ForcesEdexcel GCSE Physics: Revision notes
Section 1
What is Newton's First Law?
Newton's first law: if the resultant (overall) force on a body is zero, it either stays at rest or moves at a constant velocity. If the resultant force is not zero, the object's speed and/or direction changes.
This means a moving object doesn't need a force to keep moving at constant velocity — it needs a resultant force to change how it moves.
Students often think a constant force is needed to keep something moving at a steady speed — in fact a zero resultant force is exactly what allows constant velocity.
Section 2
What is Newton's Second Law and how is weight calculated?
Newton's second law: F = m × a (force in N, mass in kg, acceleration in m/s²). A larger resultant force or a smaller mass produces a greater acceleration.
Weight is the force of gravity on an object's mass: W = m × g. Weight is measured using a newtonmeter (spring balance) that reads in newtons. Weight depends on the local gravitational field strength (g), so the same object weighs less on the Moon than on Earth, even though its mass is unchanged.
Core Practical: the relationship between force, mass and acceleration is investigated using trolleys — varying the mass added to the trolley while keeping the accelerating force constant (or vice versa), and measuring the resulting acceleration, e.g. using light gates.
Always keep mass in kilograms in F = m × a calculations — a common mark is lost by not converting grams to kg.
Section 3
What is inertial mass and what causes circular motion?
Inertial mass is a measure of how difficult it is to change the velocity of an object. It is defined as the ratio of force to acceleration (mass = F ÷ a).
An object moving in a circle at constant speed still has a changing velocity, because its direction is constantly changing even though its speed is not. This changing velocity means there must be an acceleration, and therefore a resultant force.
This resultant force is called the centripetal force — it acts towards the centre of the circle, constantly pulling the object off its straight-line path and around the curve.
Think of a ball on a string swung in a circle — the string provides the centripetal force pulling the ball towards your hand at the centre.
Section 4
What is Newton's Third Law?
Newton's third law: when two objects interact, they exert equal and opposite forces on each other. These forces act on different objects, are the same type of force, and act along the same line.
In equilibrium situations (e.g. a book resting on a table), the weight of the book and the normal contact force from the table are NOT a Newton's third law pair — they act on the same object. The true third law pair is the book pushing down on the table, and the table pushing up on the book.
In collisions, Newton's third law connects directly to conservation of momentum: the equal and opposite forces act for the same time on each object, producing equal and opposite changes in momentum, so total momentum is conserved.
A common error is pairing a weight force with a normal contact force as a 'third law pair' — true pairs always act on two different objects and are the same type of force.
Must Know
- Newton's first law: zero resultant force means constant velocity or rest; non-zero resultant force means changing speed/direction
- Newton's second law: F = m × a
- W = m × g, where g is gravitational field strength (N/kg)
- Inertial mass = F ÷ a, a measure of resistance to changing velocity
- Circular motion at constant speed still has changing velocity, requiring a centripetal force towards the centre
- Newton's third law: equal and opposite forces act on two different interacting objects, and underpins conservation of momentum in collisions
That's the notes covered.
Carry on to the next subtopic.