The Mechanics of Friction

Key Concepts

When you slide a hockey puck on the street, it quickly comes to a stop. Take that same puck and slide it over a sheet of ice and you'll find it zooms a lot farther. What gives?

ACVs require at least two engines: one for the lift (hovering action), and the other for forward thrust. The hovering motor pushes air out the bottom, which creates a pocket of higher pressure to accumulate. As the higher pressure escapes out the bottom, it lifts the vehicle up, creating the ‘hovering’ effect. Although some hovercraft utilize air ducts to use one engine for both jobs (thrust and hover), most require two or more. In addition to small vehicles, two hover trains are currently in operation (one in Japan, the other in Austria) since 1985, using an underground cushion of air to reduce track friction and increase speed.

Experiment & Video

Did you know that you can make a simple hovercraft using a balloon, an old CD, and the top from a water bottle? Although this particular hovercraft only has a 'hovering' option, I'm sure you can quickly figure out how to add a 'thruster' to make it zoom down the table!

Materials:

• old CD
• paper cup
• 7-9" latex balloon
• tack
• top of a disposable plastic water bottle (sport-top type)
• hot glue
• scissors or razor

Here's what you need to do:

What's Going On?

The balloon is shoving air through the tiny holes in the cap, which escapes out the sides of the CD. Make sure your CD and table are both pretty flat, or you'll have drag issues. The air is a lubricating layer between the CD and table that allows the hovercraft to slide a lot easier by reducing the friction between the CD and the table.

Friction is the force between two objects in contact with one another. Friction is dependent on the materials that are in contact with one another: how much pressure is put on the materials, whether the materials are wet or dry, hot or cold. In other words, it's quite complicated! The friction between the puck and the street are a lot higher than with ice.

Friction happens due to the electro-magnetic forces between two objects. Friction is not necessarily due to the roughness of the objects but rather to chemical bonds "sticking and slipping" over one another.

Questions to Ask

1. Does the shape of the balloon matter?

2. Why bother using the paper cup?

3. What happens if you open the cap of the bottle to allow greater airflow from the balloon?

4. Is there higher pressure inside or outside the balloon?

5. What else can you use besides a CD?

6. Does it matter if the air is heated or chilled?

7. How (and where) can you add a thruster to your design?

8. What is the longest hover time your hovercraft can do?

9. What happens if you poke more (or less) holes in the sport top?