Keeping time
Old clocks count each swing. A longer pendulum makes the clock tick more slowly.
Key idea: length changes swing timeThe Pendulum
Find out what makes a pendulum swing faster or slower. Change one thing at a time and watch what happens.
Why it matters
Why learn about pendulums?
A pendulum swings in a regular way. If we understand that swing, we can use it to keep time, detect movement, and control large hanging objects.
Detecting earthquakes
An earthquake sensor has a hanging mass. When the ground moves, the mass moves differently and the sensor records the shaking.
Key idea: a hanging mass reveals movementWorking safely
A load hanging from a crane can swing like a pendulum. Knowing how it will move helps an operator stop it from hitting people or buildings.
Key idea: predicting a swing helps control itMeasuring gravity
Measure the length and swing time carefully, and a pendulum can tell you how strong gravity is in that place.
Key idea: swing time depends on gravityOur driving questionWhat makes a pendulum swing faster or slower?
Explore
Get a feel for the system
Drag the bob sideways and release it. Change the planet or length and watch the swing time, T. Start by noticing what changes.
θ 45°T ≈ 2.20 s
Notice: one period is a complete trip from one side, to the other side, and back again.
Investigate
Three guided experiments
Make a prediction before touching the controls. Then test it and explain the evidence in your own words.
Procedure
- Choose Earth and set the length to 0.50 m.
- Write down the swing time T.
- Change the length to 1.00 m and compare.
Record
Write down both times. Which pendulum took longer?
Explain
The model behind the motion
Gravity restores
Gravity pulls the bob back toward the middle whenever it is moved to one side.
It keeps moving
The bob is moving fastest at the bottom. It keeps moving and rises on the other side.
Energy trades form
At the top, the bob stores energy. At the bottom, it has motion energy. Air resistance slowly carries some of that energy away.
For small angles
T = 2π √(L / g)- T
- period in seconds
- L
- length in metres
- g
- gravitational field strength
Worked example
A 1.00 m pendulum on Earth takes about 2.01 seconds for one complete swing. A 4.00 m pendulum takes twice as long because √4 = 2.
Check your model
Can you predict the pendulum?
Extend
Turn the lab into an instrument
A pendulum can measure an unknown gravitational field. Measure its period and length, then rearrange the model: g = 4π²L / T². How closely can you recover the preset value for Mars?