Notebook Science 노트북 사이언스 · MYP Physical & Earth Sciences 1–3
Unit 04 · Physics

Potential Energy, Kinetic Energy and Gravity

위치·운동에너지와 중력

Where does the energy of a falling ball or a speeding car actually come from?

Big idea Energy cannot be made or destroyed — only transferred between stores, such as kinetic (movement) energy and gravitational potential (height) energy.

01Energy stores

Energy is stored in different ways. Two important stores in this unit are:

  • Kinetic energy store — energy of anything that is moving
  • Gravitational potential energy store — energy of anything raised up, due to gravity

02Kinetic energy

Kinetic energy depends on an object's mass and its speed. Doubling the speed makes the kinetic energy four times bigger, because speed is squared.

KE = ½ × mass × speed² (joules, J)
Example. A 2 kg ball moving at 3 m/s has KE = 0.5 × 2 × 3² = 9 J.

03Gravity and gravitational potential energy

Gravity is a force of attraction between any two masses. On Earth's surface, gravity gives objects a downward acceleration of about 9.8 m/s² (often rounded to 10 m/s²), called g.

Gravitational potential energy (GPE) is the energy an object has because it is raised to a height.

GPE = mass × g × height (joules, J)
Example. A 1 kg book lifted 2 m has GPE = 1 × 10 × 2 = 20 J.

04Conservation of energy

The Law of Conservation of Energy says total energy stays constant — it only transfers between stores, or spreads out (often as heat), it is never created or destroyed.

As an object falls, GPE transfers into KE. As a pendulum swings, energy transfers back and forth between GPE and KE.

Example. A skater at the top of a ramp has maximum GPE and zero KE. At the bottom, GPE has mostly transferred into KE (some is lost to friction as heat).
Key vocabulary
Term한국어Meaning
energy에너지The capacity to do work or cause change
kinetic energy (KE)운동 에너지Energy of a moving object
gravitational potential energy (GPE)위치 에너지Energy stored due to height in a gravity field
gravity중력The attractive force between masses
g중력 가속도Gravitational field strength, about 9.8 m/s² on Earth
conservation of energy에너지 보존Energy is never created or destroyed, only transferred
Quick check — tap a question to reveal the answer
A 4 kg object moves at 5 m/s. Calculate its kinetic energy.
KE = ½ × 4 × 5² = ½ × 4 × 25 = 50 J
A 2 kg object is lifted 3 m. Using g = 10 m/s², find its GPE.
GPE = mgh = 2 × 10 × 3 = 60 J
As a ball falls, what happens to its GPE and KE?
GPE decreases and KE increases — GPE is transferred into KE.