Definition of Work
From the physical point of view, you can talk about work only when a force acts
along a certain distance. In a more precise way, the work done by a force is the
product of the component of the force along the line of motion of the object
multiplied by the distance that the object moves under the influence of the
force.
The unit of Work is
Look at the example on page 98 in the text-book.
Definition of Power
Power is simply the rate of doing work:
It is the ratio between the work done and the time required to do it.
The unit of power is
For instance, a 70Kg man goes upstairs to the sixth floor of a building block:
he needs 90 seconds to cover a difference in height of 20 meters on foot. What
is the power developed by him during this action?
In doing it, the man works against the force of gravity for a 20 meter distance,
hence
Using a lift, the time elapsed going up to the sixth floor is only 30 seconds.
What is the power developed by the lift when carrying the same man to the sixth
floor?
As before, the lift works against gravity for a distance of 20 meters, hence the
power will be:
As you can see, the power developed by the lift is three times the one developed
by the man. This happens because the lift and the man do the same work against
gravity, but the lift needs only one third of the time than the man to do it. In
ordinary we can say "in lifting weights, a lift is more powerful than a
man".
Theorem of kinetic energy
We can refer to the kinetic energy of an object as to the movement energy this
object has. If an object is at rest it doesn’t have any kinetic energy; if it
moves it has; every time it changes its way of moving it changes its own kinetic
energy. The Kinetic energy depends on the mass and on the speed of an object in
the following manner:
note that it depends on the velocity squared.
The kinetic energy is strictly related to work in Physics.
The theorem of kinetic energy states that the change in kinetic energy
experienced by an "m" mass object because of work done on it, is
exactly equal to the amount of this work.
If we refer to an ideal system where frictional effects are negligible, the
previous statement has a general validity and doesn’t depend on the kind of
force which is doing work.
For a better understanding, look at the sample exercise in the text-book on page 100.