Mass, Velocity, and Kinetic Energy Any object in motion has kinetic energy. The amount of kinetic energy depends on the object s mass and how fast it is moving. In this activity, you will see how a constant kinetic energy affects speed when mass varies. INQUIRY FOCUS Control Variables 1. Place one book on the skateboard. 2. Push the skateboard, and observe the velocity at which the skateboard travels. skateboards or toy trucks 3 large books 3. Place three books on the skateboard, push the skateboard with the same force as in Step 2. Observe the velocity of the skateboard. What can you determine about the kinetic energy of the skateboard in the two trials? How do the velocities in the two trials compare? Explain. What factor was held constant in this experiment? What factors varied? 109
What Makes a Flashlight Shine? Energy is the ability to cause motion or change in matter. Sometimes that motion or change takes place among the atoms and molecules that make up matter. In this activity, you will observe changes in forms of energy. INQUIRY FOCUS Infer 1. Remove the batteries from a flashlight and examine them. Think about what type of energy is stored in the batteries. 2. Replace the batteries and turn on the flashlight. What type of energy do you observe? flashlight batteries 3. After a few minutes, place your hand near the bulb of the flashlight. What type of energy do you feel? Describe how you think a flashlight works in terms of energy. Where does the energy come from? Where does the energy go? What change in energy takes place in the bulb of the flashlight? 110
Determining Mechanical Energy An object has energy if it has the ability to do work on another object. In this activity, you will investigate the relationship between mechanical energy and work. INQUIRY FOCUS Interpret Data 1. Drop the ball into the box of clay from a height of 25 cm. Measure the height of the drop from the surface of the clay not from the floor or the tabletop. Record this height in the data table below. 2. Carefully remove the ball from the clay and measure and record the diameter of the crater that the ball formed. Smooth the crater out of the clay. 3. Repeat Steps 1 and 2, dropping the ball from heights of 50 cm, 75 cm, and 100 cm. small steel ball shoebox-bottom lined with soft modeling clay metric ruler Height Diameter According to your data, how are crater diameter and the height of the ball related? How does potential energy affect the ball s mechanical energy? Why? How does mechanical energy relate to the work the ball can do? How do you know? 111
Soaring Straws The gravitational potential energy of an object depends on how far it can fall, and can be increased by raising the object to a greater height. INQUIRY FOCUS Control Variables 1. Make a launcher by cutting a rubber band and taping it across an open end of a toilet paper tube. 2. Tape the straws together at one end. Starting at the untaped end, make marks every 1 cm on one straw. 3. Measure the mass of the straws. 4. Hold the launcher in one hand with your fingers over the rubber band ends. Place the rocket in the launcher, rest it on the rubber band, and pull down. Measure the stretch by the marks on the straw. 5. Align the top of the rocket with the bottom of a taped to a wall. Let go to launch the rocket. CAUTION: Aim the rocket in the air. 6. Record the height to which the rocket rises. Repeat Steps 4 and 5 two more times. 7. Repeat Steps 4 through 6 using a different amount of stretch. 8. Calculate the average height and gravitational potential energy for each trial. scissors rubber band 2 plastic straws marker metric ruler balance masking tape empty toilet paper tube graph paper Data Table Amount of stretch (cm) Trial 1 (m) Trial 2 (m) Trial 3 (m) Average height (m) Gravitational potential energy (J) Graph the gravitational potential energy versus amount of stretch. In this experiment, what measurement is related to elastic potential energy? What relationship between the elastic potential energy of the rubber band and the gravitational potential energy of the rocket does your graph show? 114 ENERGY TRANSFORMATIONS AND CONSERVATION
Law of Conservation of Energy Does a ball always rise to the same height after it is dropped from a height of 1 m? In this activity, you will find out. You will use the law of conservation of energy to explain your observations. INQUIRY FOCUS Develop a Hypothesis 1. Drop a tennis ball from a height of 1 m. Use the to measure how high it bounces. Record your result. 2. Suppose you place the tennis ball on top of the basketball and drop both together from a height of 1 m. Predict whether the tennis ball will rise to a lower, the same, or a higher height than it did in Step 1. Develop a hypothesis to explain your prediction. basketball tennis ball 3. Test your hypothesis, and record your observations. In Steps 1 and 3, how does the kinetic energy of the tennis ball just after the bounce compare to just before the bounce? How do you know? Explain why your answer to Step 3 agrees with the law of conservation of energy. 115