# Homework Help: A question in circular motion

1. May 12, 2010

### m.medhat

1. The problem statement, all variables and given/known data
if a body is in a uniform circular motion . is it in equilibrium?

2. Relevant equations

3. The attempt at a solution
i think it is not in equilibrium ...... please i want the true solution .

2. May 12, 2010

### tiny-tim

Hi m.medhat!
When you say "a body is in a uniform circular motion", do you mean like a bar rotating about its centre, or do you mean like a planet rotating around a star?

3. May 12, 2010

### RTW69

Is the object's rotational motion changing with time? If not it is in equilibrium, for an object to be in "mechanical" equilibrium the net torque acting on the object must be zero.

4. May 16, 2010

### m.medhat

i know that a body in equilibrium , the net force acting on it must be zero .
but for the body in uniform circular motion , there is a force acting on it , it is the centripetal force , and the net force here -as i think- not equal zero but equal to centripetal force .
i still need help please .

5. May 16, 2010

### tiny-tim

Yes, that's exactly correct

a body moving with constant angular velocity round a circle (ie not simply rotating on its own centre of mass) is not in equilibrium …

F = ma for that body shows, on the left, the gravitational force or the tension or whatever is forcing it towards the centre, and on the right, (mass times) the centripetal acceleration.

(The left is physics, the right is just geometry … a body moving round a circle must have centripetal acceleration, because the geometry says so. )

Because of F = ma, since (for constant angular velocity) the acceleration is centripetal, that means that the net force must be centripetal also.

6. May 17, 2010

### m.medhat

then this means that the body moving with a uniform circular motion is not in equilibrium , isn't it ?