 | | Picture 3.1 How do the pressures compare? |
| | Looking at force | |  | | | | To make it a fair test, we will keep the area constant (the area of the acrobats' feet). | | | | Look at the acrobats in picture 3.1. The brother at the front is on his own. The twin brothers behind are doing a piggy back. So the force on the ground is bigger than for their brother - it has been doubled. What effect will this have on the pressure? Show answer | | The pressure will be doubled. If we had three brothers, the pressure would be tripled. We can say: pressure is proportional to force. |
| | | | | Looking at area | | | | | | To make it a fair test, we will keep the force the same by using just one acrobat each time. | | | | One of the twins stands on one foot whilst another one stays on two feet. Which one produces a bigger pressure? Show answer | | The one on two feet produces a smaller pressure. A bigger area reduces the pressure. In fact, doubling the area halves the pressure. We say that: pressure is inversely proportional to area. |
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| | An equation for pressure | | | | | |  | | Picture 3.2 An equation for pressure. Click here for help remembering formulae. |
| | | | | | Picture 3.2 shows the equation for pressure. The force is on the top of the fraction. Therefore a bigger force will produce a bigger pressure - which is what we expected. Area is on the bottom of the fraction so a bigger area will produce a smaller pressure. | | | | | What are the units? | | | We usually measure force in newtons (N) and area in square centimetres (cm2). The unit for pressure will then be N/cm2. Notice that we get the new unit by putting the old units into the equation (newtons ÷ centimetres squared = N/cm2). | | | | | The pascal | | | The pascal (Pa) is a standard unit for pressure. One pascal is equivalent to 1 N/m2. So, to get the pressure in pascals, you need to measure the force in newtons and the area in square metres (rather than square centimetres).
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