# How to tell if an object is moving or not?

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1. Nov 4, 2014

### Kawaii Physics

Say that there's a ramp with a block on it, with a string attached to the block and to a massless, frictionless pulley at the top of the ramp, and the pulley connecting the string to another block. What if both of the masses are really close and there is friction on the incline but you can't tell if it's moving or not. What would you do to tell if it's moving or not?

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2. Nov 4, 2014

### mathman

Watch it for a long time? If you are serious take a precise measurement, wait a while and take another measurement.

3. Nov 4, 2014

### Kawaii Physics

@mathman

No, I mean by calculations, not by visual. Say you're given the mass of each block, the kinetic/static friction coefficient, and the angle of the incline.

What would you do to calculate if it's moving or not, so you can know whether to use static or kinetic friction?

4. Nov 4, 2014

### Staff: Mentor

The wording of the question tells you you need to calculate static friction; static friction is what tells you if (starting from a stop), the system will begin to move or not.

5. Nov 5, 2014

### mtworkowski@o

Cos of angle x M has to be larger than Cf static

6. Nov 5, 2014