If you're swinging a mass on a string around in a circle, you're doing no work on the mass.

This statement is generally true in physics, assuming that the swinging is happening at a constant speed and the string is remaining taut. Here's why:

1. Work is defined as the force that is applied to an object times the distance over which it is applied, and it is also dependent on the angle between the force and the direction of motion. In equation form, this is: Work = Force x Distance x cos(θ).

2. In the case of a mass swinging in a circle, the force being applied (the tension in the string) is directed towards the center of the circle. This is known as centripetal force.

3. However, the actual motion of the mass is perpendicular to this force – it's moving in a circle around the center, not towards the center.

4. Because the force and the motion are perpendicular, the angle θ in the work equation is 90 degrees. The cosine of 90 degrees is zero.

5. Therefore, according to the equation for work, no work is being done on the mass because the force is perpendicular to the direction of motion.

This is a simplification and assumes a perfect system with no air resistance, friction, or changes in speed. In a real-world system, these factors would come into play and work would be done to overcome them.