You are viewing a free preview of this lesson.
Subscribe to unlock all 10 lessons in this course and every other course on LearningBro.
Newton's laws of motion are the foundation of classical mechanics. They explain why objects move the way they do, turning the descriptive language of kinematics (displacement, velocity, acceleration) into a predictive theory of dynamics. This lesson opens OCR A-Level Physics A Module 3.5 (Newton's laws of motion and momentum) with the First Law — the deceptively simple statement that objects keep doing what they are already doing unless something forces them to change.
The First Law is not really a "law" in the modern sense; it is a definition of an inertial frame of reference and a statement that inertia is a real property of matter. Once you understand it properly, the Second and Third Laws (and the whole of mechanics) snap into place.
An object will remain at rest, or continue to move with constant velocity, unless acted upon by a resultant (net) external force.
Three key ideas are packed into this one sentence:
Inertia is the tendency of an object to resist changes to its state of motion. More massive objects have more inertia and therefore are harder to start, stop, or turn. Mass itself is often defined as a quantitative measure of inertia — specifically, the inertial mass m that appears in F = ma.
In everyday life inertia is obvious:
Before Newton, the prevailing view (inherited from Aristotle) was that objects naturally came to rest and that a force was needed to keep them moving. Newton — building on Galileo — inverted this. The reason cars coast to a stop is not that motion is unnatural, but that friction and air resistance are forces acting on them. Remove those forces (as in deep space) and a coasting spacecraft will travel for billions of years in a straight line.
A body is in translational equilibrium when the resultant force acting on it is zero:
ΣF = 0
In this state, by the First Law, the body is either stationary or moving with constant velocity. There is no way to tell the difference by a mechanics experiment done inside the body (this is the Galilean principle of relativity).
To check for equilibrium in two dimensions, resolve all forces into perpendicular components — conventionally horizontal (x) and vertical (y) — and demand that each component sum is zero:
ΣFₓ = 0 and ΣFᵧ = 0
If both are satisfied, the body is in equilibrium.
A lamp of weight W = 25 N hangs from two wires, each making an angle of 30° with the vertical. Find the tension T in each wire.
By symmetry, both tensions are equal. Resolving vertically (upwards positive):
2 T cos 30° − W = 0 T = W / (2 cos 30°) = 25 / (2 × 0.866) = 14.4 N
Resolving horizontally, the two horizontal components (T sin 30° to the left and T sin 30° to the right) automatically cancel. The lamp is in equilibrium — it hangs at rest, in perfect agreement with Newton's First Law.
flowchart LR
A[Object] --> B{Resultant force?}
B -- "ΣF = 0" --> C[Stays at rest OR continues at constant velocity]
B -- "ΣF ≠ 0" --> D[Velocity changes - accelerates]
To apply the First Law you almost always need a free-body diagram: a sketch showing only the forces acting on the body of interest, not the forces it exerts on anything else. Each force is drawn as an arrow starting on the body, with a label for its origin and magnitude.
Typical forces in A-Level questions:
| Symbol | Name | Origin |
|---|---|---|
| W | Weight | Earth's gravity, W = mg, always downwards |
| N or R | Normal contact force | Perpendicular to the surface of contact |
| T | Tension | Along a rope, cable or wire |
| F_f | Friction | Parallel to surface, opposing relative motion |
| F_d | Drag or air resistance | Opposes motion through a fluid |
| U | Upthrust | Buoyancy, upwards from a fluid |
Once the diagram is drawn you can check equilibrium by inspection: if the arrows form a closed polygon (tip-to-tail), the forces balance.
A 2.0 kg textbook sits on a flat horizontal table. Find the normal contact force between the book and the table.
This is an application of the First Law: because the book is in equilibrium, we immediately know that N = W. Students often confuse this with the Third Law (see Lesson 3) — in fact the table also pushes on the Earth through the book, but those are a different action–reaction pair.
A 70 kg person stands in a lift. The lift is moving upwards at a constant 2.0 m s⁻¹. Find the normal contact force from the floor of the lift on the person.
Constant velocity → equilibrium → ΣF = 0.
Students often wrongly guess that the lift being in motion makes a difference. It does not — constant velocity is the same as rest from Newton's point of view. The normal force only changes if the lift accelerates, which we will handle with the Second Law in Lesson 2.
An inertial frame is one in which Newton's First Law holds — where an isolated body moves in a straight line at constant speed. A non-inertial frame is one that is accelerating (for example, a braking bus or a rotating roundabout). Inside a non-inertial frame, objects appear to accelerate even with no real force acting on them, so physicists invent "fictitious" (pseudo) forces like the centrifugal force to make the maths work.
For A-Level OCR you will always work in an inertial frame, typically the laboratory frame or the ground frame. But it is worth knowing why Einstein later generalised the First Law into the foundations of general relativity: free-falling observers in a gravitational field are also "inertial".
OCR mark schemes are strict about the wording of Newton's First Law. Acceptable versions include:
Unacceptable versions that will lose marks:
Lesson 2 introduces Newton's Second Law in its full form, F = Δp/Δt, and shows when the familiar F = ma version is valid. Lesson 3 deals with the Third Law — the source of countless exam marks and countless misconceptions. From there we move into momentum (Lesson 4), impulse (Lesson 5), conservation of momentum (Lesson 6), collisions (Lessons 7–9) and finally explosions and recoil (Lesson 10). Nail the First Law now and the rest of the module will follow.