# Pushing a box (forces)

1. Nov 28, 2007

### jesuslovesu

1. The problem statement, all variables and given/known data
You exert a constant horizontal force on a large box. As a result the box moves across a horizontal floor at a constant speed v0.

If you double the constant horizontal force on the box, how would the box then move?

2. Relevant equations
F = ma = m dv/dt

3. The attempt at a solution
Well, this question is a tad confusing to me, if the box is moving at a constant speed v0 then I have to assume that I would just push the box for an instant & then get it to move at a constant v0.

If I were to double the force, then I would think that the speed would then be 2v0, however this is not the case... (dv/dt is proportional to F)

Can anyone explain how to determine the motion?

2. Nov 28, 2007

### Staff: Mentor

You are misinterpreting the problem. The only reason the box continues moving at constant speed is because you keep pushing with the constant horizontal force. Hint: What's the net force acting on the box? What other force must be acting on the box?

3. Nov 28, 2007

### jesuslovesu

Alright so if I draw a free body diagram and add in friction I can see
ma = F - Ff
Since the velocity is constant a = 0 so
F = Ff

So if I double F to 2F, then it's accelerating?
2F - Ff = ma
F = ma
a = F/m ?

Last edited: Nov 28, 2007
4. Nov 28, 2007

Very good!