1. Work IN, Work OUT The Work/Energy Principle

2. Kinetic Energy KE depends on mass and velocity Work done on an object will change KE

3. Example A 2 kilogram toy car starts from rest and attains a velocity of 3 meters per second as it travels a distance of 4.5 meters. –What is the car’s initial KE? 0 Joules –What is the car’s final KE? 9 Joules –How much work was done on the car? 9 Joules –What was the average force on the car? 2 Newtons

4. Example A 2 kilogram toy car changes its speed from 5 meters per second to 2 meters per second within 7 meters of travel. –What is the car’s initial KE? 25 Joules –What is the car’s final KE? 4 Joules –How much work was done on the car? -21 Joules –What was the average force on the car? -3 Newtons (friction force)

5. Gravitational Potential Energy How much energy does an object in a gravitational field have? –Strength of the field (g) –Mass of object (m) –Height in the field (h) GPE can be changed by changing “h” –Increase height  Increases GPE –Decrease height  Decrease GPE

6. Example An model aircraft with a mass of 2.0 kilograms is flying at an altitude of 15 meters. –What is its gravitational potential energy? 294.3 Joules –How much work needed to be done against gravity for the aircraft to reach this altitude? 294.3 Joules

7. Elastic (Spring) Potential Energy (PE S ) How much energy can be stored in an elastic object? –Strength of the spring (spring constant - k) –Distance of stretch/compression (x)

8. Example The spring in a toy dart gun has a spring constant of 50 newtons per meter. When a dart is loaded into the gun, the spring compresses 0.05 meters. –What amount of energy is stored in the spring when it is compressed? 0.063 Joules The amount of work needed to compress the spring is the SAME as the energy stored. This is also the same amount of KE that the dart will gain when fired.

9. Work/Energy Principle Summary Work in a mechanical system (horizontal) –Becomes KE (if speed increases) Work in a gravitational field (vertical) –Becomes PE g if moving upward –PE g becomes KE if allowed to fall Work in a spring system (horizontal or vertical) –Becomes PE S if spring is stretched/compressed –PE S becomes KE when the spring is released