1 There are many different forms of energy however all of which are measured in units of Joules. In this chapter we will look at two different forms of energy. Kinetic energy and potential energy. As well as how they are related to the concept of work. Energy

2 Kinetic Energy Kinetic energy: The energy of an object due to its motion. Kinetic energy is directly proportional to the mass and the the velocity squared of a moving object.

3 Example: A kg hockey puck, initially at rest, is accelerated to 27.0 m/s. Calculate the kinetic energy of the puck both at rest, and in motion. At Rest In Motion

4 Do Practice Problems Pg 238 (pdf 34) #’s 19-21

5 The Work and Kinetic Energy Theorem In order to do work on object there must be a force applied to the object. When a force is applied to an object it will accelerate. When it accelerates there is an increase in velocity. An increase in velocity will cause an increase in kinetic Energy. Therefore, the work done on object is equal to the change in the kinetic Energy of the object.

6 Example: A shot putter heaves a 7.26 kg shot with a final speed of 7.51 m/s. b) If the shot was initially at rest how much work was done on it to give its it this kinetic energy? a) What was the kinetic energy of the shot?

7 Example: A physics student does work on a 2.5 kg curling stone by exerting a 40 N force to it horizontally over a distance of 1.5 m. a) Calculate the work done by the student on the stone.

b) Assuming that the stone started from rest, calculate the velocity of the stone at the point of release. Consider the ice surface to be frictionless.

9 Example: A 75 kg skateboarder initially moving at 8.0 m/s, exerts an average force of 200 N by pushing on the ground, over a distance of 5.0 m. Find the new kinetic energy of the skateboarder.

10 Do Practice Problems Pg 245 (pdf 35) #’s Section Review Pg 246 (pdf 35) #’s 1 - 3

11 What do all of these things have in common?

12 Potential energy: is stored energy, or when an object has the potential to do work. Often we refer to what is called the total mechanical energy of the system. Which is simply the total combined kinetic and gravitational potential energies. There are many different types of potential energy such as a battery, a waterfall, a compressed spring, gasoline or anything that has the potential to do work. In this chapter will concentrate on what is called gravitational potential, or energy due to an objects position on earth. Potential Energy

13 Gravitational potential energy is directly proportional to an object’s mass and height. The higher an object is lifted the more gravitational potential energy it will have. Also a more massive object will have a larger gravitational potential energy that a less massive object at the same height. Gravitational potential energy can be found using the following formula.

14 Example: While setting up a tent you use a 3.0 kg rock to drive the tent pegs into the ground. If you lift the rock to a height of 0.68 m, what gravitational potential energy will the rock have?

15 *** Caution *** When talking about gravitational potential energy you have to specify what the height is relative to. ie: the ground, the table, the top of the hill, the bottom of the hill, ect....

16 Example: A 2.0 kg textbook is lifted from the floor to a shelf 2.1 m above the floor. a) What is the gravitational potential energy relative to the floor? b) What is the gravitational potential energy relative to the head of a 1.65 m tall person?

17 Do Practice Problems Pg 250 (pdf 36) #’s 27 & 29

18 Gravitational Potential Energy and Work When you do work on an object by lifting it to a new relative height. The object will as a result have an increase in gravitational potential energy thus the work done on an object is equal to the change in the gravitational potential energy of the object.

19 Example: A 65 kg rock climber did 16 kJ of work against gravity to reach a ledge. How high did the rock climber ascend?

20 Question: You carry a heavy box up a flight of stairs. Your friend carries an identical box on an elevator to reach the same floor as you. Which one, you or your friend, did the greatest amount of work on the box against gravity? Because the change in gravitational potential energy of the two different boxes is the same, the work done on the two boxes are equal.

21 So far we have discussed both the work/kinetic energy theorem and the work/potential energy theorem. In both cases the amount of work done on the system was equal to the change in energy of the system. As it turns out both theorems are a part of a single all encompassing theorem called the work /energy theorem. Where the work done on a system is equal the the change in the total mechanical energy of the system.

22 Do Practice Problems Pg 254 (pdf 36) #’s