Forces & Motion

This page follows the learning objectives from your IGCSE Edexcel book: motion graphs, forces, F = m a, stopping distance, momentum and moments. Use it with your textbook and past papers.

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Topic 1 Paper 1 & Paper 2 High exam frequency

Overview

In this topic you must be able to:

Exam idea: markers love questions where you combine more than one of these ideas in the same problem (for example graphs + F = m a + stopping distance).

1. Motion Graphs & Basic Formulas

Distance–Time Graphs

You should be able to:
  • Plot and explain distance–time graphs.
  • Describe what a flat, rising or steeper line means.

On a distance–time graph:

  • Gradient (slope) = speed.
  • Flat line = object is stationary (distance not changing).
  • Steeper line = higher speed.
distance time constant speed

Average Speed

Know and use the relationship between average speed, distance moved and time taken:
average speed = distance moved ÷ time taken

vavg = d / t

Always convert minutes to seconds (and hours to seconds) before using the formula.

Velocity–Time Graphs

You should be able to:
  • Plot and explain velocity–time graphs.
  • Determine acceleration from the gradient.
  • Determine distance travelled from the area under the graph.
velocity time gradient = acceleration area = distance

Acceleration

Know and use the relationship between acceleration, change in velocity and time taken:
acceleration = change in velocity ÷ time taken

a = (v − u) / t

u = initial velocity, v = final velocity.

Linking Speed, Acceleration and Distance

Use the relationship between final speed, initial speed, acceleration and distance moved:
(final speed)² = (initial speed)² + (2 × acceleration × distance moved)

v² = u² + 2 a s

Quick Check

Q1. A car travels 600 m in 30 s. What is its average speed?

Show answer

v = d / t = 600 / 30 = 20 m/s

Q2. A cyclist speeds up from 4 m/s to 10 m/s in 3 s. Calculate the acceleration.

Show answer

a = (v − u)/t = (10 − 4) / 3 = 6 / 3 = 2 m/s²

2. Forces, Friction & Elastic Behaviour

Types of Force

You should be able to:
  • Describe the effects of forces between bodies such as changes in speed, shape or direction.
  • Identify different types of force such as gravitational or electrostatic.
  • Understand how vector quantities differ from scalar quantities.
  • Understand that force is a vector quantity.
  • Calculate the resultant force of forces that act along a line.
driving force air resistance / friction weight normal reaction

Friction & Stretching

You should be able to:
  • Know that friction is a force that opposes motion.
  • Describe practicals: investigate how extension varies with applied force for helical springs, metal wires and rubber bands.
  • Know that the initial linear region of a force–extension graph is associated with Hooke’s law.
  • Describe elastic behaviour as the ability of a material to recover its original shape after the forces causing deformation have been removed.
force extension linear (Hooke's law)

Quick Check

Q3. A 6 N force to the right and a 4 N force to the right act on a trolley. What is the resultant force?

Show answer

Both forces are in the same direction, so resultant = 6 + 4 = 10 N to the right.

3. F = m a, Weight & Stopping Distance

Unbalanced Force & Acceleration

Know and use the relationship between unbalanced force, mass and acceleration:
force = mass × acceleration

F = m a

Weight

Know and use the relationship between weight, mass and gravitational field strength:
weight = mass × gravitational field strength

W = m g

Stopping Distance & Falling Objects

You should be able to:
  • Know that the stopping distance of a vehicle is the sum of the thinking distance and the braking distance.
  • Describe the factors affecting vehicle stopping distance, including speed, mass, road condition and reaction time.
  • Describe the forces acting on falling objects and explain why falling objects reach a terminal velocity.
stopping distance thinking braking

Quick Check

Q4. A 1 200 kg car has an unbalanced braking force of 4 800 N. What is its deceleration?

Show answer

a = F / m = 4 800 / 1 200 = 4 m/s² (deceleration).

4. Momentum & Newton’s Third Law

Momentum

Know and use the relationship between momentum, mass and velocity:
momentum = mass × velocity

p = m v
Conservation of momentum (no external forces, e.g. no friction or air resistance):
total initial momentum = total final momentum

m1u1 + m2u2 = m1v1 + m2v2
You should also be able to:
  • Use the idea of momentum to explain safety features (air bags, crumple zones, seat belts).
  • Apply initial momentum = final momentum to calculate the mass, velocity or momentum of objects in collisions and explosions.

Force & Change in Momentum

Use the relationship between force, change in momentum and time taken:
force = change in momentum ÷ time taken

F = (m v − m u) / t
Newton’s Third Law
  • Demonstrate an understanding of Newton’s third law: for every action, there is an equal and opposite reaction.
equal & opposite forces

Quick Check

Q5. A 1 000 kg car moving at 12 m/s crashes to a stop in 0.4 s. What average force acts on it?

Show answer

Change in momentum Δp = m v − m u = 1 000 × 0 − 1 000 × 12 = −12 000 kg m/s.
F = Δp / t = −12 000 / 0.4 = −30 000 N (opposite to the motion).

5. Moments & Turning Effects

Moment of a Force

Know and use the relationship between the moment of a force and its perpendicular distance from the pivot:
moment = force × perpendicular distance from the pivot
You should also:
  • Know that the weight of a body acts through its centre of gravity.

Principle of Moments

You should be able to:
  • Use the principle of moments for a simple system of parallel forces acting in one plane (clockwise moments = anticlockwise moments).
  • Understand how the upward forces on a light beam, supported at its ends, vary with the position of a heavy object placed on the beam.
weight (300 N) distance from pivot

Quick Check

Q6. A 300 N child sits 2.0 m from the pivot of a seesaw. What moment does the child produce?

Show answer

Moment = F × d = 300 × 2.0 = 600 N m.

What Next?

When you are comfortable with these sections, try:

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