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Weight of Water

In this demonstration, students observe a very simple hydraulic system at work: the use of water to lift a person.

The pressure at the bottom of a column of water depends on the depth of the water. Imagine diving into a swimming pool. As you dive deeper, there's more water pushing down on you, so the pressure on you is greater.

Pressure is force per unit area. The pressure at the bottom of a container doesn't depend on the shape or volume of the container, only on the depth of the water. The pressure at the bottom of the long hose in this experiment is large, because of the depth of the water above it.

The hot water bottle has a much bigger area than the opening of the hose. Since pressure is force per unit area, the same pressure on a much bigger area creates a big force that can lift the person standing on the hot water bottle.

When you push on the smaller piston (on the left) you create pressure in the liquid. The same pressure on a much bigger piston exerts a much greater force, so the second piston could lift a car or an elephant!

A system like this, where a fluid is used to transfer force, is called a hydraulic system. Hydraulic systems are used to lift heavy objects.

Objectives

  • Describe how water can be used to perform work.

  • Relate weight and pressure in a column of water.

Materials

  • Per Class:
    hot water bottle
    hose (5–6 feet)
    funnel
    duct tape
    pitcher containing 1 litre of water
    chair (for volunteer student to stand on )

Key Questions

  • Which is heavier, the volunteer or a litre of water?
  • What would happen if the funnel was kept at a low level e.g. knee height?
  • Could you lift an elephant the same way?

What To Do

Preparation

  1. Tape the funnel to the hose so that it is securely fastened.

Instructions

  1. Place the empty hot water bottle flat on the floor.
  2. Choose a volunteer to stand on the hot water bottle.
  3. Attach the end of the hose to the water bottle (with tape if necessary to prevent water escaping).
  4. Pour the water into the funnel while it is at waist height.
  5. Once a small amount of water has entered the hot water bottle, lift the funnel as you continue pouring (you will need to stand on a chair).
  6. The class will see the volunteer struggle to maintain their balance as they are lifted up (this may take a few minutes as the water takes a while to fill the apparatus).

Extensions

  • How could you use this principle to do physical work, like to move things?