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What are Newton's three laws, and how do resultant force, mass and weight determine how an object moves?

Newton's three laws of motion, resultant force, the difference between mass and weight, the equations for weight and resultant force, friction and drag, and terminal velocity.

A focused answer to OCR Gateway GCSE Physics A topic P2 on Newton's laws, covering the three laws of motion, resultant force, the difference between mass and weight, the equations for weight and force, friction and drag, and terminal velocity.

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  1. What this topic is asking
  2. Newton's three laws
  3. Resultant force
  4. Mass and weight
  5. Friction, drag and terminal velocity
  6. Try this

What this topic is asking

OCR wants you to state and apply Newton's three laws, calculate resultant force and acceleration, distinguish mass from weight, and explain friction, drag and terminal velocity. This is topic P2.2 of the OCR Gateway Physics A (J249) specification.

Newton's three laws

You must recall F=maF = ma. The first law explains why a moving object at constant velocity has balanced forces (zero resultant), and why you need a resultant force to start, stop, speed up, slow down or change direction. The third-law pairs act on different objects (for example, a swimmer pushes the water backwards and the water pushes the swimmer forwards).

Resultant force

OCR often gives a free-body diagram and asks for the resultant. For example, a 500 N500\,\text{N} driving force forward and a 200 N200\,\text{N} resistive force backward give a resultant of 500−200=300 N500 - 200 = 300\,\text{N} forward, which then accelerates the object by F=maF = ma.

Mass and weight

So a 2 kg2\,\text{kg} bag has a weight of W=mg=2×10=20 NW = mg = 2 \times 10 = 20\,\text{N} on Earth. On the Moon, where gg is about 1.6 N/kg1.6\,\text{N/kg}, its mass is still 2 kg2\,\text{kg} but its weight is only about 3.2 N3.2\,\text{N}. Weight is measured with a force meter (newtonmeter) and is directly proportional to mass.

Friction, drag and terminal velocity

Friction and air resistance (drag) act against the direction of motion. For a falling object, drag increases with speed. A falling object speeds up while its weight is greater than the drag; as it goes faster the drag rises until it equals the weight, the resultant force becomes zero, and the object falls at a constant terminal velocity by Newton's first law.

Try this

Q1. Calculate the weight of a 60 kg60\,\text{kg} student on Earth, where g=10 N/kgg = 10\,\text{N/kg}. [2 marks]

  • Cue. W=mg=60×10=600 NW = mg = 60 \times 10 = 600\,\text{N}.

Q2. State Newton's first law of motion. [1 mark]

  • Cue. An object stays at rest or moves at constant velocity unless a resultant force acts on it.

Exam-style practice questions

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

OCR 20183 marksA car of mass 1200 kg1200\,\text{kg} has a resultant forward force of 3600 N3600\,\text{N}. Calculate its acceleration, and state which of Newton's laws you have used.
Show worked answer →

A P2 Calculate question on the recall equation F=maF = ma. Rearrange for acceleration: a=Fm=36001200=3 m/s2a = \dfrac{F}{m} = \dfrac{3600}{1200} = 3\,\text{m/s}^2 (2 marks for the rearrangement and answer with units). This uses Newton's second law, which states that the resultant force equals mass times acceleration (1 mark). Markers reward the correct rearrangement, the answer in m/s2\text{m/s}^2, and naming Newton's second law. A common error is to multiply instead of divide.

OCR 20224 marksA skydiver falls from a plane and eventually reaches terminal velocity. Explain, in terms of the forces acting, why the skydiver speeds up at first and then falls at a constant terminal velocity.
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A P2 Explain question worth four marks linking forces to motion. At first the skydiver's weight is greater than the air resistance, so there is a resultant force downwards and the skydiver accelerates (1 mark). As the speed increases, the air resistance increases (1 mark). Eventually the air resistance grows until it is equal to the weight, so the resultant force is zero (1 mark). By Newton's first law, with balanced forces the skydiver now falls at a constant speed, the terminal velocity (1 mark). Markers reward weight greater than drag at the start, drag increasing with speed, balanced forces, and constant velocity by Newton's first law.

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