A screw-jack has the distance between its successive threads as P and the length of its tommy bar as r. Its velocity ratio is given by the expression

Answer Details

A screw jack is a simple machine used for lifting heavy loads. The velocity ratio of a machine is defined as the ratio of the distance moved by the effort to the distance moved by the load. In the case of a screw jack, the effort is applied through a tommy bar, which is used to turn the screw. The distance moved by the effort is the circumference of the circle described by the end of the tommy bar, which is 2πr. The load is lifted by the screw, which has a thread pitch of P, which is the distance between successive threads. The distance moved by the load is equal to the pitch of the screw. Therefore, the velocity ratio of a screw jack is given by the expression: velocity ratio = distance moved by effort / distance moved by load velocity ratio = 2πr / P Thus, the correct option is: \(\frac{2 \pi r}{P}\)