Movement and Interactions: Forces and Motion — AQA Combined Science: Synergy
This topic covers speed and velocity, motion, Newton's laws, kinetic energy and stopping distances.
Speed and velocity
- Speed is scalar; velocity is speed in a given direction (a vector).
- Typical speeds: walking ~1.5 m/s, running ~3 m/s, cycling ~6 m/s.
$$\text{speed} = \frac{\text{distance}}{\text{time}}$$
Distance, speed and time
On a distance–time graph the gradient is the speed (a curve means changing speed). On a velocity–time graph the gradient is the acceleration and the area under the line is the distance travelled.
Acceleration: $a = \dfrac{\Delta v}{t}$, and for uniform acceleration $v^2 - u^2 = 2as$.
Circular motion (Higher Tier)
An object moving in a circle at constant speed has a changing velocity because its direction changes, so it is accelerating. This requires a centripetal force directed towards the centre.
Free fall
Near Earth, objects accelerate at about 9.8 m/s² due to gravity. A falling object reaches terminal velocity when air resistance equals its weight (resultant force zero), so it falls at constant speed.
Newton's laws of motion
- First law: an object stays at rest or at constant velocity unless acted on by a resultant force.
- Second law: $F = m \times a$ — acceleration is proportional to force and inversely proportional to mass. Inertial mass measures how hard it is to change velocity.
- Third law: interacting objects exert equal and opposite forces on each other.
Momentum (Higher Tier)
$$p = m \times v$$ In a closed system, total momentum is conserved in collisions and explosions (total before = total after).
Kinetic energy
$$E_k = \tfrac{1}{2} m v^2$$ A moving object's kinetic energy depends on its mass and the square of its speed.
Stopping distances
Stopping distance = thinking distance + braking distance.
- Thinking distance depends on reaction time (increased by tiredness, alcohol, drugs, distractions).
- Braking distance depends on speed and road/vehicle conditions (increases sharply with speed). Hard braking transfers large amounts of kinetic energy to the brakes, which heat up.
Exam tips
- Velocity–time graph: gradient = acceleration, area = distance.
- Learn F = ma, E_k = ½mv² and a = Δv/t.
- Explain terminal velocity using balanced forces.
- Link thinking distance to reaction time and braking distance to speed/conditions.