38. As shown on the right, a ball of mass m is attached to a pivot fixed in place by means of a string of length L. The ball is in a state of rest. A constant force F is applied horizontally to cause the ball to swing upwards. What is the kinetic energy of the ball when the string makes an angle of θ with the vertical line?
(A) FL(1-cosθ)
(B) FLsinθ 
(C) FLtanθ+ mgLsineθ
(D) FLsinθ + mgL(cosθ-1)
(E) 0

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統計: A(0), B(0), C(0), D(5), E(0) #3239586

詳解 (共 1 筆)

#6349908
dW = F*ds*cosθ = F*L*dθ*cosθ
∫dW = ∫F*L*dθ*cosθ = F*L*sinθ (0到θ)
推力做工為F*L*sinθ
位能為mgL(1-cosθ)
所以動能為 = 推力做工 - 位能
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