Calculating Force on a Ramp Using GPE: Clive's Question

[elvinc]

Hi,
I'm not quite getting this.

Say we lift a block of weight 10N through a vertical height of 1 metre. So the work done is 10 x 1 = 10J. This 10J equals the g.p.e. Of the block.

Now we pull the block up the same height of 1 metre but along a smooth, straight ramp (no friction between block and ramp surface) for a sloping distance 5 metres. The force is acting parallel to the surface of the ramp.

At the end of the pulling up the ramp the block still has a g.p.e. of 10J. So we can work out the force required to pull the block = g.p.e gained / horizontal distance moved = 10 / 5 = 2N

But I feel I am cutting fast and loose to use the g.p.e which is gained by vertical motion to calculate a force in a horizontal (or at least sloping) direction. It just doesn't feel valid to me. What am I missing? Sorry question is a bit vague but that reflects my view on this.

Thanks

Clive

 

[Doc Al]

But I feel I am cutting fast and loose to use the g.p.e which is gained by vertical motion to calculate a force in a horizontal (or at least sloping) direction. It just doesn't feel valid to me. What am I missing? Sorry question is a bit vague but that reflects my view on this.

Try calculating the force required parallel to the ramp to just overcome gravity: F = mg sinθ

Use that to figure out the work done and compare that to the increase in gravitational potential energy.