Potential Energy — Gravitational and Elastic
In 2021, Snowy Hydro released 9 billion joules of stored gravitational energy in a single day by dropping water 900 m through turbines — proving that height equals hidden energy.
Printable Worksheets
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Q1 · Hold a book high above the floor. It's sitting still — does it have energy? What happens when you drop it?
Q2 · A stretched rubber band just sits there quietly. Where is the energy stored? How do you know it has energy?
● Know
- That gravitational PE is stored energy due to height above the ground
- That elastic PE is stored energy in stretched or compressed materials
- That PE and KE continuously convert between each other
● Understand
- Why greater height and greater mass mean more gravitational PE
- Why stretching further (up to the elastic limit) stores more elastic PE
- How energy converts through a pendulum, roller coaster, or bouncing ball cycle
● Can do
- Identify whether a situation involves gravitational or elastic PE
- Describe the PE ↔ KE conversions in a roller coaster or pendulum
- Give Australian home examples of each type of stored energy
- A ball held high above the ground
- A compressed spring in a toy
- Water in a dam reservoir
- A stretched rubber band
- A skier at the top of a slope
- Elastic potential energy
- Gravitational potential energy
- Elastic potential energy
- Gravitational potential energy
- Gravitational potential energy
A book on a high shelf has potential energy. The higher the shelf and the heavier the book, the energy it stores. A stretched spring stores potential energy. If stretched past the , the spring will not return to its original shape.
A book sitting on a high shelf looks like it's doing nothing. But the moment it falls, all that "doing nothing" becomes very dramatic, very fast. The energy was there all along — stored by its position.
Gravitational potential energy (GPE) is stored energy due to an object's height above the ground. Two things make it bigger:
- More height → more GPE. A ball on a 3 m shelf has three times the GPE of the same ball on a 1 m shelf.
- More mass → more GPE. A 4 kg bowling ball on the same shelf has more GPE than a 0.5 kg tennis ball.
Australian everyday examples of gravitational PE:
- Water in a dam (Snowy Mountains, Lake Eucumbene) — enormous GPE that converts to KE as water flows through penstocks.
- A skier at the top of a slope — all that GPE converts to KE as they descend.
- A roller coaster car at the top of the first drop — maximum GPE of the whole ride.
- A mango in a tall rainforest tree — stores GPE before it falls.
As a skier descends a slope, gravitational potential energy converts to energy. The higher the starting point, the GPE the skier begins with. A heavier skier at the same height also has GPE. At the bottom of the slope, all the GPE has been converted to energy (ignoring friction).
Elastic potential energy is stored in materials that are stretched or compressed from their natural shape. When released, that stored energy converts to kinetic energy.
Common examples:
- Stretched rubber band — flick it and the elastic PE becomes KE of the flying band.
- Compressed spring — a jack-in-the-box stores elastic PE; releasing the catch converts it to KE (the figure pops up).
- Bent archery bow — the bow stores elastic PE; releasing the string converts it to KE of the arrow.
- Bending pole vault pole — the pole bends as the athlete plants it, storing elastic PE; the unbending pole launches the athlete upward (elastic PE → GPE + KE).
The further you stretch or compress a material — up to its elastic limit — the more elastic PE is stored. Beyond the elastic limit, the material is permanently deformed and energy is wasted as heat (the material can't fully spring back).
Australian examples: car suspension springs absorb bumps (store elastic PE, release it gradually), pogo sticks (elastic PE ↔ GPE ↔ KE cycle), trampolines (elastic PE in the mat → GPE + KE of the jumper).
- An archer pulling back a bow
- A roller coaster car at the top of a hill
- A compressed car suspension spring
- A diver standing on a 10 m platform
- A stretched slingshot
- Elastic potential energy
- Gravitational potential energy
- Elastic potential energy
- Gravitational potential energy
- Elastic potential energy
Potential energy and kinetic energy constantly convert between each other in moving systems.
Pendulum:
- At the highest point (end of the swing): maximum GPE, zero KE (momentarily stationary).
- At the lowest point (bottom of the swing): zero GPE (reference level), maximum KE (fastest speed).
- The total energy stays the same (ignoring air resistance and friction).
Roller coaster: The same pattern repeats: at each peak, GPE is maximum and KE is minimum; at each trough, GPE is minimum and KE is maximum. Each successive peak must be lower (due to friction losses).
Bouncing ball: GPE → KE (falling) → elastic PE (squashing on impact) → KE (rebounding) → GPE (rising). Each bounce reaches a lower height because some energy is lost to heat and sound on impact.
Australian context — Snowy Scheme pumped hydro (Snowy 2.0): The upper reservoir stores enormous GPE. This converts to KE of flowing water → electrical energy. In reverse, surplus electricity pumps water uphill, converting electrical energy back to GPE. It's the world's biggest "energy battery" in the ground.
At the top of the first hill, the roller coaster has maximum energy and minimum energy. As it descends, GPE converts to energy, reaching maximum speed at the of the valley. Going up the next hill, KE converts back to energy.
A skier starts at the top of a steep 200 m slope. At the bottom they are moving very fast. Predict: where did all that kinetic energy come from? And does ALL the gravitational potential energy become kinetic energy? Where might some energy go?
How close was your prediction?
Earlier you were asked: A stretched rubber band just sits there quietly. Where is the energy stored? How do you know it has energy?
Now that you've worked through the lesson, write a fuller answer. Use the words elastic potential energy, elastic limit, and kinetic energy at least once each.
Q1. A ball is thrown upward and comes back down. Describe the energy transformations from the moment it leaves the hand to the moment it hits the ground. (3 marks)
Q2. Explain the difference between gravitational PE and elastic PE. Give one example of each that you would find in a typical Australian home. (3 marks)
Q3. Describe the energy transformations of a ski jumper from the top of the ramp through the jump to landing on the slope. Identify each type of energy at each stage. (4 marks)
Answers
▾MCQ 1
C — GPE depends on mass and height. Speed relates to kinetic energy, not GPE. A heavier object at greater height has the most GPE.
MCQ 2
C — As height decreases, GPE decreases and converts to kinetic energy. The roller coaster speeds up as it descends — a direct sign that GPE is becoming KE.
MCQ 3
B — A stretched rubber band stores elastic potential energy in its deformed molecular structure. When released, this elastic PE converts to kinetic energy of the band (it flies).
MCQ 4
B — At the lowest point, the pendulum is moving fastest (maximum KE) and is at the reference height (zero GPE). At the highest point, the opposite applies: zero KE and maximum GPE.
MCQ 5
B — GPE ∝ mass × height. A (1×10=10), B (2×10=20), C (2×5=10), D (4×1=4). B has the greatest value — same height as A but twice the mass.
Short Answer 1
Model answer: When the ball leaves the hand, it has kinetic energy (it is moving). As it rises, KE converts to gravitational potential energy — the ball slows down and gains height. At the highest point, KE is zero (momentarily stationary) and GPE is maximum. As it falls back down, GPE converts back to KE — the ball speeds up. When it hits the ground, all the GPE has been converted to KE (which then converts to heat, sound and deformation on impact).
Short Answer 2
Model answer: Gravitational PE is stored energy due to an object's height above the ground — it depends on mass and height. An Australian home example: books on a high shelf or a potted plant on a balcony. Elastic PE is stored energy in a stretched or compressed material. A home example: a spring in a door hinge or a rubber band around a bunch of vegies in the fridge. The key difference is what stores the energy: position (GPE) vs deformation (elastic PE).
Short Answer 3
Model answer: At the top of the ramp the ski jumper has maximum gravitational PE (high position) and some KE (moving slowly). As they ski down the ramp, GPE converts to KE — they accelerate. At the launch point (bottom of the ramp), KE is at maximum and GPE is lower. During flight, KE converts back to GPE as they rise, then GPE converts to KE as they fall. The elastic PE stored in the bending of the ski and body during the jump converts partially to KE and GPE. On landing, KE is absorbed through leg muscles and ski flexion (elastic PE), then dissipated as heat and sound.