The same weight, spread differently

Megha and her brother Pawan walked to a picnic spot carrying bags packed with exactly the same things. On the way Pawan kept shifting his bag and looked uncomfortable. It's hurting my shoulders, he said. Both bags were equally heavy, so why did his hurt and hers didn't?

Pawan worked it out himself: his bag had narrow straps, and Megha's had broad straps.

A bag feels heavy because gravity pulls it down, and that force presses on your shoulders. With narrow straps the weight presses on a small area. With broad straps the same weight is spread over a larger area.

The force is the same; only the area is different. Because area matters so much, science has a quantity for it: pressure, which is the force on each unit of area.

Pressure = Force ÷ Area

An easy way to remember it: force goes up, pressure goes up; area goes up, pressure goes down. Broad straps spread the force over more area, so they press less on your shoulders and feel more comfortable.

You see the same trick everywhere:

• a bucket with a broad handle is easier to carry than one with a thin handle;
• people carrying pots or baskets on their heads put a round cloth pad under the load;
• if a bag's handle is thin, wrapping a handkerchief round it helps.

Each one makes the area bigger, so the pressure gets smaller.

For now we count only forces that push straight onto a surface, at right angles to it, not ones sliding along it.