Newton's laws of motion · GCSE Physics

Newton's first law

If the resultant force acting on an object is zero: a stationary object stays stationary, and a moving object continues to move at the same speed in the same direction (constant velocity).
So a resultant force is needed to change the speed or direction of an object.
(Higher) Inertia is the tendency of an object to continue in its state of rest or of uniform motion.

The acceleration of an object is proportional to the resultant force acting on it, and inversely proportional to its mass.
F = m a: resultant force (N) = mass (kg) × acceleration (m/s²).
(Higher) Inertial mass is a measure of how difficult it is to change the velocity of an object: inertial mass = force ÷ acceleration.

Whenever two objects interact, the forces they exert on each other are equal in size and opposite in direction.
The two forces act on different objects and are the same type of force. Example: when you push on a wall, the wall pushes back on you with an equal force.

Pull a trolley along a bench with a string over a pulley attached to hanging masses. The weight of the hanging masses is the accelerating force.
Measure the acceleration using light gates, or a stopwatch and ruler.
Vary the force by moving masses from the trolley to the hanger (keeping the total mass constant), then vary the mass of the trolley with a constant force.
Acceleration is proportional to force, and inversely proportional to mass.

Newton's first law: An object's velocity stays the same unless a resultant force acts on it.
Newton's second law: Resultant force = mass × acceleration (F = ma).
Newton's third law: When two objects interact, they exert equal and opposite forces on each other.
Inertia: The tendency of an object to stay at rest or keep moving at constant velocity.
Inertial mass: How difficult it is to change an object's velocity: force ÷ acceleration.
Resultant force: The overall force on an object when all the forces are combined.
Light gate: A sensor that measures when an object passes, used to calculate speed and acceleration.