# Forces on pendulum

1. Nov 18, 2005

### Werg22

Is there a centripetal force with a pendulum? My view is that mg acts on the mass (for angles lower than 90) with two components. One that is parrallel to the string and another that is perpendicualr to it. Since the center of the pendulum mechanism stays still, by action reaction this component is canceled out on the mass. For a time that goes to zero the mass moves in the direction that was perpendicular to the string. But since the string moves along with the mass, the direction of the mass (which is perpendicular to the string) changes constantly and the result is a circular motion. Is this perception correct or not?

2. Nov 18, 2005

### James R

Yes, there's a net centripetal force on the mass, as there must be since the mass moves in a circle around the pivot. The force is NOT constant, though, since the speed of the pendulum varies during the swing. The centripetal force at any time is the vector sum of the weight force and the string tension force.

3. Nov 18, 2005

### Staff: Mentor

While there's certainly a centripetal force on the mass, it's not the net force. (It's the net force in the radial direction.) The acceleration of the mass--and thus the force on the mass--has both tangential and radial components.

4. Nov 18, 2005

### Werg22

And this force would be the weight?

5. Nov 18, 2005

### Staff: Mentor

Two forces act on the mass in a pendulum: the weight and the tension in the string. The vector sum of these forces is the net force on the mass.

6. Nov 18, 2005

### James R

You're right, Doc Al. The centripetal force is not the net force, since the net force has a tangential component, too.

7. Nov 18, 2005

### Werg22

Okay. And this tension is due to the fact that the center of the pendulum stays still, right?

8. Nov 19, 2005

### Staff: Mentor

It's due to the fact that one end of the string is fixed while the other end is attached to the swinging mass.