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1.2Motion, Forces and Energy

Motion — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions

25 min read 51 parts Lesson 8 of 91 1 quiz

📚 Cambridge IGCSE Physics 0625 Hub / Topical Revision
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Lesson summary

1.2 Motion — Notes — IGCSE PHYSICS notes: Define speed, velocity and acceleration, and interpret distance-time and speed-time graphs and free fall.Speed = distance travelled per unit time, a scalarEquation — v = s ÷ t for constant speedUnits — v in metres per second (m/s), s in m…

  • Formula triangle
  • Average speed
  • Variable journeys
  • Typical values
  • What the symbols mean
  • Change in velocity

Part 1 of 8Speed

Speed
Speed distance travelled per unit time, a scalar
Equation
  • Equation v = s ÷ t for constant speed
Units
  • Units v in metres per second (m/s), s in metres (m) and t in seconds (s)
Formula triangle
  • Formula triangle cover the quantity to find

Part 2 of 8Average speed

1Average speed
Average speed total distance travelled ÷ total time taken
2Variable journeys
  • Variable journeys a car speeds up, slows down and stops, so average speed is more useful than instantaneous speed
3Typical values
  • Typical values walking 1.5 m/s, running 3 m/s, cycling 6 m/s, a plane about 250 m/s

Part 3 of 8Velocity

Velocity
Velocity speed in a stated direction, a vector
Equation
  • Equation v = s ÷ t, where s is displacement
Direction
  • Direction it must be stated with the answer
Sign
  • Sign either direction can be positive if the opposite direction is negative and the choice is consistent

Part 4 of 8Acceleration

Acceleration
Acceleration the change in velocity per unit time, the rate of change of velocity
Equation
  • Equation a = Δv ÷ Δt
    What the symbols mean: a = acceleration (m s−2) · v = final velocity · u = initial velocity (m s−1) · Δv = change in velocity · t = time taken (s)
Units
  • Units metres per second squared (m/s²)
Change in velocity
  • Δv = v − u, where v is the final and u the initial velocity

Part 5 of 8Distance-time graphs

Gradient
  • Gradient the gradient of a distance-time graph is the speed
Horizontal line
  • Horizontal line the object is stationary
Straight sloping line
  • Straight sloping line constant speed
Curve
  • Curve changing speed, with a steeper curve meaning increasing speed

Part 6 of 8Speed-time graphs

Gradient
  • Gradient the gradient of a speed-time graph is the acceleration
Horizontal line
  • Horizontal line constant speed
Straight sloping line
  • Straight sloping line constant acceleration
Area under the graph
  • Area under the graph it equals the distance travelled

Part 7 of 8Free fall

1Acceleration of free fall
  • Acceleration of free fall g is about 9.8 m/s² when air resistance is absent
2Terminal velocity
  • Terminal velocity an object falling through air reaches constant speed when air resistance equals weight

Part 8 of 8More from the specification

More from the specification
Speed and velocity
  • Speed and velocity speed is distance over time with no direction, whereas velocity is a vector giving displacement per unit time.
Gradient of a distance-time graph
  • Gradient of a distance-time graph the slope represents speed, and a steeper line means faster motion;
    • a flat line means the object is stationary.
Falling object and skydiver
  • Falling object and skydiver weight accelerates a falling object until air resistance rises to equal it,
    • giving terminal velocity with zero resultant force.
Decreasing gradient
  • Decreasing gradient a curve that flattens on a speed-time graph shows the acceleration is decreasing as the resistive force grows.
Speed and velocity
  • Speed and velocity speed is distance over time with no direction, whereas velocity is a vector giving displacement per unit time.
Gradient of a distance-time graph
  • Gradient of a distance-time graph the slope represents speed, and a steeper line means faster motion;
    • a flat line means the object is stationary.
Falling object and skydiver
  • Falling object and skydiver weight accelerates a falling object until air resistance rises to equal it,
    • giving terminal velocity with zero resultant force.
Decreasing gradient
  • Decreasing gradient a curve that flattens on a speed-time graph shows the acceleration is decreasing as the resistive force grows.
Resultant force
  • Another name for net force.
Gravitational field
  • Region where mass experiences gravitational force.
Field strength
  • G = Fg/m.

Key terms

TermMeaning
SpeedDistance per unit time.
VelocitySpeed in a given direction.
AccelerationChange in velocity per unit time.
GradientSteepness of a graph line.
Terminal velocityConstant speed when air resistance equals weight.
Free fallFalling under weight alone.

Avoid these mistakes

1
Mixing up the two graphs — the gradient is speed on a distance-time graph and acceleration on a speed-time graph.
2
Using the formula for speed with an average of two speeds — use total distance divided by total time.
3
Writing deceleration as a positive speed change — it is a negative acceleration.
4
Using the area on a distance-time graph — area under a speed-time graph gives distance.

Last updated:

Physics 0625 & 9702 SUVAT Mastery Graph Interpretation

Constant Acceleration Equations (SUVAT) & Slopes

v = u + at  |  s = ut + ½at2  |  v2 = u2 + 2as  |  s = ½(u + v)t

  • Displacement-Time Graph: Gradient = Velocity (curve = accelerating).
  • Velocity-Time Graph: Gradient = Acceleration; Area under curve = Distance travelled / Displacement.
  • Free Fall in vacuum: a = g ≈ 9.81 m/s2 downwards (air resistance negligible).
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