LUX MIDDLE SCHOOL. 8 th grade Science Objective 8.3.4: Investigate energy and power: b. Describe potential and kinetic energy

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1 LUX MIDDLE SCHOOL 8 th grade Science Objective 8.3.4: Investigate energy and power: b. Describe potential and kinetic energy Scientist: Dorina Marta Mihut Lead Teacher: Angela Zabawa 1

2 KINETIC AND POTENTIAL ENERGY Potential Energy An object can store energy as the result of its position. For example: the heavy ram of a pile driver is storing energy when it is held at an elevated position. This stored energy of position is referred to as potential energy. Potential energy is the stored energy of position possessed by an object. 2

3 Gravitational potential energy is the energy stored in an object as the result of its vertical position (i.e., height). The energy is stored as the result of the gravitational attraction of the Earth for the object. The gravitational potential energy of the heavy ram of a pile driver is dependent on two variables - the mass of the ram and the height to which it is raised. There is a direct relation between gravitational potential energy and the mass of an object; more massive objects have greater gravitational potential energy. There is also a direct relation between gravitational potential energy and the height of an object; the higher that an object is elevated, the greater the gravitational potential energy. These relationships are expressed by the following equation: PE grav = mass * g * height Similarly, a drawn bow is able to store energy as the result of its position. When assuming its usual position (i.e., when not drawn), there is no energy stored in the bow. Yet, when its position is altered from its usual equilibrium position, the bow is able to store energy by virtue of its position. This stored energy of position is elastic potential energy. 3

4 Kinetic Energy Kinetic energy is the energy of motion. An object which has motion - whether it is vertical or horizontal motion - has kinetic energy. There are many forms of kinetic energy: translational (the energy due to motion from one location to another). vibration (the energy due to vibration motion), rotational (the energy due to rotational motion) We will focus upon translational kinetic energy. The amount of translational kinetic energy of an object depends upon two variables: 1. the mass (m) of the object and 2. the speed (v) of the object. The following equation is used to represent the kinetic energy (KE) of an object: where m = mass of object v = speed of object 4

5 Analysis of a Situation in Which Mechanical Energy is Conserved A roller coaster operates on the principle of energy transformation. Work is initially done on a roller coaster car to lift to its initial summit. Once lifted to the top of the summit, the roller coaster car has a large quantity of potential energy and virtually no kinetic energy (the car is almost at rest). If it can be assumed that no external forces are doing work upon the car as it travels from the initial summit to the end of the track (where finally an external braking system is employed), then the total mechanical energy of the roller coaster car is conserved. As the car descends hills and loops, its potential energy is transformed into kinetic energy (as the car speeds up); as the car ascends hills and loops, its kinetic energy is transformed into potential energy (as the car slows down). Yet in the absence of external forces doing work, the total mechanical energy of the car is conserved. 5

6 EXPERIMENT In this lab we will explore the effect of the height of a ramp and the mass of an object on the potential and kinetic energy. Hypothesis As the height of a ramp increases, potential and kinetic energy will As the mass increases, potential and kinetic energy will Materials 3 balls ( different mass) ramp (a piece of plywood) meter stick balance stop watch Procedure 1. Weigh each ball on the balance to determine its mass (in grams). Record the mass in the data table. 2. Draw a starting line from the top of the plywood. 3. Place one block of wood (1 book) under the end of the plywood to make a ramp. Measure the height of the ramp (m) and record it. 4. Place one of the balls on the starting line. 5. Release the ball and start the stop watch. 6. When the ball has used all its energy, i.e., when it comes to a complete stop, record the time. 7. Measure and record the distance (in meters) that the ball traveled. 8. Repeat steps 4-7 with the other two balls. 9. Place one additional block (2 books) under the end of the plywood. Measure the new height of the ramp and record it on the data table. 10. Repeat steps 4-7 with each of the three balls. 11. Using the third block of wood (3 books), raise the plywood ramp still higher. Measure the new height and record it on the data table. 12. Repeat steps 4-7 with each ball. Observations 1. Height of the ramp (m) =.. 6

7 Ball 1 Mass (g) Distance (m) Time (sec) Velocity (m/s) Ball 2 Ball Height of the ramp (m) = Ball 1 Mass (g) Distance (m) Time (sec) Velocity (m/s) Ball 2 Ball 3 3. Height of the ramp (m) = Ball 1 Mass (g) Distance (m) Time (sec) Velocity (m/s) Ball 2 Ball 3 Questions 1. When in this investigation did each ball have potential energy? 7

8 2. When did each ball have kinetic energy? 3. What is the relationship between energy, mass and height (potential and kinetic)? 5. What evidence do you have that supports your hypothesis? 6. The velocity of an object (V) is calculated by dividing the distance (d) traveled by time (t). Using the formula V = d/t, calculate the velocity of each ball traveled down the ramp. 7. As the mass of the ball increased, did the balls speed up or slow down? Why/Why not? 8. As the height of the ramp increased, did the balls speed up or slow down? Why/Why not? 9. Calculate the potential energy of the ball at the starting line. Calculate the kinetic energy of each ball traveling down the ramp elevated with one block of wood. Ball 1 Ball 2 Ball When a car is going downhill, the driver must apply more pressure on the brakes to stop than if the car was on ground level. Why? 11. Why is it harder to stop a four-person bobsled than a three-person bobsled? 8

KINETIC AND POTENTIAL ENERGY

KINETIC AND POTENTIAL ENERGY UNIT 1 - ENERGY SECTION 1 - ENERGEIA Background Information Energy can be in one of two states: potential or kinetic. Energy can be transferred from potential to kinetic and between objects. Potential