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

These notes condense 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 to P2.3i are assessed on both tiers. P2.3j, P2.3n and P2.3o are Higher Tier only. For full explanations read the study guide, then test yourself with the practice questions. Course pages: OCR GCSE Physics hub, printable checklist and free diagnostics. For the wider topic see Forces revision notes.

Key definitions

Term	Meaning	Unit
Gravitational field	Region where a mass feels an attractive force from another mass
Gravitational field strength, g	Force per kilogram in the field. 10 N/kg at the Earth’s surface	N/kg
Mass, m	Amount of matter. Same everywhere	kg
Weight, W	Force on a mass due to the gravitational field it is in	N
Free fall	Motion with weight as the only force. Acceleration about 10 m/s² near Earth	m/s²
Pressure, p	Force at right angles to a surface per unit area	Pa
Fluid	A gas or a liquid

Formulae

Statement	Equation	Symbols and units	Tier
P2.3h	W = m g	W in N, m in kg, g in N/kg	Both
P2.3j	m = W / g and g = W / m	rearrangements of the above	Higher Tier only
P2.3o	p = F / A	p in Pa, F in N, A in m²	Higher Tier only
P2.3o	F = p A and A = F / p	rearrangements	Higher Tier only
P2.3o	F₁ / A₁ = F₂ / A₂	pressure equal at both pistons	Higher Tier only

P2.3g: gravitational fields

• All matter has a gravitational field.
• The force on another mass is always attraction.
• Field strength is much greater for massive objects, so only planets and stars give noticeable forces.
• The pull acts on both objects: Earth pulls you and you pull Earth.

P2.3h: weight and the newtonmeter

Weight is a force. Mass is not.

Method to find weight:

1. Write W = m g.
2. Check the mass is in kg (change grams by dividing by 1000).
3. Use g = 10 N/kg for Earth unless the question gives another value.
4. Substitute and give the answer in N.

Measuring: hang the object on a newtonmeter. The spring stretches until it balances the weight and the pointer shows the force in newtons.

Small reminder: a 2.4 kg mass weighs 24 N on Earth. On the Moon, with g = 1.6 N/kg, it weighs 2.4 × 1.6 = 3.84 N. Its mass is still 2.4 kg.

Must-know distinction: mass versus weight

	Mass	Weight
Quantity type	Amount of matter	A force
Unit	kg	N
Changes with planet?	No	Yes
Measured with	Balance	Newtonmeter
Direction	None	Down, towards the centre of the planet

P2.3i: free fall

• Recall: acceleration in free fall is about 10 m/s² (9.8 m/s² more precisely) at the Earth’s surface.
• All masses accelerate equally when air resistance is absent.
• Speed gained per second is about 10 m/s, so v ≈ 10 t from rest.
• In air, air resistance grows with speed and reduces the acceleration.
• N/kg and m/s² are equivalent units for g.

P2.3j: applying the formulae (Higher Tier only)

If this changes	Then
Mass doubles, same g	Weight doubles
g halves, same mass	Weight halves
Weight fixed, g smaller	Mass is larger
Graph of W against m	Straight line through origin, gradient = g

Method for a multi-step problem:

1. List what is given with units.
2. Find mass first, since it does not change: m = W / g.
3. Use the new g to find the new weight.
4. If the weight is not balanced, find resultant force, then use F = m a.

Worked reminder: an object weighs 7.4 N where g = 3.7 N/kg. Its mass is 7.4 / 3.7 = 2.0 kg, so on Earth it weighs 2.0 × 10 = 20 N. The mass is found first because it is the quantity that stays the same.

P2.3n: pressure force in fluids (Higher Tier only)

• Fluid particles collide with surfaces and each collision pushes on the surface.
• The net force from fluid pressure acts at right angles to the surface.
• On a curved surface the force on each patch points straight out from that patch.
• When fluid is on both sides, the net force depends on the difference in pressure.

P2.3o: p = F / A and hydraulics (Higher Tier only)

Method:

1. Convert area to m² (1 m² = 10 000 cm²).
2. Pick the arrangement: p = F / A, F = p A or A = F / p.
3. Substitute, calculate and give the unit.

Small reminder: 500 N on 0.020 m² gives p = 500 / 0.020 = 25 000 Pa. A pressure of 3.0 × 10⁴ Pa on 0.50 m² gives F = 3.0 × 10⁴ × 0.50 = 15 000 N.

Idea behind the practical suggestion: one drawing pin concentrates the force on a tiny area, giving a high pressure that bursts a balloon. Many pins share the force over a bigger total area, so the pressure is lower.

Hydraulic systems

• Enclosed liquid cannot be compressed, so it passes pressure on equally.
• Same pressure at both pistons: F₁ / A₁ = F₂ / A₂.
• Force multiplier = A₂ / A₁.
• Distance moved by the large piston is smaller by the same factor.
• Work done is equal in the ideal case: F₁ d₁ = F₂ d₂.

Worked reminder: pistons of area 3.0 × 10⁻⁴ m² and 9.0 × 10⁻³ m², with 50 N on the small one. The area ratio is 9.0 × 10⁻³ / 3.0 × 10⁻⁴ = 30, so the force on the large piston is 50 × 30 = 1500 N. If the small piston moves 0.12 m, the large one moves 0.12 / 30 = 0.0040 m.

Quick self-test

Questions 1 to 4 are both tiers. Questions 5 to 12 are Higher Tier only.

1. What does a newtonmeter measure, and in what unit?
2. Find the weight of a 0.75 kg object at the Earth’s surface.
3. A 6.0 kg rock is taken to the Moon, where g = 1.6 N/kg. State its mass and weight there.
4. Recall the acceleration in free fall near the Earth’s surface.
5. An object weighs 90 N on Earth. Find its mass.
6. A sample weighs 21 N where g = 3.5 N/kg. Find its mass and its weight on Earth.
7. A graph of weight against mass is a straight line through the origin. A 3.0 kg mass has a weight of 24 N. Find the gradient and say what it represents.
8. State the direction of the force a gas exerts on the wall of its container.
9. A force of 250 N acts at right angles on 0.050 m². Find the pressure.
10. A force of 600 N is produced by a pressure of 2.4 × 10⁵ Pa. Find the area.
11. Convert 400 cm² to m².
12. A hydraulic system has small piston area 5.0 × 10⁻⁴ m² and large piston area 2.0 × 10⁻² m². A force of 40 N acts on the small piston. Find the force on the large piston.

Answers

1. Weight (a force), in newtons (N).
2. W = 0.75 × 10 = 7.5 N.
3. Mass 6.0 kg; weight = 6.0 × 1.6 = 9.6 N.
4. About 10 m/s² (9.8 m/s² more precisely).
5. m = 90 / 10 = 9.0 kg.
6. m = 21 / 3.5 = 6.0 kg; Earth weight = 6.0 × 10 = 60 N.
7. Gradient = 24 / 3.0 = 8.0 N/kg, the gravitational field strength.
8. At right angles to the wall.
9. p = 250 / 0.050 = 5000 Pa.
10. A = F / p = 600 / (2.4 × 10⁵) = 0.0025 m².
11. 400 / 10 000 = 0.04 m².
12. Ratio = 2.0 × 10⁻² / 5.0 × 10⁻⁴ = 40, so F = 40 × 40 = 1600 N.

Where marks are usually lost

• Giving weight in kg or mass in N.
• Saying “gravity” is 10 without the unit N/kg.
• Forgetting to find the mass first when moving between planets.
• Using area in cm² or mm² without converting to m².
• Saying pressure force acts “along” or “parallel to” the surface.
• Ignoring the pressure outside when a surface has fluid on both sides.
• Dividing instead of multiplying when finding force on the large hydraulic piston.
• Claiming a heavier object falls faster when there is no air resistance.
• Giving a long unrounded decimal instead of a sensible number of significant figures.

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.