Object moving in a uniform gravitational field
- The work done on a basketball moving in a uniform gravitational field.
- The work done by the gravitational force is the same for any path from a to b: w = – Δ U = mgh
In mechanics, potential energy of a body is described in detail; it was due to its
position or internal atomic arrangement. The two main types of potential energy
are elastic and gravitational potential energies. When there is no external source
of energy, if an object is in motion, its kinetic energy and its potential energy
are interchanged between each other and the object’s energy is conserved. For example,
motion of a pendulum bob or motion of a ball falling from a height.
When a charge is moving in an electric field that is produced by a source charge,
what happens to its potential energy and kinetic energy? Consider a ball held at
a height h. Its gravitational potential energy is mgh.
If the ball is released, it will fall towards the earth, which has a gravitational
potential energy of zero. The ball moves from a higher potential energy level to
a lower potential level.
Its potential energy mgh is converted to kinetic energy 1/2mv2.
Now replace the ball with a negative charge –q which has
a mass mq. Its gravitational potential is mqgh.
It will also behave in the same manner as the ball when it falls towards the earth.
Place a charge +Q on an insulating pole at a horizontal distance
and distance h from –q. The negative charge
–q is placed equidistant from the earth and the positive charge +Q.
What would happen to the motion of the negative charge?
The negative charge –q would of course be attracted to +Q,
since the electric force or Coulomb force is at least 1038 times stronger
than the gravitational force.