How Position Stores Energy That Becomes Motion
Atlas stands at the top of a curved skate ramp in an outdoor park, one hand resting on a skateboard poised at the rim, gesturing downward along the slope to show the path energy will travel as the board drops and speeds up.
- Explain what gravitational potential energy is and identify what two factors determine how much an object stores.
- Describe how potential energy converts into kinetic energy as an object falls or rolls downward.
- Compare the energy stored at the top of a ramp to the speed gained at the bottom.
- Predict what happens to kinetic energy when a moving object rises back up a slope.
- Identify real-world examples where potential and kinetic energy trade back and forth.
Key terms
- Gravitational potential energy
- Energy stored in an object because of its mass and height above the ground.
- Kinetic energy
- The energy an object has because it is moving, increasing with speed.
- Energy transformation
- The conversion of energy from one form into another, such as potential into kinetic.
- Mechanical energy
- The combined total of an object's potential energy and kinetic energy.
What Stores the Energy
Gravitational potential energy depends on two factors: how much mass an object has and how high it sits above the ground. Lift a heavier object or raise it higher and you store more energy in it, ready to be released. This is why a skateboard at the very top of a tall ramp holds the most stored energy of the whole ride — its height is greatest there, even though it is not moving at all in that instant.
Trading Position for Speed
As the board rolls down, its height shrinks so its potential energy decreases, but its speed grows so its kinetic energy increases by the same amount. Every bit of lost height becomes gained speed, which is why the board is slowest at the top and fastest at the bottom. Rolling up the far side reverses the trade, turning speed back into height. In an ideal frictionless case the total mechanical energy stays constant throughout.
Worked examples
Describe the energy of a roller coaster car at the top of the tallest hill.
- At the top the car is nearly stopped, so kinetic energy is very low.
- Its height is greatest, so gravitational potential energy is at its peak.
- Therefore stored potential energy dominates at the top.
Answer: Maximum potential energy and almost no kinetic energy.
Two identical balls drop from 4 m and 2 m; which lands faster and why?
- The ball from 4 m starts with more gravitational potential energy because it is higher.
- All that extra potential energy converts to kinetic energy by the ground.
- More kinetic energy at the bottom means greater speed.
Answer: The ball dropped from 4 m lands faster.
Activity
Arrange these four snapshots of a pendulum swing in order from the moment of greatest stored potential energy to the moment of greatest kinetic energy.
Practice
Arrange pendulum snapshots from greatest stored energy to greatest kinetic energy.
Predict where on a swing's arc the rider is moving the fastest and why.
Common mistakes to avoid
- A still object has no energy.A still object up high stores gravitational potential energy from its position, ready to become motion.
- Heavier objects always fall faster.Without air resistance, height not mass sets the landing speed, so equal heights give equal speeds.
Check your understanding
A roller coaster car is at the very top of the tallest hill on the track before it begins to descend. Which statement best describes its energy at that moment?
As a ball rolls down a hill and gains speed, what is happening to its energy?
Two identical balls are dropped from different heights. Ball A is dropped from 4 meters and Ball B from 2 meters. Which ball will be moving faster just before it hits the ground, and why?
Recap
Gravitational potential energy depends on mass and height, and it trades back and forth with kinetic energy as an object rises and falls, so total mechanical energy stays the same except for small losses to friction.
Reflect
Where on a swing or roller coaster would you expect the most stored energy, and where the most motion?