f.4 phy,, about mechanics

[匿名]

2009-03-01 7:15 pm

我有條mechanics唔識..
question: http://xs536.xs.to/xs536/09096/phy_3628.png
可唔可以教我做晒(a) - (d) ?
唔好直接寫答案比我.. 因為我想知個計數ge過程..
in fact,, 我唔係好識畫free-body diagram同拆力..
可唔可以教埋我?
THX*

回答 (1)

Prof. Physics

2009-03-01 7:44 pm

✔ 最佳答案

For the free-body diagrams, please visit:

http://i726.photobucket.com/albums/ww265/physicsworld2010/physicsworld01Mar011139.jpg

c. Consider the whole system

By Newton's 2nd law of motion

F = (mA + mB)a (no need to consider T, since it is an internal force in the system)

50 = (5 + 3)a

acceleration of the system, a = 6.25 ms^-2

Consider block A

Tension provides the force for A to accelerate.

By Newton's 2nd law of motion

T = mAa

T = 5(6.25)

Tension in the string, T = 31.25 N

d. When the block starts to move, it has a tendency to move up the plane. Hence, the friction on A should act down the plane.

So, F = mgsin30* + f

35 = (5)(10)sin30* + f

Friction, f = 10 N

參考: Physics king

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