# How to explain gravity and friction

Written by lee johnson
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Physics is a fascinating subject, but the concepts are often difficult to visualise and understand, so explaining them can be quite a challenge. If you’re trying to teach your child or anybody you know how gravity and friction work, there are plenty of simple ways to convey the general concepts without delving too deep into the details of each. Gravity, for example, can be simply understood using the classic Newtonian understanding, but the addition of Einstein’s general relativity does make things a little more confusing. Friction can be divided into ordinary friction and air resistance.

Skill level:
Moderately Easy

### Things you need

• Large sheet (such as a towel or blanket)
• Ball (around the size of a football)
• Any book
• Different surfaces (carpet and table-top, for example)

## Gravity

1. 1

Start with the basics of gravity. Explain that the “stuff” around us has mass, and any object with mass exerts a “pulling” force on every other object. Things that are bigger (have more mass) or are closer together have a bigger force acting between them, so you won’t notice your own gravity, but are constantly reminded that a big object like the Earth has gravity.

2. 2

Clarify that the term “weight” is technically a description of the amount of force something has due to gravity, and that “mass” is an accurate measure of the amount of matter an object has. You would weigh less on the Moon because of the reduced gravity, but you would still have the same mass.

3. 3

Talk about Einstein’s theory of general relativity. Explain that the universe is composed of a “fabric” of space and time, commonly referred to as spacetime. Things with mass warp spacetime, which creates a sort of “sinkhole” which pulls other things down into it.

4. 4

Ask somebody to help you hold your sheet out horizontally for a two-dimensional representation of spacetime. Place the ball on the centre and watch as the sheet droops under its mass. Explain that this is similar to the Earth or the Sun distorting the fabric of space time. Anything in the crevice would essentially fall in. This is difficult to visualise in three dimensions, so stick to the simple model.

## Friction

1. 1

Explain that although objects might appear smooth, at the atomic level there are essentially vast mountains and valleys. Describe how these can interlock and cause some resistance to motion. Tell them that if you can see the uneven surface on our “macroscopic” level, the friction is likely to be much harder to overcome.

2. 2

Demonstrate the basic principle of friction using a book and different surfaces. Place the book on an obviously rough surface such as a carpet. Push the book gently to feel the resistance created by friction, and let the people you’re explaining the concept to have a try. Move the book to a smoother surface (such as a wooden tabletop) and repeat the demonstration, noting that it’s easier to move this time but friction is still present.

3. 3

Explain that air resistance is also a form of friction. Describe how the apparently empty air is actually a “sea” of gas molecules, bumping into one another and being breathed in and out by us all to keep our bodies functioning. Point out that this creates a type of friction when things travel through the air in a similar way to the book on the rough surface, with gas molecules bombarding it and slowing it down.

4. 4

Demonstrate how friction increases the internal energy of the objects and produces heat by rubbing your hands together. The atomic unevenness of the surfaces means that the atoms basically bang into each other, which increases their energy and can be felt as heat.

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