This mini-lesson walks you through the whole of CCEA section 1.2 — Force: resultant forces, Newton's laws, mass and weight, free fall, Hooke's law, moments, the Principle of Moments and centre of gravity.
Work through each screen, answer the questions as you go (some are wordy, some are calculations) and collect ⭐ stars. Throughout, CCEA uses g = 10 N/kg. Press Start when you're ready.
Forces & resultant force
Forces come in pairs
A force is a push or a pull, measured in newtons (N). Whenever two objects interact, the forces on them are equal in size and opposite in direction — forces always arise in pairs.
Friction is a force that always opposes motion (it acts against the direction an object is trying to move).
A force in one direction can be given a positive value, and one in the opposite direction a negative value.
The resultant force is the single force that has the same effect as all the forces added together (using those + and − signs for forces along one line).
Watch out: the equal-and-opposite pair act on two different objects, never on the same one. The book pushes down on the table; the table pushes up on the book.
Quick check
Find the resultant
?A sledge is pulled forwards by a 40 N force. Friction pulls backwards with 15 N. Taking forwards as positive, what is the resultant force on the sledge?
Newton's first law
Balanced forces, no change
Newton's first law: in the absence of an unbalanced (resultant) force, an object will stay at rest or keep moving in a straight line at constant speed — that is, with constant velocity.
Balanced forces (resultant zero) do not mean stationary — a car can cruise at steady speed.
Inertia: objects resist changes to their motion. A bigger mass takes a bigger force to start, stop or turn.
Quick check
Reading the forces
?A skydiver is falling so that the air resistance pushing up exactly equals her weight pulling down. What is her motion?
Newton's second law
A resultant force accelerates
If the resultant force is not zero, the object accelerates. The acceleration is proportional to the resultant force and depends on the mass:
F = m aresultant force (N) = mass (kg) × acceleration (m/s²)
Bigger force → bigger acceleration. Bigger mass → smaller acceleration for the same force.
Worked example
A trolley of mass 4 kg feels a resultant force of 12 N.
a = F ÷ m = 12 ÷ 4 = 3 m/s²
Calculate
Your turn — Newton's second law
1A 1200 kg car accelerates at 2 m/s². Calculate the resultant force needed.
N
Hint: F = m × a = 1200 × 2.
Mass and weight
Mass is not weight
Mass is the amount of matter in an object, measured in kilograms (kg). It is the same everywhere. Weight is the force of gravity pulling on that mass, measured in newtons (N):
W = m gweight (N) = mass (kg) × gravitational field strength (N/kg)
On Earth the pull of gravity is 10 N on each 1 kg, so CCEA uses g = 10 N/kg. A 5 kg bag has a fixed mass of 5 kg, but its weight on the Moon (g ≈ 1.6 N/kg) would be far less than on Earth.
Worked example
Find the weight of a 7 kg watering can (g = 10 N/kg).
W = m × g = 7 × 10 = 70 N
Calculate
Your turn — weight
2An apple has a mass of 0.2 kg. Using g = 10 N/kg, calculate its weight on Earth.
N
Hint: W = m × g = 0.2 × 10.
Gravity & free fall
Falling under gravity
Gravity gives every falling object the same acceleration, whatever its mass:
Ignoring air resistance, all objects fall at the same rate regardless of mass — a feather and a hammer would land together in a vacuum.
An object dropped from rest speeds up by 10 m/s every second as it falls. This is the acceleration of free fall, g = 10 m/s².
An object thrown straight up experiences a retardation of 10 m/s² — gravity slows it by 10 m/s each second until it stops at the top.
Two faces of g: as a field strength it is 10 N/kg (used in W = mg); as the acceleration of free fall it is 10 m/s². They describe the same gravity.
Quick check
Two balls drop
?In a vacuum tube, a heavy metal ball and a light plastic ball are released from the same height at the same instant. What happens?
Hooke's law · Prescribed Practical P2
Stretching a spring
Hang masses on a spring and it stretches. Hooke's law says the extension is directly proportional to the applied force, provided the limit of proportionality is not exceeded:
F = k eapplied force (N) = spring constant (N/m) × extension (m)
P2: a force–extension graph is a straight line through the origin while Hooke's law holds; its gradient equals the spring constant k. It curves once the limit is passed.
Worked example
A spring of spring constant 25 N/m is stretched by an extension of 0.4 m.
F = k × e = 25 × 0.4 = 10 N
Calculate
Your turn — Hooke's law
3A force of 6 N stretches a spring by an extension of 0.15 m. Calculate the spring constant k.
N/m
Hint: rearrange F = k e to k = F ÷ e = 6 ÷ 0.15.
Moment of a force
The turning effect
A force can make something turn about a pivot. The size of this turning effect is the moment, measured in newton metres (N m):
moment = F × dmoment (N m) = force (N) × perpendicular distance from the pivot (m)
A longer spanner (bigger d) gives a bigger moment for the same push.
Watch out: CCEA only sets problems where the force and distance are perpendicular. Always measure the perpendicular distance from the pivot, not the length along the force.
Calculate
Your turn — moment
4A child pushes a door with a force of 25 N at a perpendicular distance of 0.8 m from the hinge. Calculate the moment.
N m
Hint: moment = F × d = 25 × 0.8.
Principle of Moments · Prescribed Practical P3
Balancing the seesaw
When an object is balanced (in equilibrium), the Principle of Moments applies:
sum of clockwise moments = sum of anticlockwise momentstaken about the same pivot
Anticlockwise: 20 × 3 = 60 N m. Clockwise: 30 × 2 = 60 N m. They match, so the beam balances.
P3 verifies this with a suspended metre rule and attached weights. The Principle also lets you find an unknown weight, a missing force, or its distance from the pivot.
Calculate
Your turn — balance it
5A seesaw balances. A 50 N child sits 1.2 m left of the pivot. A second child sits 1.5 m right of the pivot. What is the weight of the second child?
N
Hint: balanced means 50 × 1.2 = W × 1.5, so W = 60 ÷ 1.5.
Centre of gravity & stability
Where the weight acts
The centre of gravity is the single point where all of an object's weight can be considered to act. For simple symmetrical shapes it sits at the centre:
For a ring the centre of gravity is at its centre — a point with no material in it.
An object is more stable when it has a low centre of gravity and a wide base. It topples once its centre of gravity passes outside the base — that is when its weight begins to have a turning effect that tips it over.
Quick check
Which is most stable?
?A racing car designer wants the car to be as hard to tip over as possible. Which design is best?
Sort it
Spot the idea
Tap the law or idea that best matches each statement.
Sort it
Mass or weight?
Tap a card, then tap the box it belongs in.
⚖️ Mass (kg)
⬇️ Weight (N)
Recap
The equations to know
Resultant force: add one-dimensional forces with + and − signs
Newton's second law: F = m a
Weight: W = m g (g = 10 N/kg)
Free fall: g = 10 m/s²
Hooke's law: F = k e (graph gradient = k)
Moment: moment = F × perpendicular distance
Principle of Moments: clockwise moments = anticlockwise moments
You've covered all of CCEA 1.2 Force — resultant forces, Newton's three ideas, mass and weight, free fall, Hooke's law, moments, the Principle of Moments and centre of gravity. Press Finish to see your score.
🏆
Mini-lesson complete!
⭐⭐⭐
You've worked through Forces for CCEA GCSE Physics. 🎉
Your stars: 0 / 0
Next: test yourself in the Evaluate stage Confidence Quiz, then lock it in with Verify.
📣 Smashed it? Share your score
Challenge a mate to beat your stars, or show a parent how you got on.