Why Don't Basketballs Bounce Forever?

PS3.B: Conservation of Energy and Energy Transfer
  • Conservation of energy means that the total change of energy in any system is always equal to the total energy transferred into or out of the system. (HS-PS3-1)
  • Energy cannot be created or destroyed, but it can be transported from one place to another and transferred between systems. (HS-PS3-1), (HS-PS3-4)
  • Mathematical expressions, which quantify how the stored energy in a system depends on its configuration (e.g. relative positions of charged particles, compression of a spring) and how kinetic energy depends on mass and speed, allow the concept of conservation of energy to be used to predict and describe system behavior. (HS-PS3-1)
  • The availability of energy limits what can occur in any system. (HS-PS3-1)
  • Uncontrolled systems always evolve toward more stable states—that is, toward more uniform energy distribution (e.g., water flows downhill, objects hotter than their surrounding environment cool down). (HS-PS3-4)
Nothing is perfect and nothing is free. The first statement about ‘nothing is free’ is a way of saying the first law of thermodynamics. Really, that idea shows up in all of science. There is the law of conservation of mass. There are the laws of conservation momentum, charge, and rotational inertia.

The first law of thermodynamics is about the law of conservation of energy. Energy cannot be created or destroyed. That means when a ball drops it cannot bounce higher than it starts. This is also why most roller coasters have no peaks higher than the first one.

The second law of thermodynamics says that ‘nothing is perfect’. When any process occurs some energy will turn into a form you don't want.

For a car the energy you put in, gasoline or ethanol, is chemical in nature. Living things produced these chemicals using energy from the sun. The energy you get out is kinetic: energy of motion. You speed up going down the road. The process isn't perfect though. Energy is generally lost to heat. You can feel that the hood of a car is warm after driving somewhere. How much energy we get out divided by how much we put in is called efficiency.

If I push a car and do 75 Joules of work, but the car only gains 60 Joules of kinetic energy because of friction in the gears causing heat, I can calculate my efficiency. Efficiency is the amount of energy that is output, divided by the amount of energy input (times 100 to make a percent.) Because of the first law of thermodynamics, it is never more than 100%. Because of the second law of thermodynamics, it is almost always well under 100%. In the case of pushing my car: 60 Joules ÷ 75 Joules × 100 % = 80 % Efficiency.

Kinetic Energy is the energy that an object has because it is moving. The equation for kinetic energy is Ek = ½ mv². Therefore something more massive has more energy than something less massive. Obviously it takes more work to push a heavy boulder than to toss a pebble. Also as something moves faster, it gets a lot more energy.

Therefore if I toss a 200 g pebble at 35 m/s, I can calculate its energy of motion. I convert 200 g to kg, the unit of mass for physics. (200 g × 1 kg/1000 g = 0.2 g) The math for the new calculation is Ek = ½ (0.2 kg) × (35 m/s)²=122.5 Joules

Gravitational Potential Energy, however, is energy that something has because it is at a given height. Just like if I drop a pile of books that I'm holding, I can know that they will fall and make a lot of noise, I can calculate the amount of energy that they have the potential to release. The equation for potential energy is Ep = m × g × h. Mass is the m and g is the acceleration due to gravity on earth, 9.8 m/s². Lastly h is the height something is lifted to in meters.

For example if I pick up a 34 g pebble and lift it 3 m. I can calculate it's potential energy. Ep = (0.034 g) × (9.8 m/s²) × (3 m) = 1 Joule.

Energy changes form, escapes our use, and powers our world. Also like everything in science, energy follows rules that we can calculate.

CCSS.ELA-Literacy.RST.11-12.3 Follow precisely a complex multistep procedure when carrying out experiments, taking measurements, or performing technical tasks; analyze the specific results based on explanations in the text.
1. If a 2 kg ball is travelling at 5 m/s, what kinetic energy must it have? (measured in Joules)
2. When energy is “lost” in transfer, where does much of it go?
3. If 950 kilojoules of energy are added to a 1325 kg car engine in form of gasoline and it speeds up the car up from rest to 30 m/s, what was the efficiency of the engine?

4. If the engine is turned off on a moving car and it doesn't hit something first, it will slowly come to a stop. Which law of thermodynamics explains this?

5. If a 1 kg book is lifted 1.4 meters into the air. What potential energy does it now have?

Read the following transcript or play the audio. It tells about cellulosic fuels and ethanol. Energy can be derived from many sources, although the ultimate source for almost all energy we use is the sun. Here is your resource.
CCSS.ELA-Literacy.RST.11-12.7 Integrate and evaluate multiple sources of information presented in diverse formats and media (e.g., quantitative data, video, multimedia) in order to address a question or solve a problem.


6. What is cellulosic fuel made out of?
7. What is ethanol produced from?
8. What organization determines rules about fuels?

Use our attached Kinetic Energy webApp. Try bouncing the balls on the screen and watch their behavior. Keep track of the data tabs at the bottom of the screen. One of the balls follows the laws of thermodynamics, and one does not. Once you feel like you've figured out the physics of a bouncing ball, answer the questions below.

CCSS.ELA-Literacy.RST.11-12.9 Synthesize information from a range of sources (e.g., texts, experiments, simulations) into a coherent understanding of a process, phenomenon, or concept, resolving conflicting information when possible.
9. Which ball is losing energy to its surroundings with each bounce?
10. How is the mechanical energy of each ball tied to its kinetic and potential energy?
11. If I pick up a ball and hold it high in the air, have I increased its potential or kinetic energy?