Grade 11 · ⚛️ Physics · Mechanics · PHY-G11-M3.2
Pendulum: watch energy trade places
Build a pendulum from a string and a key, and watch potential energy turn into kinetic energy and back again.
Why this matters
Water held behind the Grand Ethiopian Renaissance Dam stores potential energy. When it falls through the turbines, that becomes kinetic energy and then electricity for homes in Bahir Dar and Addis Ababa. Energy isn't created — it's converted, exactly as in your pendulum.
Watch the demo
Video by Mr. D'Antuono
The science
Kinetic and potential energy
KE = ½·m·v² is the energy of motion. Gravitational PE = m·g·h is the stored energy of height.
Conservation of mechanical energy
When only gravity acts, KE + PE stays constant. A swinging pendulum keeps trading one for the other. Friction and air drag slowly turn some of it into heat.
You'll need
- A string about 50 cm long
- A small heavy object (key, washer, or small stone)
- A doorframe, chair, or table edge to hang from
Safety first
- Tie the weight firmly so it can't fly off.
- Keep faces clear of the swing.
Step by step
0/4 doneWhat you should see
The weight rises almost to its starting height on the other side, then comes back. It is fastest at the bottom and stops for an instant at each end. Each swing is slightly lower than the last.
Why it happens
At the highest points the weight has maximum potential energy and no kinetic energy. At the bottom, all that potential energy has become kinetic energy, so it moves fastest. A higher start means more PE and therefore more speed at the bottom. Air resistance removes a little energy each swing, so the motion slowly dies down.
Didn't work? Try this
The pendulum wobbles in circles.
Release it gently without pushing sideways, and make sure the support doesn't move.
It stops after only a few swings.
Use a heavier, smaller weight and a thin string so air resistance matters less.
Still stuck? Ask the lab assistant
Check your understanding
1. Where does the pendulum have the most kinetic energy?
2. Why does each swing get a little lower?
Your lab notebook
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