How Does a Pulley with a Mechanical Advantage of 2 Affect Force?

[moomoocow]

Mechanical Advantage : Please Help!

hello

if there was a pulley, mechanical advantage of 2, and the force of the weights dangling below the pulley was 20N.
to find the 'upward' force, or force of the pulley
would it be 20N multiply by 2, due to mechanical advantage, making it 40N
or would it also be 20N?

can somebody please explain this simply?
thank you

 

[HallsofIvy]

That depends upon what you mean by "'upward' force, or force of the pulley'. "Mechanical advantage" depends upon the fact that there is a law of conservation of energy, not of force. If you were to lift a weight of 20 Newtons using this pulley, the "mechanical advantage" is 2 means that you would only have to apply 10 Newtons force- but you would have to pull on the rope twice as far as the weight is lifted in order to conserve energy. If the weights are just "dangling" there, no being lifted, the pulley doesn't come into play at all- there is no motion so no work done. The force downward on the weights is 20 N. Since they are not moving the force upward, due to the pulley must also be 20 N.