Cliff Drop
A free kinetic and potential energy simulation. Lift anything from a feather to a semi truck over a cliff, drop it, and watch stored energy turn into motion.
What students do
Students pick a character standing on top of a cliff, then choose an object from an inventory of 11 items, from a 5-gram feather to a 36,000 kg semi truck. They drag the object onto their character, who lifts it overhead (straining more the heavier it gets), and then drop it off the cliff. Live metrics show mass, height, speed, and a pie chart of potential versus kinetic energy as the object falls. Students can switch air resistance on or off. With it on, a feather drifts down slowly while a semi truck barely slows. With it off, the scene becomes a vacuum chamber where everything falls at the same rate. After each crash, a stats screen graphs exactly how the energy changed.
The DROP button stays locked until students correctly calculate GPE = m × g × h.
The fall pauses partway down. Students use the speed shown to calculate KE = ½ × m × v².
An energy graph and data table show GPE falling, KE rising, and the total staying the same.
Toggle air resistance to compare a real fall with one inside a vacuum chamber.
The science behind Cliff Drop
Gravitational potential energy
GPE = m × g × h. Gravitational potential energy is energy stored because of an object's height.
When a student's character lifts an object over the edge, the object gains gravitational potential energy. The heavier the object (mass m) and the higher it is (height h), the more energy is stored. On Earth, g is 9.8 m/s². Lifting the object from the ground beside the character to over their head even adds a little GPE, which students can see in the metrics.
Kinetic energy
KE = ½ × m × v². Kinetic energy is the energy of motion.
As the object falls, it speeds up, so its kinetic energy grows. Because speed is squared, KE grows quickly: doubling the speed makes the kinetic energy four times larger. Students see this directly in the stats graph, where the KE curve bends upward over time.
Conservation of energy
Energy is not created or destroyed. It changes form.
At every moment of the fall, the potential energy lost equals the kinetic energy gained. In Lab Mode, students prove this themselves: after calculating KE at the mid-fall pause, they see that the GPE left plus the KE they calculated equals the GPE they started with. At impact, all of the original GPE has become kinetic energy.
Air resistance and the vacuum chamber
With air resistance on, every object feels an upward drag force that grows as it speeds up. A feather reaches its top speed, or terminal velocity, of about 1.2 m/s almost instantly and takes around 17 seconds to fall 20 meters. A bowling ball barely notices the air. The energy the air takes away becomes heat, and the stats graph shows it as a separate "lost to air" line, so GPE + KE + air still adds up to the starting energy.
Turn air resistance off and the scene becomes a vacuum chamber. With no air, a feather and a semi truck dropped from the same height land at exactly the same time and speed, just like the Apollo 15 hammer-and-feather drop on the Moon. The truck simply carries far more energy because of its mass.
NGSS alignment
Cliff Drop supports the following Next Generation Science Standards for middle school physical science.
| Standard | What it says | How the simulation supports it |
|---|---|---|
| MS-PS3-1 | Construct and interpret graphical displays of data to describe the relationships of kinetic energy to the mass of an object and to the speed of an object. | The stats screen graphs KE over time and distance, with a data table. Students compare objects of different mass dropped from the same height. |
| MS-PS3-2 | Develop a model to describe that when the arrangement of objects interacting at a distance changes, different amounts of potential energy are stored in the system. | Changing the drop height, or lifting an object overhead, changes the GPE stored in the object-Earth system. |
| MS-PS3-5 | Construct, use, and present arguments to support the claim that when the kinetic energy of an object changes, energy is transferred to or from the object. | The mid-fall check and energy graph show GPE transferring into KE throughout the fall. |
| DCI PS3.A, PS3.B | Definitions of Energy; Conservation of Energy and Energy Transfer | Every drop, plus the live pie chart and energy bars. |
| Crosscutting concepts | Energy and Matter; Scale, Proportion, and Quantity | Objects range across seven orders of magnitude in mass, from 0.005 kg to 36,000 kg. |
How to use it in class
- Predict (5 min). Ask: "If you drop a bowling ball and a semi truck off the same cliff, which hits the ground first? Which hits harder?" Have students commit to an answer.
- Explore (10 min). Students use Explore Mode to drop several objects and test their predictions.
- Lab (20 min). Students switch to Lab Mode and complete drops with at least three different objects, calculating GPE before each drop and KE mid-fall. The calculator is built in.
- Submit (2 min). Students open their lab report and take a screenshot to turn in.
- Debrief (5 min). Use the discussion questions below.
Discussion questions
- With air resistance on, why did the feather fall so much slower than the bowling ball? What changed in the vacuum chamber?
- In the vacuum chamber, why did the feather and the semi truck land at the same time, even though the truck had millions of times more energy?
- At the mid-fall pause, how did GPE + KE compare to the GPE you started with? What does that tell you?
- What happens to the kinetic energy when the object hits the ground?
- How did the fall change on the Moon and on Jupiter? Why did the same drop have less energy on the Moon and more on Jupiter?
Key vocabulary
Energy, joule (J), gravitational potential energy, kinetic energy, mass, gravity, height, speed, velocity, conservation of energy, free fall, air resistance.
Frequently asked questions
Is Cliff Drop free?
Yes. It runs free in any web browser with no ads, no download, and no login.
What is the difference between Lab Mode and Explore Mode?
In Lab Mode, students must correctly calculate gravitational potential energy before every drop and kinetic energy at a mid-fall pause, and each completed drop is added to a lab log. Explore Mode removes the math gates so students can drop anything from any height, on Earth, the Moon, Mars, or Jupiter.
Does the simulation include air resistance?
Yes, and students can turn it on or off. With air resistance, light and spread-out objects like a feather fall much more slowly than heavy ones. With it off, the scene becomes a vacuum chamber where every object falls at the same rate, just like the Apollo 15 hammer-and-feather drop on the Moon.
Are the numbers accurate?
Yes. The simulation uses real free-fall physics with g = 9.8 m/s² on Earth. With air resistance on, it uses the standard quadratic drag model with realistic terminal velocities for each object. Heights, speeds, fall times, and energies are calculated exactly from those equations.