Swinging pendulums
On a farm, forces are everywhere. An ox pulls a plough or a cart. A donkey pulls a cart to the market. A worker pushes a wheelbarrow. A machine that throws seeds gives each seed a push, and a stronger push sends a seed farther. A pendulum is a weight on a string that swings back and forth. Gravity pulls the weight down and the string holds it, so it swings. We count one swing each time the weight goes out and comes back to where it started. To test a pendulum fairly we change one thing, the length of the string, and keep the rest the same: the same weight, the same start place and the same time, 15 seconds. We do each length three times, because one try can be a mistake. With a shorter string, the pendulum swings faster, so we count more swings in 15 seconds.
| String | Trial 1 | Trial 2 | Trial 3 |
|---|---|---|---|
| 60 cm | 9 | 10 | 9 |
| 30 cm | 14 | 13 | 14 |
| 15 cm | 19 | 20 | 19 |
Pendulum test, with a grown-up. Tape a pencil to the edge of a desk so that half of it hangs over. Tape on a string with a washer or nut at the end; a grown-up prepares strings of 60 cm, 30 cm and 15 cm. First predict which string will give the most swings. Pull the weight back about a hand’s width, let go, and count the full swings in 15 seconds while a grown-up watches the clock. Do three trials for each string and fill in the table. Keep faces and fingers away from the swinging weight, and never swing it towards a person. Then write the next part of a farm story: oxen pull a seed machine, the driver slows them down, and the seeds fall near. Use dialogue between two children.
| String | Swings in 15 s |
|---|---|
| 60 cm | 8 |
| 30 cm | 12 |
| 15 cm | 17 |