When work is done by the force of gravity, the path that the object takes isn't important. What matters
is the object's change in height, regardless of the path taken to get there. We have already discussed
that when there is a net force on an object, it's kinetic energy changes. We can see that in the
previous example of the brick falling towards the Earth over a distance h. As it falls the net force on it
is F,-mg, and it gains kinetic energy as it falls. Let's consider what must happen before the brick
falls. If the brick starts on the ground, let's assume that someone has to lift it up a distance h before it
falls.
10) If you lift a brick (mass m) up a distance h, what is the minimum force you need to apply to the
brick to lift it up?
11) If you apply that much force to the brick, how much work does your force do on the brick over the
distance h?
12) While lifting the brick, what is the net force acting on the brick?
13) While you are lifting up the brick, the net force on it is zero. Therefore, the net work done on the
brick is zero, and its kinetic energy remains the same. However, your force is still doing work on
the brick. If it's not causing a gain of kinetic energy then what is that work doing?