Question

The velocity ratio (VR) for a machine is-
(A). Always constant
(B). Varies with effort
(C). Varies with machine input
(D). Varies with machine output

Answer 1

A machine performs two functions primarily; motion modifying and force modifying. The motion and force modifications are inversely proportional .Usually the force gets amplified while the motion gets reduced. The performance of the machine is calculated by its efficiency.

Complete answer:
When a machine works, both forces and motion undergoes transformations inside it. The parts of a machine are connected so as to give an output motion from a given input motion. This is known as the mechanism of the machine.
Similarly forces usually undergo amplification to give an output force in the machine. The forces and motion are dependent on each other and are usually inversely proportional to each other. The output force is usually greater than the output while the output speed is less than the input speed.
The ratio of input speed to the output speed is called the velocity ratio (VR) of a machine. For a given machine, the velocity ratio always remains constant and in ideal conditions, it will be equal to the mechanical advantage of the machine.

The velocity ratio is a constant quantity. Therefore, the correct option is (A).

Note:
Work done is the product of force and displacement. The ratio of output force to input force for a machine is called its mechanical advantage. The efficiency of a machine is calculated by dividing the mechanical advantage by velocity ratio and multiplying by 100. It is necessary for all machines to perform motion- modifying functions; otherwise it is a structure not a machine.