Physics: force and motion

code PLUS

2012-09-04 12:13 am

A bob of mass 0.5 kg is connected by an inextensible light string to a fixed point on a smooth horizontal table. The length of the string is 1 m. The bob rotates in a circular path with a constant speed of 0.8 m per sec.

1. Find the work done by tension on the bob when it completes 1 revolution.
2. If a lump of plasticine of mass 0.3 kg falls vertically onto the bob and sticks on it, find the new tension in the string.

True or false + explanation: A net force acting on an object always does work on it.

回答 (1)

天同

2012-09-04 5:43 am

✔ 最佳答案

1. Work done by string tension = 0, because there is no radial displacement of the tension.

2. Use conservation of momentum
0.5 x 0.8 = (0.5+0.3)v
where v is the speed of the bob and plasticine
i.e. v = 0.5 m/s

Hence, new tension
= (0.5+0.3) x 0.5^2/1 N = 0.2 N

The statement is false.
A net force does work if and only if the force has a displacement.

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