OCR GCSE Physics A: Gravity, weight and pressure force in fluids (J249)

This guide covers statements P2.3g to P2.3j and P2.3n to P2.3o of Topic P2 Forces in the OCR GCSE (9-1) Physics A (Gateway Science) specification, Version 5.0 (July 2026), first assessment 2018. P2.3g, P2.3h and P2.3i are assessed on both tiers. P2.3j, P2.3n and P2.3o are marked in bold in the specification, so they are Higher Tier only. Other pages cover springs and moments: see the Forces study guide for the wider topic.

Related pages: revision notes, practice questions, the OCR GCSE Physics hub, the printable checklist and the free diagnostics.

What this unit covers

Statement	What you must be able to do	Tier
P2.3g	Describe that all matter has a gravitational field that causes attraction, and that the field strength is much greater for massive objects	Both
P2.3h	Define weight, describe how it is measured, and describe the relationship between weight and the gravitational field strength, g	Both
P2.3i	Recall the acceleration in free fall	Both
P2.3j	Apply formulae relating force, mass and physical constants, including g, to explore how changes in these are inter-related	Higher Tier only
P2.3n	Recall that pressure in fluids (gases and liquids) causes a net force at right angles to any surface	Higher Tier only
P2.3o	Use the relationship between force, pressure and area in contact, including how simple hydraulic systems work	Higher Tier only

Gravitational fields (P2.3g)

Every object that has mass has a gravitational field around it. The field is a region where another mass feels a force. The force is always an attraction: it pulls masses towards each other, never apart.

The strength of the field depends on the mass that creates it. A very massive object such as a planet or a star has a field strong enough to hold other objects in orbit. A pencil also has a field, but it is so weak that the pull between two pencils on a desk is far too small to notice. So in everyday life you only notice the Earth’s pull.

The attraction acts on both objects, so the Earth is pulled towards you as well as you towards the Earth.

Weight and the newtonmeter (P2.3h)

Weight is the force on an object caused by the gravitational field it is in. It is a force, so it is measured in newtons (N) and it acts downwards towards the centre of the planet. Mass is the amount of matter in an object. It is measured in kilograms (kg) and it does not change when the object moves to another planet.

Weight is measured with a newtonmeter. The object hangs from the hook, the spring inside stretches until its upward pull balances the weight, and the pointer reads the force on a scale in newtons. A balance that reads in kilograms measures mass.

The relationship between weight and gravitational field strength is:

weight = mass × gravitational field strength, or W = m g

where W is in newtons (N), m is in kilograms (kg) and g is in newtons per kilogram (N/kg). The specification gives g as 10 N/kg at the Earth’s surface. The unit N/kg tells you what g means: each kilogram of mass feels a gravitational force of g newtons. Field strength varies from place to place, so the same mass has different weights on different planets.

Worked example 1: weight on Earth

A diver has a mass of 64 kg. Find her weight at the Earth’s surface.

1. W = m g
2. W = 64 × 10
3. W = 640 N

Worked example 2: rearranging for mass

A probe weighs 48 N on the Moon, where g = 1.6 N/kg. Find its mass and its weight on Earth.

1. Rearrange: m = W / g
2. m = 48 / 1.6 = 30 kg
3. Mass is unchanged on Earth, so W = 30 × 10 = 300 N

Free fall (P2.3i)

An object is in free fall when the only force acting on it is its weight. In that case it accelerates downwards. Near the Earth’s surface you should recall that the acceleration in free fall is about 10 m/s², which matches g = 10 N/kg. A more precise value is 9.8 m/s². The specification gives g as 10 N/kg, so use 10 unless a question supplies another value.

Two facts about free fall:

• All objects fall with the same acceleration, whatever their mass.
• In air, air resistance acts upwards and grows with speed, so real objects do not keep accelerating at 10 m/s². Free fall is the idealised case with no air resistance, as on the Moon or in a vacuum chamber.

Mass does not matter because a heavier object has a bigger weight but also needs a bigger force to accelerate it. The effects cancel.

Worked example 3: two masses, one acceleration

A 0.25 kg stone and a 4.0 kg stone are dropped together in a vacuum. Show that they accelerate equally.

1. Weight of small stone = 0.25 × 10 = 2.5 N. Acceleration = force / mass = 2.5 / 0.25 = 10 m/s².
2. Weight of large stone = 4.0 × 10 = 40 N. Acceleration = 40 / 4.0 = 10 m/s².
3. Both accelerate at 10 m/s², so they reach the floor together.

A free-falling object gains about 10 m/s of speed every second. After 3.0 s from rest its speed is 10 × 3.0 = 30 m/s.

Applying the formulae (P2.3j, Higher Tier only)

At Higher Tier you must use W = m g flexibly and explore how its quantities depend on each other. The four ideas below cover the usual questions.

• Direct proportion: at one place, weight is directly proportional to mass. Doubling the mass doubles the weight. A graph of weight against mass is a straight line through the origin, and its gradient is g.
• Changing g: for the same mass, weight is directly proportional to g. A planet with half the field strength gives half the weight.
• Fixed weight: if the weight is fixed, mass and g are inversely proportional. The same 12 N reading means a larger mass where g is smaller.
• Weight and acceleration together: when the weight is not balanced, use resultant force = mass × acceleration, with the weight as one of the forces.

Worked example 4: moving between planets

A sample reads 12 N on a newtonmeter on a planet where g = 4.0 N/kg. Find its mass and its weight on Earth.

1. m = W / g = 12 / 4.0 = 3.0 kg
2. Earth weight = 3.0 × 10 = 30 N

Worked example 5: weight in a force calculation

A 5.0 kg crate is lifted by a rope with an upward pull of 70 N. Find its acceleration.

1. Weight = 5.0 × 10 = 50 N downwards.
2. Resultant force = 70 − 50 = 20 N upwards.
3. a = F / m = 20 / 5.0 = 4.0 m/s² upwards.

Pressure in fluids and the force it causes (P2.3n, Higher Tier only)

A fluid is a gas or a liquid. Its particles move constantly and collide with any surface they touch. Each collision pushes on the surface. Billions of collisions each second add up to a steady pressure.

The key result: pressure in a fluid causes a net force at right angles to any surface in contact with it. At each collision the particle rebounds from the surface, so the push is perpendicular to the surface. On a curved surface such as a balloon, the force on each small patch points directly outwards from that patch. This is why a balloon stays round.

If fluid acts on both sides of a surface, it is the difference in pressure that matters. A tyre pressure that is higher than the air pressure outside gives a net outward force on the tyre wall.

Pressure, force and area (P2.3o, Higher Tier only)

The relationship is:

pressure = force normal to a surface / area of that surface, or p = F / A

where p is in pascals (Pa), F is in newtons (N) and A is in square metres (m²). “Normal” means at right angles. The rearrangements are F = p A and A = F / p.

A spread force gives a small pressure and a concentrated force gives a large one. This explains the specification’s practical idea of pushing a balloon onto a single drawing pin or onto many pins. The same force is shared over a larger total area with many pins, so the pressure on the balloon is lower and it does not burst.

Worked example 6: calculating pressure

A force of 360 N pushes at right angles onto an area of 0.0045 m². Find the pressure.

1. p = F / A
2. p = 360 / 0.0045
3. p = 80 000 Pa

Worked example 7: a drawing pin

A thumb pushes with 30 N. The pin head has area 1.5 × 10⁻⁴ m² and the tip has area 2.5 × 10⁻⁷ m². Compare the pressures.

1. Head: p = 30 / (1.5 × 10⁻⁴) = 2.0 × 10⁵ Pa.
2. Tip: p = 30 / (2.5 × 10⁻⁷) = 1.2 × 10⁸ Pa.
3. The tip pressure is 600 times the head pressure, because its area is 600 times smaller.

Worked example 8: net force from a pressure difference

A lid of area 0.012 m² closes a container. The pressure inside is 1.8 × 10⁵ Pa and the pressure outside is 1.0 × 10⁵ Pa. Find the net force on the lid.

1. Pressure difference = 1.8 × 10⁵ − 1.0 × 10⁵ = 8.0 × 10⁴ Pa.
2. F = p A = 8.0 × 10⁴ × 0.012
3. F = 960 N, outwards and at right angles to the lid.

Simple hydraulic systems (P2.3o, Higher Tier only)

A hydraulic system uses a liquid to transmit pressure. It has a small piston (the effort) joined by a pipe to a large piston (the load). Liquids are almost impossible to compress, and the pressure applied to an enclosed liquid is passed on through it. So the pressure is the same at both pistons:

F₁ / A₁ = F₂ / A₂

Because A₂ is bigger, F₂ is bigger. The system is a force multiplier, with a multiplication factor equal to A₂ / A₁. The price is distance. The liquid volume leaving one cylinder equals the volume entering the other, so the small piston moves further than the large piston by the same factor.

Worked example 9: a hydraulic press

The small piston has area 4.0 × 10⁻⁴ m² and the large piston has area 6.0 × 10⁻³ m². A force of 120 N acts on the small piston, which moves 0.30 m.

1. Pressure in the liquid = 120 / (4.0 × 10⁻⁴) = 3.0 × 10⁵ Pa.
2. Force on the large piston = 3.0 × 10⁵ × 6.0 × 10⁻³ = 1800 N.
3. Area ratio = 6.0 × 10⁻³ / 4.0 × 10⁻⁴ = 15, so the force is 15 times bigger.
4. The large piston moves 0.30 / 15 = 0.020 m.
5. Check: work in = 120 × 0.30 = 36 J; work out = 1800 × 0.020 = 36 J.

Common errors

• Writing weight in kilograms or mass in newtons. A newtonmeter reads newtons.
• Saying mass changes on the Moon. Only the weight changes.
• Saying heavy objects fall faster in a vacuum.
• Leaving area in cm² when the equation needs m². One square metre is 10 000 cm².
• Drawing the force from fluid pressure along the surface instead of at right angles to it.
• Forgetting to subtract the outside pressure when fluid acts on both sides.

Test yourself with the practice questions, then condense with the revision notes.

Official syllabus

OCR, GCSE (9-1) Gateway Science Physics A (J249) specification, Version 5.0 (July 2026), first assessment 2018, Topic P2 Forces, statements P2.3g to P2.3j, P2.3n and P2.3o, with the equation list in the appendix.