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AQA GCSE Physics · Topic 4.5 Forces · Combined and Triple
Balanced and Unbalanced Forces, Resultant Force and Weight
In this lesson you'll learn to: sort forces into contact and non-contact; read a force diagram and
decide whether the forces are balanced or unbalanced; calculate the resultant force and its direction;
and calculate weight with W = m g, from a mass in kilograms or in grams.
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Prefer to watch? Scroll to the bottom for the video on this topic
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A video walkthrough is coming soon — it will appear at the bottom of this page
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Start here — have a play
Don't read anything yet. Work through the six force diagrams: balanced or unbalanced, and what's the resultant force? Then push a box from both sides and see what an unbalanced force does to it, and finish with some weight calculations.
1 · Are the forces balanced?
2 · Work out the resultant force
N
Forces on the box
smooth floor: no friction
Use g =
Calculate the weight
N
Resultant force: add forces in the same direction, subtract forces in opposite directions. Zero resultant = balanced: the object stays still or keeps moving at a steady velocity. Not zero = unbalanced: the velocity changes (speeds up, slows down or changes direction). Weight: W = m g, with m in kg.
Physics With Kate · physicswithkate.com — GCSE & International GCSE Physics
Balanced doesn't mean stopped: it means the velocity doesn't change.
Resultant zero? Nothing changes. Not zero? The velocity changes.
And weight = mass × gravitational field strength, with the mass in kilograms. The notes show you how to set it out.
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Revision notes
Contact and non-contact forces
A force is a push or a pull that acts on an object because it interacts with another object. Force is a vector: it has a size and a direction, so we draw it as an arrow. The length shows the size; the way it points shows the direction.
| Contact forces (the objects touch) | Non-contact forces (the objects are apart) |
|---|---|
| normal contact force (reaction) | weight (gravitational force) |
| friction | magnetic force |
| air resistance and drag | electrostatic force |
| tension, thrust, upthrust |
Resultant force
⭐ Learn this
The resultant force is the single force that has the same effect as all the forces acting together.
Forces in the same direction: add them. Forces in opposite directions: subtract them, and the resultant points the way of the bigger one.
Balanced or unbalanced?
| Balanced | Unbalanced | |
|---|---|---|
| Resultant force | 0 N | not 0 N |
| If it’s still | it stays still | it starts to move |
| If it’s moving | it carries on at a steady velocity (same speed, same direction) | its velocity changes: it speeds up, slows down or changes direction |
Weight
⭐ The equation
weight = mass × gravitational field strength W = m g
W in newtons (N), m in kilograms (kg), g in newtons per kilogram (N/kg). On Earth g ≈ 10 N/kg (use 9.8 N/kg if the question gives it).
- Mass is the amount of matter in an object. It’s a scalar, in kg, and it’s the same everywhere.
- Weight is the force of gravity on that mass. It’s a vector, in N, acting downwards through the object’s centre of mass. It changes if g changes: you weigh less on the Moon.
- Weight and mass are directly proportional: double the mass, double the weight. Weight is measured with a calibrated spring balance (a newtonmeter).
⚠ Trap — grams
W = m g needs the mass in kilograms. Divide grams by 1000 first: 250 g = 0.25 kg, so W = 0.25 × 10 = 2.5 N, not 2500 N.
The answer that gets the marks
The forces are balanced / unbalanced because the resultant force is
___ N (direction), so the object stays still or moves at a steady velocity /
speeds up / slows down / changes direction.
decide size and direction what happens to the velocity
🗣 Words to use in a written answer
Tick each one off as you use it.
How to write it in the exam
A boat has a thrust of 900 N forwards and a drag of 500 N backwards. Calculate the resultant force on the boat and describe its motion.
Opposite directions, so subtract: 900 − 500 = 400 N forwards. The forces are unbalanced, so the boat speeds up (accelerates forwards).
A skydiver of mass 70 kg has just jumped. Air resistance is 200 N. Calculate the resultant force on the skydiver. (g = 10 N/kg)
W = m g = 70 × 10 = 700 N down. Resultant = 700 − 200 = 500 N downwards, so the skydiver speeds up.
Calculate the weight of a 250 g apple. (g = 10 N/kg)
m = 250 ÷ 1000 = 0.25 kg. W = m g = 0.25 × 10 = 2.5 N
A car moves at a steady 20 m/s along a straight, flat road. The driving force is 1500 N. State the size of the drag on the car and explain your answer. (2 marks)
Drag = 1500 N. The car moves at a constant velocity, so the forces are balanced and the resultant force is zero.
✅ Check your answer
Resultant force: a size and a direction · subtract opposite forces, add forces in the same direction · mass in kg before W = m g · weight in N · say what happens to the velocity.
📘 Now do it in your workbook
Six force diagrams: label the forces, tick balanced or unbalanced and contact or non-contact, calculate the weight, then the resultant force with its direction.
AQA GCSE Physics Workbook · FORCES · page 4✅ Can you do it? Tick as you go
0% complete🎉 Nice work! You've ticked off every objective for this topic.
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Prefer to watch? Here's the whole thing
Video walkthrough — coming soon
Everything above, explained out loud — useful for a last-minute recap, or if you'd rather hear it than read it.
Everything above, explained out loud — useful for a last-minute recap, or if you'd rather hear it than read it. The video is on its way.
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Video coming soon
The walkthrough for this lesson is being filmed and will appear right here. Until then, the notes and the free workbook page above cover every mark.
Now put it into practice
Page 4 of the Forces workbook is free: six force diagrams to label, decide and calculate.
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