1. Physics Section 5.2 Define and apply forms of mechanical energy. Energy is the ability to do work. Kinetic energy is the energy of an object due its motion. The kinetic energy of a moving object is directly proportional to its mass and the square of its velocity Forms of mechanical energy 1.Kinetic energy 2. Potential Energy a. Gravitational b. Elastic KE = ½ mv 2 KE = kinetic energy (J) m = mass (kg) v= velocity (m/s)

2. The unit of energy is the Joule. Example: Find the energy of a rock that has a mass of.25 kg and a velocity of 35 m/s. What happens to its energy if the mass is doubled? What happens to its energy if its velocity is doubled?

3. problem How fast would a 65 g bullet have to travel to have the same amount of kinetic energy as a 140 g bullet with a velocity of 180 m/s?

4. Work – Kinetic Energy Theorem The net work done by all the forces acting on an object is equal to the change in the objects kinetic energy. W net = ∆KE W net = ½ mv f 2 - ½ mv i 2 m = mass (kg) v f = final velocity (m/s) v i = initial velocity (m/s)

5. problem A net force of 56 N is applied to a stationary object. For what distance must the force be applied in order for the object to have a kinetic energy of 420 J?

6. problem A force of 75 N is applied to a 15 kg object causing it to accelerate. How far must it travel to have a velocity of 5.0 m/s?

7. Potential Energy is the energy associated with an object because of its position, shape, or condition of the object. Types of potential energy 1. Gravitational – due to its position in a gravitational field 2. Elastic – due to a deformation of an elastic object The gravitational potential energy of an object is directly proportional to its mass, height, and the acceleration due to gravity. PE g = mgh PE g = gravitational potential energy(J) m = mass (kg) g = acceleration (m/s 2 ) h = height (m)

8. problem A 55 kg object is raised 2.5 m above the surface of the Earth. Find its gravitational potential energy.

9. Elastic Potential Energy PE elastic = ½ k·x 2 PE elastic = elastic potential energy(J) k = spring constant (N/m) x = distance stretched or compressed (m) http://uk.youtube.com/watch?v=z8bVx43qlHo

10. problem A spring with a force constant of 18 N/m has a relaxed length of.75 m. The spring is stretched to a length of 1.00 m. Find its elastic potential energy.

11. problem A man has a mass of 75 kg and is attached to an unstretched bungee cord of length 18 m. He jumps off a 85 m high bridge and stops 22 m above the ground. If the spring constant of the bungee cord is 65 N/m, what is his total potential energy when he stops falling?

12. assignment Page 172 Problems 1 - 4