Nothing Is Free

A man wants to get a heavy drum onto the back of a truck. He cannot lift it — nobody could. So he leans a long plank from the ground up to the truck bed and rolls the drum up the plank instead. It goes up easily.

Ask him what the plank did and he will say it made the drum lighter. It did not. The drum weighs exactly what it weighed before.

What the plank did was let him trade. Instead of a huge push over a short distance, he made a small push over a long distance. The plank did not reduce the job. It rearranged it into a shape his arms could manage.

That is what every simple machine does, and it is the whole of this lesson. A simple machine changes the direction of a force, or the size of a force, or both. What it never, ever does is make the job smaller.

There is a price on every one of them, and once you know where to look you can always find it. By the end you'll be able to:

• name the four simple machines you need and say what each one is for
• state the one rule that governs all of them
• work out what a machine is costing you, not just what it is giving you
• spot the wrong answer that this topic is famous for