Kinetic and Potential Energy

1. Kinetic and Potential Energy Conservation of Energy

2. Kinetic Energy • kinetic energy - the energy objects have because they are in motion • m –mass of object (kg) • v – velocity of object (m/s) • Like work, the unit of energy is the joule (J)

3. Example • A 1400 kg car is travelling at 30 m/s. What is its kinetic energy?

4. Work-Energy Theorem • An important relationship can be found between work and kinetic energy. It is known as the work-energy theorem, • “the change in an object's kinetic energy is equal to the net work done on the object.”

5. Example • pool cue strikes a ball of mass 0.25 kg and gives it a speed of 1.5 m/s. If the force exerted by the cue is 20 N, over what distance did this force act?

6. Example 2 • A soccer player kicks a soccer ball initially at rest. She exerts a force of 500 N on the ball over a distance of 10 cm. If the ball has a mass of 450 kg, with what speed does it leave her foot?

7. Potential Energy • Potential energy - energy an object has because of its position or state. (stored work) • Use the symbol U to represent potential energy. • A battery contains chemical potential energy and when that energy is released, it can do work to power your walkman, etc.

8. Gravitational Potential Energy • The potential energy used most often in physics is gravitational potential energy. When you lift a bowling ball, you must do work against gravity. That work is transformed into gravitational potential energy which can be released as kinetic energy if you drop the ball.

9. Formula for PE • To eliminate the choosing of a reference point we talk about change in potential energy

10. Example • What is the increase in the gravitational energy of a 5 kg book that is lifted from the floor to a table 1.5 m high?

11. Example 2 • A falling ball of mass 250 g losses 25 J of gravitational potential energy. What distance has the ball fallen?

12. Potential Energy of a Spring • You can also store potential energy in a spring. The formula for this is: Us = ½kx2 • Notice that it's the same formula as the work done by a spring. Also notice that spring potential energy is always positive.

13. Example • A spring has a spring constant of 800 N/m. How much must this string be stretched to store 75 J of potential energy?

14. Displacement and PE

15. Energy and Conservation • One of the most fundamental and powerful ideas in physics is the one of energy conservation. • the total mechanical energy of a system is conserved. That is, the energy it initially has must be equal to its final energy. • mechanical energy-the sum of its kinetic and potential energies.

16. Energy Conservation • You can write the conservation of energy statement in many different mathematical forms. Here are some of them: • KEi + PEi = KEf + PEf

17. Example • A biologist uses a spring-loaded gun to shoot tranquilizer darts into a wild African hamster. The spring in the gun has a spring constant of 940 N/m. A 38 g dart is loaded into the gun and the spring is compressed a distance of 25 cm. With what speed will the dart leave the gun?