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Balanced and Unbalanced

Using force diagrams to see when forces cancel out or cause acceleration.

  • Define and explain Balanced and Unbalanced in your own words
  • Use key terms such as resultant force accurately
  • Apply what you have learned to new examples and questions
  • Avoid the common mistakes learners make with this topic

This lesson focuses on Balanced and Unbalanced: using force diagrams to see when forces cancel out or cause acceleration.

Definition: Balanced and Unbalanced

Using force diagrams to see when forces cancel out or cause acceleration.

Key ideas

Balanced forces mean no change in motion

When forces are equal and opposite they cancel, giving a resultant force of zero. A book resting on a table and a car cruising at steady speed both have balanced forces. Unbalanced forces leave a non-zero resultant, which accelerates the object in that direction — this is the heart of Newton's second law.

Forces are pushes, pulls and twists

Forces act between objects and are measured in newtons. Contact forces need touching — friction, tension and the normal push of a surface. Non-contact forces act at a distance — gravity, magnetism and static electricity. Drawing them as arrows shows both size and direction at a glance.

Key term — resultant force: The single force that has the same effect as all the forces acting on an object combined.

Worked example: Balanced and Unbalanced

Two teams pull a rope with 200 N left and 200 N right. What is the resultant force?

0 N — the forces are balanced, so the rope does not accelerate.

Answer: 0 N — the forces are balanced, so the rope does not accelerate.

Common mistakes
  • Saying balanced forces mean the object is stationary Balanced forces also allow steady-speed motion — a car at a constant speed has balanced forces.
  • Using kilograms for weight Weight is a force, so it is measured in newtons; kilograms measure mass.

Practice

A skydiver falls at a steady speed. What does this tell you about the forces?
Steady speed means no acceleration.

The forces are balanced — air resistance upwards equals weight downwards, so the resultant force is zero.

A car needs a bigger engine force to keep a constant speed uphill than on flat ground. Why?
Think about the extra force acting downhill.

Uphill, a component of the car's weight pulls it backwards down the slope, so the engine must supply extra force to balance it and keep the resultant at zero.

Is magnetism a contact or non-contact force?
Does a magnet need to touch the metal?

Non-contact — a magnet pulls iron without touching it.

Would your mass change on the Moon? Would your weight?
Mass is the amount of matter.

Mass stays the same; weight drops to about one-sixth because the Moon's gravity is weaker.

Quick check

Balanced and Unbalanced — quick check

Which of these best defines "resultant force"?

The single force that has the same effect as all the forces acting on an object combined.

A 5 kg bag has what weight on Earth? (g = 10 N/kg)

W = m × g = 5 × 10 = 50 N.
Key takeaways
  • Balanced and Unbalanced: using force diagrams to see when forces cancel out or cause acceleration.
  • Balanced forces mean no change in motion: When forces are equal and opposite they cancel, giving a resultant force of zero.
  • newton: The unit of force, symbol N.
  • Watch out for: saying balanced forces mean the object is stationary
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