1. Honors Physics Work, Gravity and Potential Energy

2. Work Done by Gravity  Consider a falling tennis ball.  As the ball falls, gravity does positive work on the ball.  This is evident by the increase in Kinetic Energy of the ball.  The definition of work can be used to calculate the magnitude of the work.  The force is constant and the angle between the force and motion is zero degrees.

3. Work done by Gravity  We let the distance, s, that it falls be the difference between initial and final heights.  This shows that the work done is dependent upon the change in vertical position only, not the actual path taken.

4. Going Up?  Is the expression valid for an object traveling upward?  Yes! The final height is greater than the initial height, yielding work to be a negative quantity.  The next question then is: Form where do we measure the height?

5. Reference Level  The position of zero height, h = 0, is called the reference level.  The choice is arbitrary. We can put the reference level wherever we like.  You will see that a wise choice can simplify many problems.  The only condition is that all heights be measured from the same reference level.

6. Gravitational Potential Energy  Notice that the work done by gravity is the difference between two like terms.  This similar to the Work-Energy Theorem.  In this case, each term is again a type of energy.  Gravitational Potential Energy.

7. Gravitational Potential Energy  This is the energy possessed by an object due to its position.  It is a result of the forces exerted between two particles of the same system.  We can show that there is energy by showing its ability to do work.  Demonstration: A massive object held above a soda can.  When the mass hits the can, it crushes the can, thus doing work.  It must have had energy!

8. Types of Forces  All forces can be placed is one of two classes: Conservative Nonconservative  Conservative forces are those for which the work they do is independent of the path taken.  Nonconservative forces are those for which the work done is path dependent.

9. Conservative Forces  The work done by conservative forces is independent of the path taken.  A potential energy is associated with a conservative force.  The total work done around any closed path is zero.  To date, Gravity is the only conservative force that we have discussed.

10. Nonconservative Forces  The work done is path dependent.  The transfer of energy by a nonconservative force is not reversible, therefore there is no potential energy associated with such forces.  To date, all forces other than gravity have been nonconservative.

11. Work-Energy Theorem Revisited