Pressure [replacing lesson: Pressure]

Philip is putting up a poster, but he’s run out of drawing pins. He tries using a needle instead of a drawing pin to hang up the poster. Ouch! The needle doesn’t work — it pricks Philip’s thumb. Maybe this wooden dowel with a flat end would be better?

The blunt end is easier on Philip’s thumb. But even though Philip presses just as hard as before, the dowel doesn’t pierce the wall. How come drawing pins go into the wall easily, with just one little push? Well, it’s all about pressure! When Philip pushes the needle, he applies a force to one end.

This end of the needle is a tiny area, where all the force is concentrated. So, Philip feels a lot of pressure on his thumb, and it hurts. When he uses the dowel, it doesn't hurt. This is because the force Philip applies spreads out over the larger area of the dowel’s flat end. There is less pressure on his thumb.

But the other end of the dowel that is pressed against the wall has a larger area too. The force Philip applies spreads out over that surface. It isn’t enough to pierce the wall. This is why Philip really needs to find some drawing pins. Like the dowel, a drawing pin has a blunt end.

Philip can press without getting hurt, because the pressure is low. And like the needle, the pin has a pointy end that touches the wall, where all the force concentrates. Where the pin’s tip touches the wall, the pressure is high, and the pin goes straight through. Did you notice? There is a relationship between the force applied, the area of contact, and the resulting pressure.

When you apply a force over an area, you create pressure. How much pressure there is, depends on how much force is applied, and over what area. Pressure equals force divided by area. If you know the amount of force, and the area of contact, you can calculate pressure. Let’s try it!

Jenny and Aki are on the beach. Both of them weigh the same: 60 kilograms. The force with which each of them acts on the sand is about 600 newtons. Aki is wearing high heels, and Jenny is wearing flat boots. The soles of Jenny’s boots have a combined area of 0.04 square meters.

The combined area of the soles and heels of Aki’s shoes is only 0.015 square meters. Can you tell which of the girls puts more pressure on the sand? Let’s use the formula to work it out! The pressure under Jenny’s boots is equal to the force — 600 newtons, divided by the area — 0.04 square meters. 15,000 newtons per square metre — or 15,000 pascals.

The pressure under Aki’s high-heeled shoes is 600 newtons, divided by the area — 0.015 square meters. That’s 40,000 pascals. The pressure under Aki’s high-heeled shoes is much higher than the pressure under Jenny’s boots. Can Aki somehow reduce the pressure she puts on the sand? You can change the pressure by changing either the force or the contact area.

Increasing the force or decreasing the area, increases the pressure. Decreasing the force or increasing the area, reduces the pressure. Aki can’t do much about the force. But she can increase the contact area. She takes off her shoes!

Her bare feet have a larger area than her high heels, so the force spreads out, and she doesn’t sink into the sand!