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How do forces change motion, and what do Newton's laws tell us?

Contact and non-contact forces, resultant forces, Newton's first, second and third laws, the equation F = ma, mass and weight, and stopping distances.

A focused answer to Edexcel GCSE Combined Science Topic 2 (CP2), covering contact and non-contact forces, resultant forces, Newton's three laws of motion, the equation F = ma, the difference between mass and weight, and stopping distances.

Generated by Claude Opus 4.89 min answer

Reviewed by: AI editorial process; not yet individually human-reviewed

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  1. What this dot point is asking
  2. Types of force and resultant force
  3. Newton's laws of motion
  4. Mass and weight
  5. Stopping distance
  6. Try this

What this dot point is asking

Edexcel wants you to distinguish contact and non-contact forces, find a resultant force, state and use Newton's three laws including F=maF = ma, distinguish mass and weight, and describe the factors affecting stopping distance.

Types of force and resultant force

The resultant force is the single force that would have the same effect as all the forces acting together. Forces in the same direction add; forces in opposite directions subtract.

Newton's laws of motion

Mass and weight

Mass is the amount of matter in an object (in kgkg) and is the same everywhere. Weight is the force of gravity on an object (in NN) and depends on the gravitational field strength:

W=mgW = mg

where g≈10 N/kgg \approx 10\,N/kg on Earth. So a 5 kg5\,kg bag has a weight of 5×10=50 N5 \times 10 = 50\,N on Earth, but less on the Moon where gg is smaller.

Stopping distance

The stopping distance of a vehicle is the distance travelled from the moment a hazard is seen to the moment the vehicle stops:

stopping distance=thinking distance+braking distance\text{stopping distance} = \text{thinking distance} + \text{braking distance}

The thinking distance (during the driver's reaction time) increases with speed, tiredness, distraction, alcohol or drugs. The braking distance (while the brakes act) increases with speed, a wet or icy road, worn tyres or brakes, and a heavier load. Braking distance increases more steeply than thinking distance as speed rises.

When a car brakes, the brakes do work on the wheels, transferring the car's kinetic energy to the thermal energy store of the brakes, so they get hot. Because kinetic energy depends on the square of the speed, doubling the speed roughly quadruples the braking distance, which is why small increases in speed have a large effect on how far a car needs to stop. A larger braking force can stop a car in a shorter distance, but a very large braking force can make the car skid or the brakes overheat, which is dangerous.

Newton's first law also explains why seatbelts and air bags matter: in a crash the passengers tend to keep moving (their inertia) until a force acts on them, so seatbelts provide that force gradually and reduce injury. Inertia is the tendency of an object to stay at rest or keep moving at constant velocity, and a more massive object has more inertia.

Try this

Q1. State Newton's second law as an equation. [1 mark]

  • Cue. F=maF = ma (resultant force = mass times acceleration).

Q2. Calculate the weight of a 3 kg3\,kg object on Earth (g=10 N/kgg = 10\,N/kg). [2 marks]

  • Cue. W=mg=3×10=30 NW = mg = 3 \times 10 = 30\,N.

Exam-style practice questions

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

Edexcel 20203 marksA car of mass 1200 kg1200\,\text{kg} experiences a resultant forward force of 3600 N3600\,\text{N}. Calculate its acceleration, stating the equation you use.
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A 3-mark calculation using Newton's second law.

Use F=maF = ma, rearranged to a=Fma = \dfrac{F}{m} (1 mark). Substitute: a=36001200a = \dfrac{3600}{1200} (1 mark). So a=3 m/s2a = 3\,\text{m/s}^2 (1 mark).

Markers reward stating and rearranging F=maF = ma, the substitution, and the correct value with units. A common error is to multiply instead of divide.

Edexcel 20224 marksExplain how the thinking distance and the braking distance of a car are each affected, giving one factor that increases each one.
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A 4-mark question on stopping distances.

The thinking distance is how far the car travels during the driver's reaction time; it increases if the driver is tired, distracted, or has taken alcohol or drugs, or if the car is going faster (2 marks). The braking distance is how far the car travels while braking; it increases if the brakes or tyres are worn, the road is wet or icy, or the car is going faster or is more heavily loaded (2 marks).

Markers reward a correct factor for each, and credit faster speed increasing both. The stopping distance is the sum of the thinking and braking distances.

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