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# What Role does Center of Gravity Play in Making Vertical lines?

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Author: Janice VanCleave

### What You Need to Know

In relation to Earth, horizontal is a direction that is parallel with the horizon, which is a left/right position. Perpendicular means at a right angle to a plane or a surface. Vertical is a direction that is in an up/down position, which is perpendicular to the horizon.

### How Does a Perpendicular Line Work?

A horizontal line goes from left to right (or right to left). A tabletop has a horizontal surface. Vertical describes a line or an object that is perpendicular (at a right angle) to a horizontal surface. For example, boards A and B in the fence diagram are vertical, and boards C and D are horizontal. This means that boards A and B are perpendicular to boards C and D, and vice versa. To indicate that lines are perpendicular to each other, a square can be drawn in the corner where the lines meet. Two of the sixteen right angles formed by the boards are identified. Can you find the others?

### Fun Fact

Gravity pulls everything, including you, toward the center of the Earth. No matter where you stand on Earth, such as at the North Pole or the South Pole, the ground beneath you is down and the sky overhead is up.

### What Does This Have to Do with Making a Vertical Line?

Gravity pulls everything toward the center of the Earth. A line pointing toward the center of the Earth would be a vertical line. Carpenters can use this fact to determine that a building is perfectly vertical. What is a plumb bob?

### Real-Life Science Challenge

Carpenters use a device called a level to determine how straight a structure is. A level has a tube of liquid with a small bubble in it. When the level is held against the side of a building, the bubble in the level will be in the center of the tube if the wall is vertical. If you turn the level so that it is horizontal, it will tell you if a floor is truly horizontal. The bubble in the level will be in the center of the tube if the floor is horizontal.

### Experiment

Now, start experimenting with a device that makes a vertical line.

Hints

• Free-hanging objects hang vertically if wind or another force does not move them.
• Weighted objects resist motion.