Newtons 2nd and 3rd law question

[The Fish]

Hello!

I am just beginning Physics 11 IB.

I am having a bit of trouble with a question. Perhaps the answers in the back of my book are wrong (as they tend to be sometimes.)

For those of you who have it, it is "Physics for the IB Diploma" by K. A. Tsokos. Page 86, #24.

Two bodies are joined by a string and are pulled up an inclined plane that makes an angle of 30 degrees to the horizontal. So it is an inclined plane. At the bottom is a 8.0kg body, then a string connecting it to a 4.0kg body, and there is a force F on the 4.0kg body, all at an angle of 30 degrees.

Calculate the tension of the string when:

a.) the bodies move with constant speed

That is easy. The tension is mg sin30, since that is the parallel force of gravity on the 8.0kg body, so 80 sin30 = 40N.

b.) The bodies move up the plane with an acceleration of 2.0 ms^-2

I thought it would be the same, but the back of the book says it is 56N. Where does this number come from?

c.) What is the value of f in each case?

I don't know how to find that either.

Do YOU?

[dessskris]

I have the same textbook and have finished this chapter.

a) T=mg sin30

b) T=mg sin30 + ma

c) case a) F=T + Mg sin30

c) case b) F=T + Mg sin30 + Ma

m=8.0 kg and M=4.0 kg

[dessskris]

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Kind regards,

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