# m2 powerWatch

Thread starter 8 years ago
#1
a book slides down a rough desk, incline = 30, and falls to the ground which is 0.8 below the edge of the lid. The coefficient of friction is 1/4. Given thatthe books starts from rest 0.4m from the bottom of the lid which is hinged at the edge of the desk, find the speed at it which it hits the floor.

So, did:

gh - 1/4 R * 0.4 = KE
9.8 * 0.4sin30 - 1/4 R * 0.4

However, how can I work out R if there is no mention of the mass of the book?
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8 years ago
#2
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Thread starter 8 years ago
#3
But I don't know what 'm' is and hence cannot calculate R
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8 years ago
#4
(Original post by SilverstarDJ)
But I don't know what 'm' is and hence cannot calculate R
It "m" term will cancel.
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Thread starter 8 years ago
#5
(Original post by electriic_ink)
It "m" term will cancel.
I can't see where it cancels out --- could you elaborate on that please?
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Thread starter 8 years ago
#6
ahhh yes, I see where they cancel out. Thanks.

However, I'm not sure how to calculate the speed.

I tried mgh - 1/4 mgcos30 * 0.4 = 1/2 m v * v

And the the value for v, I put into the suvat equation, but the I get an incorrect answer. Any advice on what I have done wrong?
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8 years ago
#7
Show me your suvat equation
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Thread starter 8 years ago
#8
v * v = u * u + 2as
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8 years ago
#9
(Original post by SilverstarDJ)
v * v = u * u + 2as
>__> I meant your working with it
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Thread starter 8 years ago
#10
(Original post by 2710)
>__> I meant your working with it
0.5mv*v = mgh - friction * distance
0.5 v*v = 9.8sin30 - 0.25 * 0.4* 9.8 cos30
v * v = 4.051....

Into suvat:

v * v = 4.051... + 2x 9.8 x 0.8 = 19.73
v= 4.41

(4.23 is the answer on the answer sheet)
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8 years ago
#11
(Original post by SilverstarDJ)
0.5mv*v = mgh - friction * distance
0.5 v*v = 9.8sin30 - 0.25 * 0.4* 9.8 cos30
v * v = 4.051....

Into suvat:

v * v = 4.051... + 2x 9.8 x 0.8 = 19.73
v= 4.41

(4.23 is the answer on the answer sheet)
The bolded parts are wrong. Firstly, you are missing the actually height of 0.4. It's meant to be 0.4x9.8sin30. It will then give you the right answer
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Thread starter 8 years ago
#12
Thank you very much for your help. It is much appreciated.
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