# Why centripetal accel is from an internal force?

1. Mar 26, 2009

### makeAwish

Why centripetal accel is from an internal force?
Usually what type of forces can be considered as external force, besides friction?

And, for static equilibrium, do we only use the eqn: sum of forces equals zero, or we can also apply sum of moments equals zero?

Thanks!

2. Mar 26, 2009

### Cantab Morgan

There is some arbitrariness in defining whether a force is internal or external, because the boundaries of the system under study are in the eye of the beholder. Typically, the entirety of a solid object is within the boundary of a system, so any forces between parts of a solid object would be internal. Centripetal forces on a spinning solid body come from the electrostatic forces holding the object rigid. So typically, centripetal forces are internal, unless some bizarre and unusual boundary for the the system was chosen. It's hard to see the usefulness of such a choice.

Any forces between an object inside the system boundary and outside the system boundary are considered external. These could be anything from magnetism to gravity to whatever.

If I understand things correctly, to have static equilibrium, the sum of the forces and the sum of the torques must be zero.

3. Mar 26, 2009

### makeAwish

Say, an object on a table and my system is the book and table, the book's and the table's weights and the normal reaction forces are considered to be internal or external forces?
And what if my system is just the book, the same forces are external?

4. Mar 26, 2009

### makeAwish

Static eqm means the object not moving..
But sum of the forces equals zero means acceleration equals to zero rite?

Am i right to say zero accel generally dont mean velocity equals to zero, but for static eqm, though sum of the forces equal to zero, the obj in fact is not moving?

5. Mar 26, 2009

### Cantab Morgan

Good questions! You tell me. In both cases, the Earth is outside of your system boundary, so would gravity be external?

What about the contact (or normal) forces between the book and the table. When do such forces cross the system boundary and when do they not?

6. Mar 26, 2009

### Cantab Morgan

It's not obvious to me that static equilibrium really means that the object is not moving. I can build a house of cards while riding on a smooth train, but I (and my deck of cards) are still moving.

7. Mar 26, 2009

### makeAwish

oh.. so in both cases, weight is external force?

if my system is just the book, normal forces are external but if my sys is both book and table, they are internal?

friction is it always considered as external? say i push the book along table and sys is both book and table..
even if i take my sys as the book only, friction is external too?

8. Mar 26, 2009

### makeAwish

hmm. but i tot static means object is at rest?

Then for both static and dynamic eqm, the equations are the same? (sum of forces equals zero, and sum of moments equals zero)

9. Mar 26, 2009

### Bob S

There are two type of static equilibrium; stable and quasi stable. Consider a solid cube of side a balanced on a cylinder of radius b. When a is small, the system is stable, when it is larger than a certain value it is quasistable; any perturbation will make the cube tip over.

10. Mar 26, 2009

### makeAwish

this is abt stability is it?

hmm.. i still dont understand..

11. Mar 26, 2009

### Bob S

Work out the stability equations for a cube with sides = 30 cm, balanced on a cylinder with radius 5 cm, perturb it and see what happens. Then increase the size of the cylinder radius to 50 cm.

12. Mar 26, 2009

### makeAwish

but i haven't learn abt stability yet.. =x i don't know the equations..

13. Mar 27, 2009

### Bob S

Work out the potential energy of my example of cube balanced on cylinder as a function of a. b. and tilt angle of cube, and if it increases as tilt angle increases, it is absolutely stable, and if it decreases, the cube is quasistable and will fall off if pushed.