Mechanical Energy Ch. 4

Energy Is the ability to do work. Energy = work Units = Joules (J) James Prescott Joule

Work Work is a force applied through a distance. W=Force(N) x distance (m) ◦ W=Fxd ◦ SI Unit = Joules (J) If you apply a force, but do not move, you are not doing work. Joe pushes the mower with 250 N of force for 20 m. How much work has he done? W = Fxd 250 N x 20 m = 5000 J

Power Power is the rate at which energy is converted. ◦ Or, it’s how fast you do work. Power = Work (J) / time (s) = Watts (W) or Power = Energy (J) / time (s) SI Unit = Watts (W) ◦ Watts = Joules / seconds If a light bulb produces 300 J of energy in 5 s, how much power does it produce? P = W/t P = 300J / 5 s P = 60 watts

Potential energy Stored energy – has the potential to do work Elastic PE – energy stored by stretching or compressing an object ◦ Ex. – rubber band, spring Chemical PE – energy stored in chemical bonds ◦ Ex. – food, gasoline, batteries

Potential energy Gravitational PE – stored energy due to gravitational attraction between objects Depends on mass, gravity, and height Formula: GPE = mass x gravity x height ◦ GPE = mgh  g = 9.8 m/s 2  Unit = Joules (J) If h, then GPE If m, then GPE

Kinetic Energy Energy of motion – an object must be moving to have KE Depends on mass and velocity Formula: KE = ½ mass x velocity 2 ◦ KE = ½ mv 2 ◦ Units are Joules (J) Depends more on velocity than mass ◦ v is squared

Work Energy Theorem Work = Change in Kinetic Energy ◦ W = Δ KE To change the KE on an object, work must be done on the object. ◦ The more KE an object has, the more work must be done to change that energy.  Car accelerating and decelerating examples

Conservation of Energy Energy can not be created or destroyed. It can only be transformed from one type of energy to another. The total amount of energy never changes!!! Pendulum Example