aramis8
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Any ideas how they got 952000N for part (iv)

Thanks

Other answers:

i) 10 m/s
ii) 24000
iii)24000
iv)952000 N
v) 0.5 m
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aramis8
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This is my train of thought so far:

During impact the pile driver imparts a force on the pile when gives it a velocity of 40m/s (using conservation of momentum)

Now we know u = 40, t = 0.025, v = 0, which enables us to find a = -1600

Now after impact what forces are acting on the pile? obviously R from the ground, but also the weight of the pile right?

so weight of pile - R = ma? this still doesn't elicit R= 952000 N
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ghostwalker
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(Original post by aramis8)
This is my train of thought so far:

During impact the pile driver imparts a force on the pile when gives it a velocity of 40m/s (using conservation of momentum)

Now we know u = 40, t = 0.025, v = 0, which enables us to find a = -1600

Now after impact what forces are acting on the pile? obviously R from the ground, but also the weight of the pile right?

so weight of pile - R = ma? this still doesn't elicit R= 952000 N
Don't see anything wrong with your reasoning - yes, you do need to take into account the weight of the pile - and the given answer is incorrect, IMO.
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aramis8
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(Original post by ghostwalker)
Don't see anything wrong with your reasoning - yes, you do need to take into account the weight of the pile - and the given answer is incorrect, IMO.
Thanks for your reply. So that would make R = 965880 N hmmm
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aramis8
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(Original post by ghostwalker)
Don't see anything wrong with your reasoning - yes, you do need to take into account the weight of the pile - and the given answer is incorrect, IMO.
Also, can we assume the impact is instantaneous, so the pile goes from zero to 30m/s in a negligible amount of time? so for most if not all of the 0.025 seconds the pile is only affected by resistance from ground and it's own weight?
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mqb2766
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(Original post by aramis8)
Any ideas how they got 952000N for part (iv)

Thanks

Other answers:

i) 10 m/s
ii) 24000
iii)24000
iv)952000 N
v) 0.5 m
* The 952000 is with g=9.8 and incorrectly forgetting about the pile weight during ground resistance deceleration. Maybe one of those different books/different g/same question "typos".
* Your new R value incorrectly uses g=9.8 and 10 in the same expression.
* Instantaneous impact - yes - the block immediately rebounds in the question.
* Yes, the net deceleration of the pile (resistance - gravity) acts for 0.025s.
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aramis8
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(Original post by mqb2766)
* The 952000 is with g=9.8 and incorrectly forgetting about the pile weight during ground resistance deceleration. Maybe one of those different books/different g/same question "typos".
* Your new R value incorrectly uses g=9.8 and 10 in the same expression.
* Instantaneous impact - yes - the block immediately rebounds in the question.
* Yes, the net deceleration of the pile (resistance - gravity) acts for 0.025s.
Thank you thank you! This makes so much sense.
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