Forces & motion

1. Distinguish scalars and vectors

   A scalar has size only - distance, speed, mass, energy. A vector has size and direction - displacement, velocity, force, acceleration. The difference matters: two 5 N forces give 10 N if they push the same way but zero if they push opposite ways. Vectors are often drawn as arrows whose length shows the size.

2. Find the resultant of forces and calculate weight

   The resultant force is the single force that has the same effect as all the forces combined. Forces in a line add or subtract by direction. Weight is the force of gravity on a mass: W = mg, with g about 9.8 N/kg, so a 10 kg bag weighs about 98 N. Balanced forces mean no change in motion.

3. Apply Newton's three laws of motion

   First law: with no resultant force, an object stays still or keeps moving at steady speed. Second law: a resultant force causes acceleration, F = ma, so 20 N on a 4 kg mass gives 5 m/s². Third law: every force has an equal and opposite reaction. Together they explain how and why objects move.

4. Calculate speed, velocity and acceleration

   Speed is distance ÷ time; velocity is speed in a stated direction. Acceleration is how fast velocity changes: a = (change in velocity) ÷ time. A car going from 0 to 20 m/s in 8 s accelerates at 2.5 m/s². Steady speed in a circle still counts as acceleration because the direction keeps changing.

5. Interpret distance–time and velocity–time graphs

   On a distance–time graph the gradient is speed - steeper means faster, flat means stopped. On a velocity–time graph the gradient is acceleration and the area under the line is the distance travelled. Reading the right quantity from the right graph is a skill worth practising until it is automatic.

6. Explain stopping distance and the factors affecting it

   Stopping distance is thinking distance plus braking distance. Thinking distance grows with speed and with slow reactions - tiredness, alcohol, distraction. Braking distance grows with speed (and rises steeply, roughly with speed squared) and with wet or icy roads, poor tyres or worn brakes. Both parts add up to how far a car needs to stop.

Scalar quantity

A quantity with magnitude (size) only, such as speed, distance or mass.

Vector quantity

A quantity with both magnitude and direction, such as velocity, displacement, force and acceleration.

Weight

The force of gravity on an object: weight = mass × gravitational field strength (W = mg). Measured in newtons.

Resultant force

The single force that would have the same effect as all the forces acting on an object together.

Acceleration

The rate of change of velocity, measured in m/s². A resultant force is what causes it: F = ma.

Terminal velocity

The constant top speed of a falling object, reached when air resistance has grown to balance its weight so the resultant force is zero.

MomentumHigher only

Mass in motion: momentum = mass × velocity (p = mv). In a closed system, total momentum is the same before and after a collision.

InertiaHigher only

The tendency of an object to stay at rest or keep moving at the same velocity unless a resultant force acts on it.

More Physics topics

• Energy
• Electricity
• Particle model of matter
• Atomic structure & radioactivity
• Waves
• Magnetism & electromagnetism

Keep reading

• What's on the GCSE Combined Science specification?
• What are the GCSE Combined Science grade boundaries?
• The GCSE Combined Science syllabus, explained