Perhaps the most familiar example of a force is the gravitational
attraction of a body to a nearby astronomical body, eg gravitation on
earth. We convinced ourselves that all bodies fall to earth with the
same acceleration which we denoted g. The force that causes this
acceleration is gravity and the force of gravity
at the surface of the earth is called
the weight of the body. To give the free fall acceleration
we determined previously the weight of a body with mass
m must have a magnitude
and be directed in the normal direction toward the surface of the
earth. Interestingly the pound is actually a unit of force, not mass.
That is 1 lb = 4.45 N. The weight is the force which we must
counteract when we lift a body. To lift a body at constant velocity we
must apply a force 
so that
