M2 Question Watch

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Sparkling_Jules
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I'm stuck. I keep getting the wrong answers for lots of questions then going back and getting the right one. This time however I went back and got an even worse answer. Can someone help?

A box, mass 10kg, on slope at 20* to horizontal, mew = 0.2. Box released from rest and slides down platform, calculate the speed of the box after it has been moving 3s.
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Gaz031
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(Original post by Sparkling_Jules)
I'm stuck. I keep getting the wrong answers for lots of questions then going back and getting the right one. This time however I went back and got an even worse answer. Can someone help?

A box, mass 10kg, on slope at 20* to horizontal, mew = 0.2. Box released from rest and slides down platform, calculate the speed of the box after it has been moving 3s.
F=ma
10gsin20 - 0.2R = 10a
R=10gcos20
10gsin20-0.2(10gcos20) = 10a
gsin20 - 0.2gcos20 = a
a = g[sin20 - 0.2cos20]

u=0, a=g[sin20-0.2cos20], t=3, v=?
v=u+at
v= 3g[sin20-0.2cos20]
I don't have a calculator to hand but that's not too tricky to evaluate.
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Sparkling_Jules
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Aha! cos for R, I was using sin... :embarasse

It's the simplest things that cost me marks.
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Sparkling_Jules
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How about this one:

Car (1400kg) pulling caravan (400kg) up slope (sin&=0.15). Power= 60kW. Resistance against car = 800N, caravan = 300N. At the instant v = 12m/s calculate:
a)acceleration
b)tension in tow-bar

I guess it would help if I could remember connected bodies stuff from M1.....
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Sparkling_Jules
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No, wait.... I've done it!
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Gaz031
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Nice one If you ever have trouble deciding whether to use cos or sin then draw a few dotted lines and mark in the angles. You know that cos resolves 'along' while sin resolves 'perpendicularly'.
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Sparkling_Jules
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I did have dotted lines and soh cah toa; I still managed to muck up!
I frequently miss negatives as well. I can do the hard maths but make silly mistakes!
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