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How do forces affect motion, and what does F = ma tell us?

Balanced and unbalanced (resultant) forces, Newton's laws of motion, and the equation force equals mass times acceleration.

A focused answer to the WJEC GCSE Science Double Award Unit 6 topic on forces, covering balanced and unbalanced (resultant) forces, Newton's laws of motion, and the equation force equals mass times acceleration.

Generated by Claude Opus 4.89 min answer

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  1. What this dot point is asking
  2. Balanced and unbalanced forces
  3. Newton's laws of motion
  4. The equation F = ma
  5. Forces on a moving vehicle
  6. Weight and mass
  7. Terminal velocity
  8. Try this

What this dot point is asking

WJEC Double Award Unit 6 wants you to describe balanced and unbalanced (resultant) forces, state Newton's laws of motion, and use the equation F=maF = ma.

Balanced and unbalanced forces

When forces are balanced (resultant zero), an object stays stationary or keeps moving at a constant velocity. When forces are unbalanced, the object accelerates (speeds up, slows down or changes direction) in the direction of the resultant force.

Newton's laws of motion

The equation F = ma

A bigger force gives a bigger acceleration; a bigger mass gives a smaller acceleration for the same force. This is why a loaded lorry accelerates more slowly than an empty one.

Forces on a moving vehicle

A car moving along has a driving force forwards and resistive forces (friction and air resistance) backwards. At a steady speed, these are balanced (resultant zero). If the driving force increases, the forces become unbalanced and the car accelerates; if the resistive forces become larger than the driving force, it decelerates. This is a common way exam questions test the link between forces and motion.

Weight and mass

It is important not to confuse mass and weight. Mass is the amount of matter in an object, measured in kilograms (kg), and it stays the same everywhere. Weight is the force of gravity on the object, measured in newtons (N), and it depends on the gravitational field strength: weight=mg\text{weight} = mg. So a 10 kg object has a weight of about 100 N on Earth (g = 10 N/kg) but less on the Moon, where gravity is weaker, even though its mass is unchanged. Keeping mass (kg) and weight (N) separate is a common exam point.

Terminal velocity

When an object falls, its weight pulls it down while air resistance pushes up. As it speeds up, the air resistance increases, until it equals the weight. The forces are then balanced, the resultant force is zero, and the object stops accelerating - it falls at a steady terminal velocity. A skydiver reaches terminal velocity, and a parachute increases the air resistance so the terminal velocity becomes much lower, allowing a safe landing. This is a good example of balanced and unbalanced forces in action.

Try this

Q1. What is the unit of force? [1 mark]

  • Cue. The newton (N).

Q2. A 10 kg object accelerates at 3 m/s squared. What resultant force acts on it? [2 marks]

  • Cue. F=ma=10×3=30 NF = ma = 10 \times 3 = 30\,\text{N}.

Exam-style practice questions

Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

WJEC style3 marksA force of 200 N acts on a mass of 40 kg. Calculate the acceleration.
Show worked answer →

A Unit 6 calculation. Use F=maF = ma, rearranged to a=Fma = \dfrac{F}{m} (1 mark). Substitute: a=20040a = \dfrac{200}{40} (1 mark) =5 m/s2= 5\,\text{m/s}^2 (1 mark). Markers reward the rearranged equation, the substitution and the unit. A common error is to multiply force and mass.

WJEC style4 marksExplain, using forces, why a car travelling at a steady speed has balanced forces, and what happens when the driving force increases.
Show worked answer →

A Unit 6 explain question worth 4 marks. Reward: at a steady speed, the driving force equals the resistive forces (friction and air resistance), so the forces are balanced and the resultant force is zero (2); when the driving force increases, there is now an unbalanced (resultant) force forwards (1), so the car accelerates (speeds up) (1). Markers credit balanced forces at steady speed and an unbalanced force causing acceleration. A common error is to say a constant force gives a constant speed.

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