Question

You can swing a ball on a string in a vertical circle if you swing it fast enough. But if you swing too slowly, the string goes slack as the ball nears the top. Explain why there’s a minimum speed to keep the ball moving in a circle.

Short answer

At the top of the swing both gravity and tension pull down, therefore if the ball isn't traveling as fast as the force is trying to keep it going (in circular motion) then it will fall.

Step-by-step solution

Given information

You can swing a ball on a string in a vertical circle if you swing it fast enough. But if you swing too slowly, the string goes slack as the ball nears the top.

Explanation

At the top of the swing both gravity and tension pull down, therefore if the ball isn't traveling as fast as the force is trying to keep it going (in circular motion) then it will fall. Basically if gravity were less, then you could swing a vertical string slower and slower.