How do Newton's laws relate force, mass and acceleration, and what is terminal velocity?
Newton's laws of motion, the equation force equals mass times acceleration, weight, balanced forces and terminal velocity.
A focused answer to WJEC GCSE Physics topic 2.2 on Newton's laws, covering balanced and unbalanced forces, the equation force equals mass times acceleration, the difference between mass and weight, inertia, and terminal velocity.
Reviewed by: AI editorial process; not yet individually human-reviewed
Have a quick question? Jump to the Q&A page
Jump to a section
What this topic is asking
WJEC wants you to use Newton's laws to relate force, mass and acceleration, distinguish mass from weight, and explain terminal velocity. This is topic 2.2 Newton's laws in Unit 2 of WJEC GCSE Physics (3420).
Newton's three laws
The first law explains inertia: the tendency of an object to keep doing what it is doing. A heavier object has more inertia and is harder to start or stop moving. The third law explains how we walk (your foot pushes the ground back, the ground pushes you forward) and how rockets work. The two forces in a third-law pair always act on different objects, are the same type of force, and are equal in size but opposite in direction; this is why they never cancel out to leave an object motionless.
A resultant force is what is left after combining every force acting on an object, taking direction into account. If the upward and downward forces are equal and the leftward and rightward forces are equal, the resultant is zero and the first law applies: the object stays still or carries on at constant velocity. WJEC often gives a free-body diagram and asks for the resultant force before any acceleration calculation, so always combine the forces first.
Force, mass and acceleration
Mass and weight
A falling object speeds up because its weight gives it a resultant downward force, so by it accelerates. Near the Earth's surface, ignoring air resistance, every object accelerates at the same rate, the acceleration due to gravity, because a larger mass has both a larger weight and a larger inertia, and the two effects cancel. Once air resistance is included, the object reaches terminal velocity: as it speeds up the air resistance grows until it equals the weight, the resultant force becomes zero, and the object falls at a steady speed. This is why a parachute, which greatly increases air resistance, lowers the terminal velocity to a safe value.
Try this
Q1. Calculate the weight of a student on Earth (). [2 marks]
- Cue. .
Q2. State Newton's first law of motion. [1 mark]
- Cue. An object stays at rest or moves at constant velocity unless acted on by a resultant force.
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 20183 marksA force of acts on a car of mass . Calculate its acceleration.Show worked answer →
A topic 2.2 Calculate question. Use , rearranged to (1 mark). Substitute and : (1 mark) (1 mark for the answer with units). Markers reward the rearrangement, the substitution and the unit. A common error is to multiply force by mass instead of dividing.
WJEC 20224 marksExplain, in terms of the forces acting, why a skydiver reaches a terminal velocity.Show worked answer →
A topic 2.2 Explain question. At first the skydiver's weight is greater than air resistance, so there is a resultant downward force and they accelerate (1 mark). As they speed up, air resistance increases (1 mark). Eventually air resistance equals the weight, so the resultant force is zero (1 mark) and they fall at a constant terminal velocity with no further acceleration (1 mark). Markers reward the growing air resistance, balanced forces and constant velocity. A common error is to say the forces become zero rather than balanced.
Related dot points
- Speed, velocity and acceleration, the equations of motion, and reading distance-time and velocity-time graphs.
A focused answer to WJEC GCSE Physics topic 2.1 on distance, speed and acceleration, covering the difference between speed and velocity, the acceleration equation, the equation of motion, and how to read distance-time and velocity-time graphs.
- Work done, power, kinetic and gravitational potential energy, energy conservation, and Hooke's law for springs.
A focused answer to WJEC GCSE Physics topic 2.3 on work and energy, covering work done, power, kinetic and gravitational potential energy, the conservation of energy, and Hooke's law for the extension of a spring.
- Momentum, the equation momentum equals mass times velocity, and conservation of momentum in collisions and explosions.
A focused answer to WJEC GCSE Physics topic 2.4 on momentum, covering the momentum equation, the conservation of momentum, how momentum applies to collisions and explosions, and the link between force, change in momentum and safety features.
- Thinking distance, braking distance and stopping distance, the factors that affect them, and how vehicle safety features work.
A focused answer to WJEC GCSE Physics on stopping distances, covering thinking distance, braking distance, total stopping distance, the factors that affect each, the energy transfer in braking, and how crumple zones, airbags and seatbelts reduce injury.
Sources & how we know this
- WJEC GCSE Physics specification (3420) from 2016 — WJEC (2016)