# Circular motion of ball and string question

1. Nov 7, 2013

### sofiasherwood

Lets say we have a ball on a string and we spin it around. The ball will undergo circular motion. The tension in the string is what provides the centripetal force, directed inwards. Then what would be the force directed outwards? (According to Newton's third law of motion). It cant be centrifugal, because that is not a real force. Or lets say if we were to draw a free body diagram of the ball, what forces would be present? Assuming there is no tangential acceleration and that the ball is travelling at a constant speed.

2. Nov 7, 2013

### Staff: Mentor

The string pulls on the ball, and at the same time the ball pulls on the string. Equal and opposite forces.

3. Nov 7, 2013

### Staff: Mentor

Both ways.
See here for more: http://en.wikipedia.org/wiki/Tension_(physics [Broken])

Last edited by a moderator: May 6, 2017
4. Nov 7, 2013

According to newtons first law,the ball tries to move in a straight line.We are constantly changing it's direction.A force is required to change the direction of an object.So a centripetal force is provided.That's it.

5. Nov 7, 2013

### Joel Martis

Hi. The centrifugal force appears in the frame of reference of the ball( which is a non inertial frame). It never arises in the frame of reference of the ground. If we draw a free body diagram of the ball in the ground frame of reference, then the only forces acting on the ball will be the tension (=centripetal force) and gravitational force.Speaking of Newton's third law , you must remember that the equal-and-opposite pair of forces (in general ) will not act on the same body . Here the string exerts an inward force on the ball, and the ball exerts an outward force on the string , both being equal in magnitude. I hope this clears things for you .

6. Nov 7, 2013

Let's think for EXAMPLE,you are spinning a ball of mass 100KG(With a very strong rope and a thread.At a certain point,you release the strong rope.The ball will try to move in a straight line at a tangent.As the momentum will be very high,the centripetal force will also be high.The thread cant withstand the force provided,so it breaks.