A marble run is a simple way to see energy transfer in action. When a marble starts high on a ramp, it has gravitational potential energy because of its height above the measuring zone. As it rolls downhill, that stored energy changes into kinetic energy of motion and rotational motion.
This project matters because it lets students measure real energy changes instead of only drawing ideal diagrams.
In a real marble run, not all starting energy becomes measured motion at the bottom. Some energy is transferred to thermal energy and sound because of friction between the marble and track, air resistance, and small bumps or flexing in the materials. By changing the track length or slope, students can test how design choices affect speed, energy loss, and percent efficiency.
The goal is to compare PE at the start with KE at the measuring zone and explain where the missing energy went.
Key Facts
- Gravitational potential energy: PE = mgh
- Translational kinetic energy: KE = 1/2 mv^2
- Percent efficiency: efficiency = useful output energy / input energy x 100%
- Energy lost: E_lost = PE_start - KE_measured
- Speed from a photogate or timer: v = distance / time
- Increasing height usually increases starting PE, while increasing friction usually decreases final measured KE
Vocabulary
- Gravitational potential energy
- Energy stored by an object because of its height in a gravitational field.
- Kinetic energy
- Energy an object has because it is moving.
- Friction
- A contact force that opposes motion and transfers mechanical energy into thermal energy and sound.
- Efficiency
- The percentage of input energy that becomes useful output energy.
- Controlled variable
- A factor kept the same during an experiment so the effect of one chosen variable can be tested fairly.
Common Mistakes to Avoid
- Using track length instead of vertical height in PE = mgh is wrong because gravitational potential energy depends on height change, not the distance along the ramp.
- Forgetting to convert grams to kilograms is wrong because joules require mass in kilograms when using PE = mgh and KE = 1/2 mv^2.
- Claiming the missing energy disappeared is wrong because energy is conserved, but some is transferred to thermal energy, sound, and deformation of the track.
- Changing slope and track material at the same time is wrong because it makes it impossible to tell which variable caused the change in speed or efficiency.
Practice Questions
- 1 A 0.020 kg marble starts 0.60 m above the measuring zone. Calculate its starting gravitational potential energy using g = 9.8 m/s^2.
- 2 A 0.025 kg marble is measured moving at 2.4 m/s at the bottom of the track. Calculate its kinetic energy. If it started with 0.90 J of potential energy, calculate the percent efficiency.
- 3 Two marble runs start at the same height, but one has a longer rough section of track. Explain which run should have less measured kinetic energy at the bottom and why.