Physics

(a) Give two reasons as to why the efficiency of a single removable pulley system is always less than 100%.

(b) Why a thick lead container is required to keep a radioactive substance?

(c) What is the principle of moments?

Machines

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Answer

(a) The efficiency of a single movable pulley system is not 100% because of the following reasons:

  1. There is friction in the pulley bearings or at the axle.
  2. The weight of the pulley and string is not zero.

(b) The radioactive substances must be kept in a thick lead container with a very narrow opening, so that the walls of the container absorb the radiations which strike on them from the inside and thus radiations only come out through the narrow opening.

(c) According to the principle of moments, if the algebraic sum of moments of all the forces, acting on the body, about the axis of rotation is zero, the body is in equilibrium. Therefore, as per the principle of moments,
sum of anticlockwise moments = sum of clockwise moments

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