# Work done on body moving in a circle

## Main Question or Discussion Point

Situation 1: completely horizontal circle
Imagine a ball being whirled in a completely horizontal circle.
The only forces acting are tension and weight

Would I be correct in assuming that in this situation NO WORK is done on the ball because the forces are directed (at all times) perpendicular to the direction of motion (there is never a resultant force along the line of motion)

Situation 2: completely vertical circle
Imagine a ball being whirled in a completely vertical circle.
The only forces acting are tension and weight

at all points (except for the top and bottom of the circle) components of the force act along the same line as the direction of motion, right?
Therefore, would I be correct in assuming that in this situation, work is ALWAYS done on the ball (except at the top and bottom)

Are both my assumptions (for situation 1 and 2) correct?

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Yes , you are right in both the situations.

Andrew Mason
Homework Helper
Situation 1: completely horizontal circle
Imagine a ball being whirled in a completely horizontal circle.
The only forces acting are tension and weight

Would I be correct in assuming that in this situation NO WORK is done on the ball because the forces are directed (at all times) perpendicular to the direction of motion (there is never a resultant force along the line of motion)
If the ball does not change tangential speed, then no work is done on the ball.

Situation 2: completely vertical circle
Imagine a ball being whirled in a completely vertical circle.
The only forces acting are tension and weight

at all points (except for the top and bottom of the circle) components of the force act along the same line as the direction of motion, right?
Therefore, would I be correct in assuming that in this situation, work is ALWAYS done on the ball (except at the top and bottom)

Are both my assumptions (for situation 1 and 2) correct?
Work is not being done on the ball by the person or machine that is whirling the ball.

As the ball moves from the bottom to the top of the circle, the ball does positive work on the earth (earth does negative work on the ball) resulting in a decrease in its kinetic energy (and an increase in its potential energy). As the ball moves from top to bottom, the earth does positive work on the ball (ball does negative work on the earth), resulting in an increase in its kinetic energy (and a decrease in its potential energy). So the total work done on or by the ball over one complete circle is 0. The work done on or by the whirler over any interval is 0.

AM