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Discussion in 'MCAT Study Question Q&A' started by inaccensa, Aug 12, 2011.
A nonideal plane decreases the force and increases the work as opposed to pulling it at a height h?
Ideal machines, such as a frictionless plane, do not change the work done because the force the work is being done against is conservative. We use machines because it changes the distance that we apply the force through, so the product of force and distance is still the same, thus the work is the same.
The picture of non-ideal machines is somewhat different because the forces involved are no longer conservative. Therefore, the work done is more than it would be if you were using an ideal machine.
For the block on the plane question, think of it this way. For an ideal machine, the work done is just change in the energy of the system. You take a box from rest and lift it to a height h and you do mgh of work on the box. That amount of energy is constant, regardless of how you get the box there - you can move it up a ramp, use a pulley or a system of pulleys, whatever you want to do. The total amount of work you do will never change because gravitation is a conservative force. If you add in friction that changes.
I'm sorry, I should have been more specific. I do understand that the work done is equal and in absence of friction, force req to pull up the plane is less, but the distance is greater and viceversa when it pull it up height "h"
My question really was why the force decreased? if the work in a non-ideal situation increases, since frictional forces must be overcome, how come the force is decreasing??
This is probably more a question of application. The net force needed increases, relative to what it would have been in the ideal case. But, as you point out, it's entirely possible that the force with the non-ideal plane could be larger than the force needed without the machine at all.
As an example, consider pushing a block up an inclined plane with a very large coefficient of friction. Let's say that the frictional force is so large that, not only did you push the block a longer distance (just as you would in the ideal case), but that you also had to exert a larger force than what it would have taken to just pick it up and move it h, straight up against gravity without the plane. Clearly, this is a very inefficient type of machine, but it's still possible.