Skateboards, Hills, and the Energy That Never Disappears
Lumi the glowing fox stands at the top of a sunny skate ramp, paw resting on a skateboard, tracing a swooping arrow with light to show energy rolling downhill and back up.
- Define kinetic energy as the energy of a moving object.
- Predict how changing mass or speed changes an object's kinetic energy, using the fact that speed has a squared effect.
- Describe potential energy as stored energy that depends on position or height.
- Explain that energy transfers and converts between forms without being created or destroyed.
Key terms
- Kinetic energy
- The energy an object has because it is moving, depending on mass and speed.
- Potential energy
- Stored energy an object has because of its position or height above the ground.
- Mass
- The amount of matter packed into an object, measured in kilograms.
- Energy conservation
- The principle that total energy is never created or destroyed, only changed in form.
Why Speed Counts Double
Mass and speed both raise an object's kinetic energy, but they do not act the same way. Doubling the mass simply doubles the energy, a one-for-one trade. Doubling the speed, however, multiplies the energy by four because speed appears as a square in the kinetic energy relationship. This squared effect is the reason high-speed crashes are so much more destructive than low-speed ones — a small jump in speed produces a large jump in the energy carried.
The Energy Trade on a Ramp
At the top of a ramp a skateboard is still, so it has maximum potential energy from its height and almost no kinetic energy. As it rolls down, the height shrinks and the speed grows, so potential energy steadily converts into kinetic energy. Rolling back up reverses the trade. A little energy slips away as heat from friction, but in an ideal frictionless world the total of potential plus kinetic energy would stay exactly constant the whole way.
Worked examples
A cart's speed doubles while its mass is unchanged; how does kinetic energy change?
- Recall that kinetic energy depends on the square of speed.
- Doubling speed means multiplying by 2, and 2 squared is 4.
- So the kinetic energy is multiplied by 4.
Answer: Kinetic energy quadruples (×4).
Two carts have equal speed but cart A has twice the mass of cart B; compare their kinetic energy.
- Kinetic energy is directly proportional to mass when speed is equal.
- Cart A has twice the mass of cart B.
- So cart A has twice the kinetic energy of cart B.
Answer: Cart A has double the kinetic energy of cart B.
Activity
Sort each item below into kinetic energy (moving) or stored potential energy (still, stretched, or elevated).
Practice
Predict how kinetic energy changes if a ball's speed triples while its mass stays the same.
Sort five everyday objects into those storing potential energy and those carrying kinetic energy.
Common mistakes to avoid
- Lighter objects always carry more energy.At the same speed a heavier object carries more kinetic energy because energy grows with mass.
- An object high up but still has no energy.A still object up high stores potential energy from its position, ready to become motion.
Check your understanding
Two carts roll at the exact same speed. Cart A has more mass than Cart B. Which cart has more kinetic energy?
A ball sits still at the top of a tall hill. What kind of energy does it mainly have?
As a skateboard rolls down a ramp and speeds up, what happens to its energy?
A cart doubles its speed while its mass stays the same. What happens to its kinetic energy?
Recap
Kinetic energy comes from motion and grows with mass and the square of speed, potential energy comes from height, and the two trade back and forth without the total energy ever being created or destroyed.
Reflect
Where in your own day do you notice stored energy turning into motion, the way it does on the ramp?