# Constant velocity assumptions

1. Mar 11, 2012

### mathsciguy

It's been a while since I've done any work-energy questions, and I just noticed how vague some questions could be (or maybe I don't understand it that well). Anyway, suppose I have a box moving down an incline where the only forces acting on it are its weight, the normal force, and friction. Now, can I assume that this box is moving in constant velocity, or there is zero net force? Or is this something that is apparent and it does not need to be assumed since in this particular question the assumption was never given, but the known variables seem fit the case where there is constant velocity.

Thanks.

2. Mar 12, 2012

### turin

There is no reason from your description to assume a constant velocity. And, from your freshman physics, constant velocity ⇔ zero net force, so there is neither reason to assume zero net force.

net force = 0 ⇔ net work = 0 ⇔ kinetic energy remains constant.

net force ≠ 0 ⇔ net work ≠ 0 ⇔ kinetic energy changes.

Which happens, and increase or decrease in kinetic energy, depends on more information that is not given in your description.

3. Mar 12, 2012

### mathsciguy

Oh yeah, that was really silly. Thanks.