A simple physics Q.(Work)

The work is done against the force of gravity (mg).

First of all, the man must lift the load with a force AT LEAST equal to 10 x 9.81 N (otherwise he cannot lift it up).

So we can consider in 2 different cases:

i) When lifting force = 10 x 9.81 N

The net force on the load is zero, so the load is moving up with a constant velocity (it should be given an initlal velocity first for this case)

As a result, the block will NOT gain any kinetic energy as it travels up and hence all work done by the man in lifting the load will be 10 x 9.81 x 2

ii) When lifting force > 10 x 9.81 N

The net force on the load is upward, so the load is moving up with acceleration.

As a result, the block will gain kinetic energy as it travels up and then the work done by the man can be divided into 2 parts: GPE gain and KE gain of the load.

For GPE part, it is still unchanged which is 10 x 9.81 x 2

For KE part, it will be equal to work done by the man - GPE gain of the load.

So no matter how much force the man applies, the gain in GPE of the load is still the same. (Indeed this question is asking the gain in GPE of the load)

Simply use the relation: work-done (by the man) = gain in potential energy of the load
This relation is just a form of the Law of Conservation of Energy, expressing the transfer of energy from the man to the load.

Since work done = F.(2)
where F is the force exerted by the man
but gain in potential energy of the load = weight of load x height raised = (10 x 9.81) x 2 J
Hence, F.(2) = (10 x 9.81) x 2
i.e. F = (10 x 9.81) N

F = mg
Work Done = Fx = mgx
p.s. Force does not equal to Work Done

2012-01-18 17:03:49 補充：
When the man lifts the load, he must apply a force equal to the weight of the load? Why?
By Newton third law, action and reaction.
Force act on the man lifting the weight = Force act on the weight by the man