Introduction to Physics: Conservation of Energy

Grade 10 Science: Conservation of Energy Lab Name:______Partner:______Materials: Metersticks, Tape, Stopwatch, Golf ball, Tennis ball, Rubber ball Objectives: 1. Explore how energy changes from potential to kinetic and then back to potential again. 2. Compare theoretical and experimental values for height dropped and velocity obtained. 3. Develop a relationship between the height a particular ball is dropped from and the height it reaches when it bounces back. Concepts: The mechanical energy of any object in a closed system is conserved. This is demonstrated by the fact that the addition of an object’s initial kinetic and potential energies is equal to the addition of that object’s final kinetic and potential energies.

ME = KEi + PEi = KEf + PEf Recall that an object’s kinetic energy is equal to one half the product of its mass and square velocity (KE = ½ mv2). While an object’s potential energy is equal to product of its mass, acceleration due to gravity and the height the object is above (or below) a reference level (PE = mgh). Also recall that the final velocity of an object that is accelerating over a know distance can be ascertained through the kinematical equation: 2 2 vf = vi + 2ad Procedure I: 1. Affix two metersticks to the wall such that one is above the other. The metersticks should be vertical and start at the floor. This will allow you to accurately measure any vertical distance from the floor up to a height of 2 meters. 2. Record the mass of each of the balls. 3. Hold one of the balls such that the bottom of the ball is at the 1 m mark and is about 15 cm away from the wall. 4. As the ball is dropped, your partner should watch at eye level to see how high the ball bounces (be sure to note the highest point that the bottom of the ball reaches). Repeat this procedure 3 more times recording each of the bounce heights. Calculate the average bounce height. 5. Repeat steps 3 and 4 by dropping the ball from three other drop heights. 6. Repeat steps 3, 4 and 5 using a different ball. Continue this until you have collected data for a golf ball, tennis ball and rubber ball. Procedure II: 1. Hold one of the balls such that the bottom of the ball is at the 1.5 m mark of the metersticks. 2. At the moment the ball is dropped begin the stopwatch. Stop the stopwatch at the moment the ball strikes the ground. This may take a few attempts to obtain an accurate result. Record the total time of free fall. Repeat this procedure 3 more times, recording each of the free fall times for this particular drop height. Calculate the average time of free fall. 3. Repeat steps 1 and 2 by timing the ball from three other drop heights. Data Processing: Procedure I:  For EACH of the drop heights and EACH of the balls, calculate the specific initial AND final potential energies.  Construct a graph of bounce height (y) versus drop height (x) for each of the balls. Find the best-fit line. Procedure II:  For each of the drop heights calculate the ball’s initial potential energy.  For each of the drop heights calculate the ball’s theoretical velocity the moment before it hit the ground. Use conservation of energy laws to obtain these values.  For each of the drop heights calculate the experimental velocity the moment before it hit the ground. Use kinematical equation(s) to obtain these values.  Construct a graph of drop height (y) versus velocity (x) for each of the balls. Find the best-fit line. Hint: Be Careful!

Note: ALL of your raw AND processed data should be presented in tables that are neat, organized, understandable and fully labeled. A person that has no prior knowledge of this lab should be able to read these tables and understand the experiment. You must show one example (a fully worked out example) of every type of calculation performed. All graphs must be neat, organized, understandable and fully labeled.

Part I Questions: 1. Which ball initially had the greatest potential energy? Which ball had the least? Why? 2. How could you have made these three different balls all have the same potential energy (without putting them on the floor)? Be specific, use exact numbers and calculations. 3. Where did the balls have the greatest kinetic energy? Explain. 4. How do you account for the differences in bounce heights of the three balls? 5. Why did the balls not bounce back to the original height from which they were dropped? Is the energy in the system conserved? Explain. 6. Using your first three graphs, find out how high each ball would bounce if it were dropped from 10 m. Part II Questions: 7. How close were your theoretical and experimental velocity values? Why do these results not match each other?