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A bouncing ball is a simple way to see energy changing form over time. At the top of its path, the ball has the most gravitational potential energy because it is highest above the ground. As it falls, that stored energy changes into kinetic energy, which is the energy of motion. This idea helps students connect motion, forces, and conservation of energy in a familiar everyday event.
When the ball hits the ground, the story becomes more interesting because the ball briefly compresses and stores energy elastically. Some of the energy then returns to kinetic energy as the ball rebounds, but not all of it comes back. A real ball loses some energy to sound, heat, and internal deformation, so each bounce reaches a lower height. The changing bounce height gives a clear visual clue that mechanical energy is not perfectly conserved in real collisions.
Key Facts
- Gravitational potential energy near Earth: PE = mgh
- Kinetic energy: KE = (1/2)mv^2
- At the highest point, v = 0 so KE = 0 and PE is maximum relative to the ground.
- During the fall, PE decreases while KE increases, so energy changes form.
- At maximum compression, the ball is momentarily at rest and much of its energy is elastic potential energy.
- If the rebound height is lower than the drop height, some mechanical energy was transferred to heat, sound, and internal motion.
Vocabulary
- Gravitational potential energy
- Energy an object has because of its height in a gravitational field.
- Kinetic energy
- Energy an object has because it is moving.
- Elastic potential energy
- Stored energy in an object that is compressed or stretched and can return to motion.
- Mechanical energy
- The total of kinetic energy and potential energy in a system.
- Energy dissipation
- The process in which useful mechanical energy is transferred into forms like heat or sound.
Common Mistakes to Avoid
- Assuming the ball has the most kinetic energy at the top, which is wrong because its speed is zero there, so its kinetic energy is zero at that instant.
- Thinking energy disappears after the bounce, which is wrong because energy is conserved overall and is transferred into heat, sound, and deformation.
- Using PE = mgh with the wrong height reference, which is wrong because h must be measured from the chosen zero level, usually the ground.
- Believing the ball stops having energy when it is most compressed, which is wrong because energy is stored as elastic potential energy even when the speed is momentarily zero.
Practice Questions
- 1 A 0.50 kg ball is dropped from a height of 2.0 m. Using g = 9.8 m/s^2, what is its gravitational potential energy relative to the ground at the release point?
- 2 Just before hitting the ground, a 0.20 kg ball is moving at 6.0 m/s. What is its kinetic energy?
- 3 A ball rebounds to a lower height than the height it was dropped from. Explain what this tells you about energy transformations during the collision.