How does a plumb bob show a vertical line?

1. A builder lays bricks, one row on another. If each row is a tiny bit off, the wall slowly leans. From close up, a small lean is very hard to see.
2. So the builder hangs a plumb bob beside the wall: a heavy, pointed weight on a thin string. At first it swings from side to side.
3. Each swing is a little smaller, because the air slows it down. Soon the bob hangs still. Now the string makes a line that is exactly up and down.
4. Why does it point down? Gravity pulls the weight toward the middle of the Earth. Anywhere in the world, a plumb line points at the center of the Earth.
5. Now compare. Measure the gap between the wall and the string at the top, then at the bottom. If the two gaps are the same, the wall is straight. If not, it leans.
6. The builder nudges the wall until both gaps match. Now it stands plumb, which means truly upright. People have built this way for thousands of years.

Tap each part of the plumb line. See what it does.

• Hanging block. The string hangs from a block or a nail at the top. The block holds the string a little way out from the wall, so the bob can hang freely.
• String. The string is thin and strong. The weight pulls it tight, so it makes a perfectly straight line from the top to the bob.
• Cap. The string goes into the bob through a small cap, exactly in the middle of the top. That way the bob hangs evenly and does not tip.
• Bob. The bob is a heavy piece of metal, often brass or steel. Because it is heavy, it pulls the string tight and a little wind hardly moves it.
• Pointed tip. The tip is a sharp point. It shows the exact spot on the ground that is straight below the place where the string is tied.
• Mark on the ground. A builder can mark the spot under the tip. That mark is exactly below the top of the string, even many floors down.

Tap each wall to hang the plumb bob beside it. When it stops swinging, compare the gap at the top with the gap at the bottom.

Experiment: A wall leans away from the string at the bottom. How will the two gaps compare?

1. Tap the first wall and wait for the bob to rest.
2. Compare the two gap bars. Then check the other walls.
3. Which wall has equal gaps?

What we found: The string always hangs straight down, whatever the wall does. So if the wall leans, the gap changes from top to bottom. Only a wall that is truly upright stays the same distance from the string all the way down.

Turn on JavaScript to try it yourself.

Move the top of the wall until both gaps match, then press Check. You cannot see the numbers, so compare the gap bars carefully.

A plumb bob is one of the simplest tools ever made: a heavy weight with a pointed tip, hanging on a string. Builders in ancient Egypt used them, and builders still use them today.

It works because of gravity. The Earth pulls everything toward its middle. A weight on a string is pulled down until the string is tight. If you give it a push, it swings like a playground swing, but the air slows it and it comes to rest. Then the string points exactly toward the middle of the Earth.

That gives the builder a perfect up-and-down line to compare with. The string hangs a little way out from the top of the wall. If the wall is straight, the gap between the wall and the string is the same at the bottom as at the top. If the gap grows or shrinks, the wall is leaning.

A heavy bob is better than a light one, because wind cannot push it around so easily. Some builders even dip the bob in a bucket of water to stop it swinging sooner. Our swing comparison is not perfect: a swing has two chains and a seat, so it can only move forward and back, but a plumb bob is free to settle in any direction.

Key words

Plumb

Standing exactly straight up and down.

Gravity

The pull of the Earth that makes things fall down.

Tension

The pull along a tight string.

Pendulum

A weight on a string or rod that swings back and forth.

Vertical

The straight up-and-down direction, in line with gravity.

Check your thinking

1. Why does the string of a plumb bob point straight down?

   Show the answer

   Gravity pulls the weight toward the middle of the Earth. The weight is pulled down until the string is tight and in line with the pull.

2. The gap between wall and string is 5 centimeters at the top and 5 at the bottom. What does that tell you?

   Show the answer

   The wall is plumb. The string is straight up and down. A wall that keeps the same distance from it must be too.

3. Why should you wait before reading a plumb bob?

   Show the answer

   It has to stop swinging before the string shows straight down. While it swings, the string points to one side and then the other. Only at rest does it point down.

4. Why is a heavy bob better than a light one on a windy day?

   Show the answer

   The wind cannot push a heavy bob around as easily. The same puff of wind is a tiny push compared with a big weight.

5. You swap a 200 gram bob for a 400 gram bob on the same string. What happens to the time for one swing?

   Show the answer

   It stays about the same. A pendulum's period depends on its length and on gravity, not on its mass.