Newton's laws of motion: the first law and resultant force, the second law F = ma and inertial mass, and the third law of equal and opposite forces.

What this dot point is asking

Edexcel statements 2.10 to 2.18 cover Newton's three laws: the first law and the idea of a resultant force, the second law $F = ma$ (with the core practical investigating force, mass and acceleration), inertia and inertial mass, and the third law of equal and opposite forces.

Newton's first law

The first law tells you that a force is not needed to keep something moving, only to change its motion. A spacecraft far from any star coasts at constant velocity forever because no resultant force acts. On Earth, moving objects slow down only because friction and air resistance provide a resultant force. The tendency of an object to keep its state of motion is called inertia.

Newton's second law

The second law makes the first law quantitative: the resultant force equals mass times acceleration. So the same force gives a heavier object a smaller acceleration, which is why a loaded lorry pulls away more slowly than an empty car under the same driving force. It is always the resultant (net) force that goes into $F = ma$, so combine all the forces first.

The Edexcel core practical for this topic investigates the relationship between force, mass and acceleration, usually by pulling a trolley with masses on a hanging string over a pulley and timing it through light gates. It confirms that acceleration is proportional to the resultant force (for fixed mass) and inversely proportional to the mass (for fixed force).

Newton's third law

The third-law pair is often misunderstood. When you push on a wall, the wall pushes back on you with an equal and opposite force; when a swimmer pushes water backwards, the water pushes the swimmer forwards. Because the two forces act on different objects, they do not cancel out on a single object, which is why motion is still possible. A common exam trap is to confuse a third-law pair with two balanced forces on the same object: balanced forces (such as weight and the normal contact force on a book resting on a table) act on the one object and happen to be equal, whereas a third-law pair always acts on two different objects and is equal by the law itself.

How Edexcel examines this

Newton's laws run right through Topic 2 and appear on both tiers. The first and third laws are usually tested as short explanation questions, where the mark scheme demands precise wording: the first law must mention a constant velocity (or rest) and a resultant force, and a third-law answer must state that the forces are equal, opposite and act on different objects. The second law is the calculation workhorse, set as a two to four mark problem using $F = ma$, often combined with the equations of motion so that you first find an acceleration and then a velocity or distance. Edexcel also examines the core practical, asking you to describe how varying the masses on a trolley shows that acceleration is proportional to force and inversely proportional to mass, and to identify the independent, dependent and control variables. Always feed the resultant force into $F = ma$, combining any forces that act along the same line first.

Try this

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

• Cue. $F = ma$.

Q2. A resultant force of $40\,\text{N}$ acts on a $5\,\text{kg}$ box. Calculate its acceleration. [2 marks]

• Cue. $a = \dfrac{F}{m} = \dfrac{40}{5} = 8\,\text{m/s}^2$.