Forces and motion · GCSE Physics
Newton’s laws
Teacher-written GCSE Physics revision on Newton’s three laws as distinct ideas: first-law inertia and zero resultant, second-law F = ma, and third-law pairs on two different objects.
Law 1: zero resultant → constant velocity. Law 2: resultant F = ma. Law 3: equal and opposite forces on two different objects. Do not mash them into one sentence.
The important bits
What you need to know
- 1
Newton’s first law: if the resultant force on an object is zero, the object stays at rest or continues at constant velocity (constant speed in a straight line). A resultant is needed to change velocity.
- 2
Newton’s first law is about resultant force, not about “no forces”. A car at steady 70 mph has large driving and resistive forces that cancel.
- 3
Newton’s second law: the acceleration of an object is proportional to the resultant force and inversely proportional to mass. In the form you calculate with, F = ma, with F in N, m in kg, a in m/s².
- 4
In F = ma, F must be the resultant force. If 20 N pulls and 5 N of drag acts, F = 15 N, not 20 N.
- 5
Newton’s third law: when two objects interact, they exert equal and opposite forces of the same type on each other. The pair acts on two different objects, so they do not cancel on one free-body diagram.
- 6
Your feet push the Earth down; the Earth pushes you up (normal contact). Those two are a third-law pair. Weight and normal contact on you are not a third-law pair; they are two different interactions that may happen to balance.
- 7
A larger mass needs a larger resultant force for the same acceleration (more inertia). In space, with no drag, a small resultant still produces acceleration; there is no “need for a force to keep you moving”.
- 8
Terminal velocity is Newton 1 and 2 together: as speed rises, drag rises, resultant falls, a falls, until resultant is zero and velocity stays constant.
Quotations worth analysing
Short evidence. Real method.
“Newton’s first law: an object remains at rest or in uniform motion unless acted on by a resultant force.”
Uniform motion means constant velocity. The law is why you keep moving when the bus stops suddenly — your body wants constant velocity.
“Newton’s second law: F = ma, where F is the resultant force.”
The word resultant is doing the work. Plug in one of several forces and the acceleration will be wrong.
“Newton’s third law: forces always come in equal and opposite pairs acting on different objects.”
Same type, same size, opposite direction, different objects. Weight of a book is Earth on book; the partner is book on Earth, not the table’s normal contact.
Go deeper
Keep the three laws in three separate boxes
Law 1 answers “what if the resultant is zero?” — constant velocity, which includes sitting still. Law 2 answers “what if it is not zero?” — a = F/m, numbers in SI units. Law 3 answers “where does a force come from?” — another object pushing or pulling back with the same type of force. A skydiver at terminal velocity is law 1: weight and drag cancel. A skydiver just after jumping is law 2: weight is bigger than drag, so there is a downward resultant. The skydiver’s weight and the Earth’s equal pull on the skydiver’s mass-partner planet is law 3, which does not appear on the skydiver’s free-body diagram as two arrows. If your paragraph uses the same example for all three laws without saying which is which, the examiner cannot award the distinct marks.
Go deeper
Third-law pairs never sit on the same diagram
A free-body diagram is one object. A third-law pair lives on two diagrams. Book on table: diagram of book has weight (Earth) and normal contact (table). Diagram of table has the book’s downward contact force and the floor’s support. The book’s weight pairs with the book’s gravitational pull on the Earth, which is not drawn on either classroom diagram. This is why “they cancel” is a dangerous sentence: they would cancel only if they acted on the same object. They do not. When a rocket pushes exhaust backwards, the exhaust pushes the rocket forwards — that pair explains thrust in empty space, where there is no air to “push against”. Law 1 then says that once the engine cuts, the rocket keeps its velocity; it does not need air or a continual force to keep going.
See the idea in action
A 1200 kg car has a driving force of 2400 N and resistive forces of 600 N. Resultant F = 2400 − 600 = 1800 N (Newton 2 uses this, not 2400 N). a = F/m = 1800 / 1200 = 1.5 m/s² forwards. If resistive forces later rise to 2400 N, resultant = 0, so velocity becomes constant (Newton 1), even though the engine is still working. The driving force is the road pushing the tyres forwards; the tyres push the road backwards with 2400 N (Newton 3). Those 2400 N on the road do not cancel the 2400 N on the car because they act on different objects.
Exam technique
Turn knowledge into marks
Name which law you are using. For law 1, say resultant force is zero, not “no forces”. For law 2, write resultant F = ma with mass in kg. For law 3, name both objects and the type of force, and refuse to put the pair on one diagram.
Common mistakes
Do not give these marks away
- 01
Saying a car at constant speed has no forces, or using F = ma with only the driving force when drag also acts.
- 02
Putting a Newton-third-law pair on the same object so they “cancel”.
- 03
Blending the three laws into one vague sentence about “equal and opposite forces keep things moving”.
A car travels at constant 30 m/s in a straight line. What is the resultant force on it?
AEqual to its weight
BZero
CEqual to the driving force
DImpossible to tell
Show the answer
Zero. Newton’s first law: constant velocity means the resultant force is zero. Driving force and resistive forces still exist; they cancel.
Quick questions
If this is the bit you searched
What is Newton’s first law?
If the resultant force is zero, an object remains at rest or continues at constant velocity. A resultant force is needed to change speed or direction.
What is Newton’s second law?
Resultant force equals mass times acceleration: F = ma. Acceleration is in the direction of the resultant force.
What is Newton’s third law?
When two objects interact, they exert equal and opposite forces of the same type on each other. The forces act on different objects.
Why does a rocket work in space?
The rocket pushes exhaust backwards; the exhaust pushes the rocket forwards (Newton 3). No air is required. Once the engine stops, Newton 1 says it keeps its velocity.