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work DONE watch

1. is it me or does work not make sense?
the definition is force*distance moved in the direction of the force, but at an angle it's completely different?! if i have a 3, 4, 5 meter right-angle triangle and i'm trying to push a 1000 N block to balance on the top of it, why isn't the work done 1000 N (force)*5 meters (distance in the direction of the force)=5000 J, instead of 4*1000=4000J. I'm definitely pushing it along 5 meters??
2. (Original post by rolf9t)
is it me or does work not make sense?
the definition is force*distance moved in the direction of the force, but at an angle it's completely different?! if i have a 3, 4, 5 meter right-angle triangle and i'm trying to push a 1000 N block to balance on the top of it, why isn't the work done 1000 N (force)*5 meters (distance in the direction of the force)=5000 J, instead of 4*1000=4000J. I'm definitely pushing it along 5 meters??
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Does the slope have friction?
The work you do pushing the mass up the slope will depend on this.

With no friction you only do work against gravity. In that case the force you push up the slope with will not be 1000N if that is the weight of the block. The work done will then just be mgh or 1000 x 4 (4m is the vertical height, g=10m/s/s) * see below

However, if there is friction and you push a block up the slope steadily with a 1000N force parallel to the slope, then the work done is 1000N x 5m.

*The difficulty lies in the way you worded the question.

A 100kg block (1000N weight) on a frictionless slope doesn't need a force of mg (=1000N) to push it up steadily. The force needed is mg sin θ where sin θ = 4/5 (from your 3,4,5 triangle)
Force required = 1000 x (4/5)
Work done is force x 5m = 1000 x (4/5) x 5 = 1000N x 4m

And 1000 x 4 is the result you get if you just use mgh where h = 4m

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Updated: April 6, 2013
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