We
define the mechanical energy (E) of a system as the sum of the KE and gravitational PE of
all its parts. Therefore: |
Wnet =  E = Ef - Ei |
So, |
Ef =
Ei + Wnet |
If no contact forces are applied Wnet is zero, no energy is transferred
into or out of the system, Ef = Ei , and mechanical energy is
conserved. Assuming that g is constant,
|
Wnet = PEg + KE
|
| becomes, |
Wnet = (1/2 mvf2
- 1/2 mvi2) + (mghf - mghi) |
|
When the only force acting is gravity, Wnet = 0 and
|
| 1/2 mvf2
- 1/2 mvi2 = mghf - mghi At every instant in the motion, the total
mechanical energy is constant; if the boy's KE increases, its PE must decrease, and vice
versa.
|
 |
The transfer of energy from one form to
another in the swing pendulum. |
|
Conservation of Mechanical Energy Demo |
