phy ce (mechanic)

[匿名]

2009-10-21 5:47 am

有2條想問 thx
1. An object, moving up a smooth inclined plane making an angle O with horizontal decreases its speed from x to y ms-1 . what is the distance , in meters, travelled in this period? (hkcee 1974)
A. (x2-y2)/2g B.(x2-y2)/2gcosO C.(x2-y2)/2gsinO D. (2(x-y))/gsinO

2. an object passing a mark with a velocity y ms-1 eastward is subjected to a constant acceleration of x ms-1 in a westward direction. how long, in seconds, will it take to return to the same mark?
A. y2/x B.y2/2x C. y/x D 2y/x

thanks.

回答 (1)

Prof. Physics

2009-10-22 2:24 am

✔ 最佳答案

1. Assume only normal reaction and the weight act on the object.

Take the direction up the plane be positive.

Acceleration, a = -gsinO

Initial speed, u = x

Final speed, v = y

By equation of motion,

v^2 = u^2 + 2as

y^2 = x^2 - 2gssinO

Distance, s = (x^2 - y^2)/2gsinO

The answer is C.

2. Let eastward be positive direction.

Initial velocity, u = y ms^-1

Acceleration, a = -x ms^-2

Displacement, s = 0

By s = ut + 1/2 at^2

0 = yt - 1/2 xt^2

t = 0 (rej.) or 2y / x

The answer is D.

參考: Physics king

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