Work, Energy and Power

Work͏� A scalar quantity which refers to product of the force and the displacement resulting from application of force.

͏� W= Fd

͏� Unit: Joule (J) or kgm2/s2 or N*m

Energy͏� Capacity to do work1. Kinetic energy- energy of motion• The net work done on an object is equal to its

change in kinetic energy.• If work is done, KE increases. But, if this object

does work on another object, KE decreases.2. Potential energy- energy at rest• Gravitational Potential Energy• Chemical Energy• Elastic Potential Energy

Formula�KE= ½ mv2

�Work= ½ m (Vf-Vi)2

�GPE= work = mgh

Law of Conservation of Energy~ Energy can neither be created nor destroyed~Energy can be changed from one form to the other~Energy can be changed into different forms but the total energy of a system is constant

KE+PE = 0KE= -PE

Power Speed of work or rate of doing work P= w/t P= Fv or F(d/t)Unit: J/s or Watt (W)

Sample problems1. A 98kg basketball player runs at a speed of 7m/s.

a) What is his KE?b) How much work was done by the player to reach this

speed starting from rest?2. A 2kg shell is shot from a cannon.

a) What is the GPE if the shell reaches a 450m in height?b) What is the change in PE of the system when the shell

falls to the height of 300m?3. Chito carries a 12kg knapsack while climbing a

mountain. After 20 minutes, he is 6m above the foot of the mountain. If Chito weighs 6.8x102 N,

a) How much total work is done?b) What is his power in kilowatts?