lot quicker actually
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origamigoose
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#7
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you know the pile driver is at 10 m/s just as it hits the surface. you know it final velocity will be 0 m/s as it comes to rest. you know the time taken is 0.025 seconds. work out acceleration and then use that to work out Force.
Last edited by origamigoose; 1 year ago
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aramis8
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Hi, I'm also stuck on the iv), any help is welcome, thanks
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aramis8
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Impulse acting on pile = 24000

So force acting on pile = 24000/0.025 = 960000N

960000 - resistance= 600a (for pile) That's as far as I got

How can I find a for the pile?
Last edited by aramis8; 1 month ago
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aramis8
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#10
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using conservation of momentum, the pile has initial velocity 40m/s

Then I can use v = u + at to get a = -1600

Then 960000 - R = 600 x - 1600, which gives R = 1920000 N which is incorrect
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