Pressure in solidsIB MYP Physics: Revision notes
Section 1
What is pressure?
Pressure tells you how concentrated a force is. It is the force acting on each unit of area:
p = F / A
where p is pressure in pascals (Pa), F is the force in newtons (N) and A is the area in square metres (m²). 1 Pa = 1 N/m².
Worked example: a crate weighing 900 N rests on a floor with a contact area of 1.5 m². p = 900 / 1.5 = 600 Pa.
The force is the weight of the object pressing on the surface, and the area is the area in contact with the surface.
Convert areas to m² first. 1 cm² = 0.0001 m², so 5 cm² = 0.0005 m².
Section 2
The effect of area
For the same force, pressure depends on area:
- Small area gives a large pressure.
- Large area gives a small pressure.
The same crate (900 N) on a face of area 0.50 m² gives p = 900 / 0.50 = 1800 Pa, three times larger than on a face of area 1.5 m².
The force (weight) stays the same. Only the area changes.
Pressure is not the same as force. The same force can make a large or small pressure depending on the area.
Section 3
Rearranging the equation
You can rearrange p = F / A:
- F = p × A (to find the force)
- A = F / p (to find the area)
Worked example: a skier weighs 700 N. Each ski has an area of 0.20 m². Pressure on the snow = 700 / (2 × 0.20) = 700 / 0.40 = 1750 Pa.
Without skis her boots have a total area of 0.04 m², so p = 700 / 0.04 = 17 500 Pa. The skis reduce the pressure by ten times, so she sinks less.
Section 4
Applications: increasing pressure
To make a large pressure, use a small area:
- Knives, needles and nails have a very small area at the tip or edge, so even a small force gives a large pressure that cuts or pierces.
- A sharper knife has a smaller edge area, so it cuts more easily.
- High-heeled shoes put the whole weight on a small area, giving a large pressure on the floor.
Section 5
Applications: reducing pressure
To make a small pressure, use a large area:
- Skis, snowshoes and snowboards spread the weight over a large area, so the person does not sink into snow.
- Tractor tyres are wide to spread the weight and stop the tractor sinking in soft soil.
- Caterpillar tracks on diggers and tanks spread the weight over a very large area.
- Camels' feet are wide so they do not sink into sand.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Pressure in solids
- A warehouse worker in Rotterdam rests a wooden crate that weighs 900 N on a concrete floor. The largest face of the crate has an area of 1.5 m² and its smallest face has an area of 0.50 m².Calculate the pressure when the crate rests on its smallest face. Explain why this pressure is different from the pressure on its largest face.2 marks
- A skier in the Swiss Alps weighs 700 N. Each ski has a contact area of 0.20 m². Without skis, her two boots have a total contact area of 0.04 m². The snow is deep and soft.Calculate the pressure the skier exerts on the snow when she stands on both skis.2 marks
- Students investigate how the area of contact affects the pressure on soft sand. A brick of weight 24 N is placed on a tray of sand, resting on a different face each time. The three faces have areas of 0.020 m², 0.010 m² and 0.005 m². The depth of the dent in the sand was 3 mm, 6 mm and 12 mm respectively.State a testable hypothesis for this investigation and give a scientific reason for it. State the dependent variable.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).