Pressure, moments and workIB MYP Sciences: Revision notes
Section 1
Pressure
Pressure is the force acting on each unit of area.
pressure = force ÷ area (p = F ÷ A)
The unit is the pascal (Pa), which is 1 N/m².
Worked example: a camel of weight 6000 N stands on four feet, each of area 0.075 m². Total area = 0.30 m², so p = 6000 ÷ 0.30 = 20 000 Pa.
For the same force:
- a small area gives a large pressure (a sharp knife, a nail point)
- a large area gives a small pressure (snow shoes, wide tyres)
Use the total area in contact with the surface, for example all four feet, not just one.
Section 2
Pressure in fluids
A fluid is a liquid or a gas. Fluids exert pressure on any surface they touch.
- Pressure in a fluid acts in all directions.
- Pressure in a liquid increases with depth, because there is more liquid above pushing down. This is why dams are thicker at the bottom and deep-sea divers feel more pressure.
- Pressure in a gas comes from the particles hitting the walls of the container.
The pressure of the atmosphere is greatest at sea level and falls as you climb higher.
Section 3
Moments
A force can make an object turn about a point called the pivot. The turning effect is the moment.
moment = force × perpendicular distance from the pivot (unit: newton metre, N m)
Worked example: a force of 30 N acts at a perpendicular distance of 0.40 m from the pivot. Moment = 30 × 0.40 = 12 N m.
A bigger force, or a bigger distance from the pivot, gives a bigger moment. This is why a door handle is far from the hinge.
The distance must be measured at right angles to the line of the force.
Section 4
The principle of moments
When an object is balanced (not turning), the total clockwise moment equals the total anticlockwise moment.
Worked example (see-saw): Amara weighs 400 N and sits 1.5 m from the pivot, giving an anticlockwise moment of 400 × 1.5 = 600 N m. For Ben (500 N) to balance, 500 × d = 600, so d = 1.2 m.
If the moments are not equal, the object turns in the direction of the larger moment. A lever uses this idea: a small force far from the pivot can balance or lift a large force close to it.
Section 5
Work done and simple machines
Work is done when a force moves an object in the direction of the force.
work done = force × distance moved in the direction of the force (W = F × d)
The unit is the joule (J): 1 J = 1 N m.
Worked example: a 50 N force moves a box 6 m. W = 50 × 6 = 300 J.
Simple machines such as levers and pulleys let a smaller force do a job over a longer distance. They do not reduce the work needed; some work is always wasted as heat because of friction.
For example, a worker pulls a pulley rope with 500 N through 24 m (12 000 J) to lift an 1800 N load 6.0 m (10 800 J).
Must Know
- p = F ÷ A, in Pa (N/m²); liquid pressure increases with depth
- moment = force × perpendicular distance (N m)
- Balanced: clockwise moments = anticlockwise moments
- W = F × d, in J
- Levers and pulleys make jobs easier but do not reduce the work done
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Pressure, moments and work
- A camel in the Arabian desert has a weight of 6000 N. Each of its four wide feet has a contact area of 0.075 m² with the sand.A tourist weighing 750 N stands on the sand on both feet. The total contact area is 0.050 m². Calculate the pressure that the tourist exerts on the sand.2 marks
- Amara and Ben sit on a see-saw, which is a uniform plank balanced on a pivot at its centre. Amara weighs 400 N and sits 1.5 m from the pivot. Ben weighs 500 N.Ben now moves to 1.5 m from the pivot. Explain what happens to the see-saw.2 marks
- Sana, a student in Colombo, investigates the principle of moments using a uniform metre rule balanced on a pivot at its 50 cm mark. She hangs a 2.0 N weight 20 cm to the left of the pivot. She then hangs different weights on the right and slides each one along the rule until the rule balances horizontally. The balancing distances from the pivot are 40 cm for 1.0 N, 21 cm for 2.0 N, 10 cm for 4.0 N and 8 cm for 5.0 N.Describe how Sana should carry out the investigation so that her results are valid, including one safety precaution.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).