A hydraulic-powered activity from Science Buddies

Key Concepts Physics Engineering Simple Machines Force Pressure

Introduction Not even the strongest human could lift a truck into the air. Our brains, however, are smart enough to create a tool that can lift heavy objects for us: hydraulic lifts! You can find them at car repair shops, in wheelchair lifts, and even on skyscraper construction sites. They use water (or other liquids) to increase the force available to lift things. And you will get to build one in this activity! Try it out, and discover how much easier it is to lift with the power of water.

Background If you have ever used a wheelbarrow, you know it can help to lift heavy objects: lifting the handles of the wheelbarrow is easier than lifting the object itself. A hydraulic lift accomplishes the same task. It allows you to move a heavy object with much less effort.

"Hydraulic" refers to tools that operate by moving liquids, such as water or oil. You cannot compress liquids (unless you have special equipment). This means that no matter how hard you press on a liquid you cannot noticeably change its volume. Try it out! Fill an oral medicine syringe with water, close its open tip with a finger, and try to press the plunger in. You will see that you cannot. Even if you could place an elephant on the plunger, it would not compress the fluid in the syringe. So what will you be able to do with this water power?

In the effort to move something, it is not only important to exert force but also to consider the full area over which the force is spread. Think of a time when you played with Play-Doh. Pushing a finger into the dough is easy, but pushing the palm of your hand into the dough is harder—you probably felt more resistance. Because your hand has a larger surface area, your effort is spread out over a larger area, and the dough does not receive as much force for each square inch as it does with a single-finger push. Now think about this problem in reverse: sometimes you want less force per square inch, such as when you are trying to lift something. Is there a way to place an object on a larger surface so it creates less pressure when you're trying to lift it? Try the activity to find out!

Observations and Results You probably found it easier to lift the jar with the large helping syringe. In that combination the jar probably also moved a much shorter distance.

When you press one plunger the liquid inside the closed system presses with an equal amount of pressure against the other plunger. Because pressure is created by applying force over an area, a small push on a small area creates the same pressure as a much larger push on a much larger area. That is why a hydraulic lift can multiply a force: you push on a small plunger, the pressure that is created moves through the liquid to a much larger plunger, and the load on the larger plunger feels a much greater push than the one you first made. For that reason it was easier and, if you tried, you could move more weight with a large helping syringe compared to a small one.

Did you see how liquid was transferred from one column to the other in the process? Because the volume of liquid does not change, the larger plunger will not move as much as the smaller plunger. But that is the tradeoff for being able to lift a heavy object!

More to Explore Jack it Up! Lift a Load Using Hydraulics, from Science Buddies The Multiplication of Force by a Hydraulic System, by Wisc Online Heavy Lifting with a Lever, from Scientific American STEM Activities for Kids, from Science Buddies

This activity brought to you in partnership with Science Buddies

Heidi Ledford and Nature magazine

Chelsea Harvey and E&E News

Sara Reardon and Nature magazine

Discover world-changing science. Explore our digital archive back to 1845, including articles by more than 150 Nobel Prize winners.

© 2022 Scientific American, a Division of Springer Nature America, Inc.

Thanks for reading Scientific American. Knowledge awaits.

Already a subscriber? Sign in.

Thanks for reading Scientific American. Create your free account or Sign in to continue.

Continue reading with a Scientific American subscription.

You may cancel at any time.