Grade 7 · Energy Transfers · Energy Stores and Transfers
Kinetic and Gravitational Potential Energy
Calculating energy in moving and raised objects.
Learning objectives
- Calculate kinetic energy from mass and speed.
- Calculate gravitational potential energy.
- Use energy transfer to predict speed at the bottom of a slope.
NGSS alignment
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.
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.
HS-PS3-2
Develop and use models to illustrate that energy at the macroscopic scale can be accounted for as a combination of energy associated with the motions of particles and energy associated with the relative positions of particles.
MYP criteria
- Criterion A — Knowing and Understanding
- Criterion C — Processing and Evaluating
ENGAGE
Doubling the speed
Doubling a car's speed does not double its stopping distance — the braking distance becomes about four times longer.
Think about it
- Which quantity in the kinetic energy equation is squared?
- What does that predict about crash severity?
EXPLAIN
Two energy equations
Key equations
Kinetic energy Ek = 0.5 x m x v squared. Gravitational potential energy Ep = m x g x h, with g about 9.8 N/kg.
| Object | Mass | Speed or height | Energy |
|---|---|---|---|
| Football kicked | 0.45 kg | 20 m/s | 90 J kinetic |
| Student running | 50 kg | 4 m/s | 400 J kinetic |
| Book lifted | 1.2 kg | 1.5 m | 17.6 J potential |
Transferring between the two
For an object falling or rolling down a smooth slope, the gravitational potential energy lost equals the kinetic energy gained. Setting m x g x h equal to 0.5 x m x v squared lets you predict the speed at the bottom, and the mass cancels.
Common misconception
Doubling speed does not double kinetic energy. Because speed is squared, doubling the speed gives four times the kinetic energy.
Watch
Work, Energy and Power: Crash Course Physics
CrashCourse · 10 min
Covers the kinetic and potential energy equations.
Before you watch: If speed doubles, what happens to kinetic energy?
- Calculate the kinetic energy of a 50 kg runner at 4 m/s.
- Why does mass cancel when finding speed at the bottom of a slope?
Interactive simulation · PhET
Energy Skate Park: Basics
Turn on the bar graph and watch kinetic and potential energy swap as the skater moves.
While you explore
- Where is potential energy greatest?
- What happens to the total when you add friction?
Key vocabulary
- Gravitational potential energy
- The energy stored by lifting something up.
- Kinetic energy
- The energy an object has because it is moving.
Practice questions
0/2 correct
Level 1 · Criterion A
Calculate the kinetic energy of a 2 kg ball moving at 3 m/s.
Level 2 · Criterion A
If the speed of a car doubles, its kinetic energy:
MYP criterion tasks
Level 3 · Criterion C
A 0.2 kg ball is dropped from 1.8 m. Calculate its speed just before landing, and explain one reason the measured speed would be lower.
Reflect & track
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