Activity 5a Potential and Kinetic Energy PHYS 010. To investigate the relationship between potential energy and kinetic energy.

Size: px

Start display at page:

Download "Activity 5a Potential and Kinetic Energy PHYS 010. To investigate the relationship between potential energy and kinetic energy."

Grant Clark
7 years ago
Views:

1 Name: Date: Partners: Purpose: To investigate the relationship between potential energy and kinetic energy. Materials: 1. Super-balls, or hard bouncy rubber balls. Metre stick and tape 3. calculator 4. table or desk 5. scale 6. stopwatch Diagram: Ruler Prediction/Discussion: 1. In layperson terms, something has potential if it has the ability to do something in the future. In physics terms, we often use the word potential to describe different forms of stored energy. In your own words, describe what you know about the potential energy of a falling object. How can you increase the potential energy of an object? Rev /6

In layperson terms, something has potential if it has the ability to do something in the future.

2 . Kinematics is the study of objects in motion. In your own words, describe what you know about the kinetic energy of a moving object. How can you increase the kinetic energy of an object? What is the kinetic energy of an object at rest? Part 1: Procedure: Data: 1. Tape the metre stick to the side of the table or desk, so the 0-cm mark is on the floor.. Hold the ball so its bottom edge aligns with the 40 cm mark, then drop the ball. Have someone in your group measure the height to which the ball bounces. Record this bounce height in the table below. 3. Measure the time the ball takes to drop from the starting point to the floor and record in table. 4. Repeat steps and 3 (ie. do two trials) 5. Drop the ball twice each for other initial heights: say 70cm and 100cm. 6. Calculate the average bounce height (m) and average drop time, as indicated in the table (in m). 7. Calculate the speed of the ball just before it hits the ground: Vground = (g)*(t) = (9.81 m/s) * (average drop time), and record in last column of the table (m/s). Initial height of ball 0.4 m Trial 1 Bounce Height (m) Average Bounce Height (m) Drop Time (s) Average Drop Time (s) Speed at Ground (m/s) 0.7 m m 1 Rev /6

. Hold the ball so its bottom edge aligns with the 40 cm mark, then drop the ball. Have someone in your group measure the height to which the ball bounces.

3 Mass of ball: kg Energy Calculations: Initial height of ball 0.4 m Average Bounce Height (m) *from 1st table* Potential Energy (initial height) (J) Potential Energy (bounce height) (J) Speed at Ground (m/s) *from 1st table* Kinetic Energy (J) 0.7 m 1.0 m Questions: 1. The equation for potential energy (PE) is: Potential Energy = (mass) x (height) x (acceleration of gravity), where mass is in kilograms, height is in meters, acceleration of gravity is 9.8 m/s, and potential energy is in joules. a) Weigh the ball on a scale, and record the mass in kilograms where indicated above the nd table. b) Calculate the potential energy of the ball at its initial height, and at its average bounce height, and fill in your answers in the second table. Aside: The formula for potential energy should make sense to you. To lift an object to a certain height, you need to do work on it. The amount of work is W = Force distance. W F d But from Newton s law: Force = Mass Acceleration. F m a and in this case a is acceleration due to gravity, usually called g. Then: PE W m d g. What happens to the average bounce height as you increase the initial height? Does this make sense? Does the bounce height equal the initial height? Why or why not? Rev /6

4 3. What happens to the potential energy of the ball as you increase the initial height? Does this make sense? What is the difference in potential energy for the initial and bounce height? Where does this energy go? 4. At what point in a drop does the ball have maximum potential energy? At what point does the ball have minimum potential energy? 5. Calculate the kinetic energy (KE) of the ball just before it hits the ground: KE = ½ mv where v is the speed of the ball just before it hits the ground, which you calculated in the first data table. Fill in your results in the last column of the nd data table. Aside: The formula for kinetic energy should also make sense to you (even if it looks completely different from the formula for potential energy). The work gained to go from zero velocity to the final velocity v is still W m d a as above, but made more general for any acceleration a. But acceleration is just average velocity divided by time, and in this example, average velocity = (initial velocity + final velocity)/, v v and so we can write a. Hence, W m d. But remember that distance/time t t is just velocity so one can replace the d /t term with v to end up with the kinetic 1 energy, KE m v Rev /6

Calculate the kinetic energy (KE) of the ball just before it hits the ground: KE = ½ mv where v is the speed of the ball just before it hits the ground, which you calculated in the first data table.

5 6. Compare the KE of the ball with its initial PE. Do they increase together as the drop height increases? Does this make sense? 7. At what point in a drop does the ball have maximum kinetic energy? At what point does the ball have minimum kinetic energy? 8. What can you conclude then about the relationship between gravitational potential energy, and kinetic energy? Part Predict/Observe: 1. Suppose you are able to drop two superballs, one above the other as shown. What do you think will happen? Rev /6

What can you conclude then about the relationship between gravitational potential energy, and kinetic energy?

6 . Try it (good luck). You can try a variety of ball size combinations to see which works best. Summarize what you see. If you get a good bounce (well lined up when they hit), how does the maximum height of the upper ball s bounce compare to the case of just dropping a single ball from the same height? 3. Can you explain qualitatively what you have observed? Summary and Suggestions for the Future: a. What were the important concepts of physics/science that you learned from this activity? What else did you learn? b. Can you think of alternative hands-on ways in which these concepts could be demonstrated? c. What changes (if any) would you make to teach these activities in a Grade 7-8 classroom Rev /6

height? 3. Can you explain qualitatively what you have observed? Summary and Suggestions for the Future: a.

Gravitational Potential Energy

Gravitational Potential Energy Consider a ball falling from a height of y 0 =h to the floor at height y=0. A net force of gravity has been acting on the ball as it drops. So the total work done on the