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標題:

Physics: force and motion

發問:

Take g=10 m s-2.A ball of mass 50 g, which is initially at rest, is dropped onto the ground from a height of 1 m. If it rises back to a height of 0.8 m after the rebound, find the change in its momentum during the rebound.A. 0.236 kg m s-1B. 0.250 kg m s-1 C. 0.424 kg m s-1 D. 0.472kg m s-1 My steps are as... 顯示更多 Take g=10 m s-2. A ball of mass 50 g, which is initially at rest, is dropped onto the ground from a height of 1 m. If it rises back to a height of 0.8 m after the rebound, find the change in its momentum during the rebound. A. 0.236 kg m s-1 B. 0.250 kg m s-1 C. 0.424 kg m s-1 D. 0.472kg m s-1 My steps are as followed: (0.5mv^2)+(mgh)=(0.5mv^2)+(mgh) i.e. 0+(0.05)(10)(1) =(0.5)(0.05)(v^2)+(0.05)(10)(0.8) v=2 m s-1 mv-mu=(0.05)(2)-(0.05)(0) =0.01 kg m s-1 Please point out my fault and give me the correct steps.

最佳解答:

The speed of the ball before striking the ground = square-root[2g x 1] m/s = 4.472 m/s where g is the acceleration due to gravity, taken to be 10 m/s2 The speed of the ball immediately after rebound = square-root[2g x 0.8] m/s = 4 m/s Hence, change in momentum = final momentum - initial momentum = 0.05 x 4 - (0.05 x (-4.472)) kg/m/s (upward direction is taken as +ve) = 0.4236 kg.m/s Option C is the answer. Your equations are all wrong.

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