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How do Newton's three laws connect force, mass, and the motion of an object?

State and apply Newton's three laws of motion, including the relationship F equals ma

A focused answer to the O-Level Physics outcome on Newton's laws. Inertia and the first law, the F equals ma relationship in the second law, action-reaction pairs in the third law, and worked calculations.

Generated by Claude Opus 4.88 min answer

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

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  1. What this dot point is asking
  2. The answer
  3. Examples in context
  4. Try this

What this dot point is asking

SEAB wants you to state Newton's three laws of motion, to understand inertia, to apply F=maF = ma in calculations, and to identify action-reaction pairs. The central idea is that a resultant force changes motion: with no resultant force an object keeps doing what it was doing, and with a resultant force it accelerates in proportion.

The answer

Newton's first law (inertia)

An object stays at rest, or keeps moving at constant velocity in a straight line, unless a resultant force acts on it. The reluctance of an object to change its motion is called inertia, and inertia increases with mass. This is why a passenger lurches forward when a bus brakes: their body tends to keep moving.

Newton's second law

The resultant force on an object equals its mass times its acceleration:

F=maF = ma

Force is in newtons, mass in kilograms, acceleration in m s−2\text{m s}^{-2}. For a fixed force a larger mass gives a smaller acceleration. The acceleration is always in the same direction as the resultant force.

Newton's third law

When object A pushes or pulls object B, object B pushes or pulls A with a force that is equal in size and opposite in direction. These action-reaction forces:

  • are always equal in size and opposite in direction,
  • act on two different objects (so they never cancel on one object),
  • are the same type of force (both contact, or both gravitational).

Putting the laws together

The first law is the special case of the second when F=0F = 0 (then a=0a = 0, so velocity is constant). The second law lets you calculate motion from forces. The third law explains how objects push off each other, from walking to rockets.

Examples in context

Example 1. A rocket lifting off. A rocket pushes hot gas downward, and by Newton's third law the gas pushes the rocket upward with an equal and opposite force. When this upward thrust exceeds the rocket's weight, the resultant force is upward and, by the second law, the rocket accelerates up.

Example 2. Seatbelts and inertia. In a sudden stop a passenger's body tends to keep moving forward because of its inertia (first law). The seatbelt provides the backward resultant force that decelerates the passenger safely, an everyday use of the second law to control acceleration.

Try this

Q1. State Newton's first law of motion. [2 marks]

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

Q2. A resultant force of 20 N20\ \text{N} acts on a 4.0 kg4.0\ \text{kg} mass. Calculate the acceleration. [2 marks]

  • Cue. a=Fm=204.0=5.0 m s−2a = \dfrac{F}{m} = \dfrac{20}{4.0} = 5.0\ \text{m s}^{-2}.

Q3. A person stands on the ground. Identify the action-reaction pair involving the person's feet and the ground. [2 marks]

  • Cue. The feet push down on the ground; the ground pushes up on the feet with an equal and opposite force.

Exam-style practice questions

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

Original4 marksA trolley of mass 3.0 kg3.0\ \text{kg} is pushed with a resultant force of 12 N12\ \text{N} along a smooth floor. (a) Calculate its acceleration. (b) If the same force acts on a 6.0 kg6.0\ \text{kg} trolley, state and explain what happens to the acceleration.
Show worked answer →

(a) Newton's second law: a=Fm=123.0=4.0 m s−2a = \dfrac{F}{m} = \dfrac{12}{3.0} = 4.0\ \text{m s}^{-2}.

(b) For the same force, a=126.0=2.0 m s−2a = \dfrac{12}{6.0} = 2.0\ \text{m s}^{-2}. Doubling the mass halves the acceleration, because acceleration is inversely proportional to mass for a fixed force.

Markers reward a=F/ma = F/m, the correct value with units, and the explanation that for a fixed force, doubling mass halves the acceleration.

Original4 marks(a) State Newton's third law of motion. (b) A swimmer pushes backward on the water with her hands. Use Newton's third law to explain how this moves her forward.
Show worked answer →

(a) Newton's third law: when object A exerts a force on object B, object B exerts a force on A that is equal in size and opposite in direction.

(b) The swimmer's hands push the water backward (the action). By Newton's third law the water pushes her hands, and so her body, forward with an equal and opposite force (the reaction), and this forward force drives her through the water.

Markers reward the correct statement of the third law, identifying the action (hands push water back) and the reaction (water pushes swimmer forward), and noting the forces are equal and opposite on different objects.

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