1.4Motion, Forces and Energy

Effects of Forces — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions

15 min read 31 parts Lesson 30 of 91 Unit quiz

📚 Cambridge IGCSE Physics 0625 Hub / Topical Revision

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Lesson summary

Resultant Forces and Newton's First Law — IGCSE PHYSICS notes: Combine forces along a line, decide whether they are balanced, and apply Newton's first law to rest and constant velocity.Definition — a force is a push or a pull on an object caused by an interaction with another obj…

Effects of a force
What it decides
Vector nature
Book on a table
Equal and opposite
Same direction

Section 1 of 31What forces do

Definition

Definition a force is a push or a pull on an object caused by an interaction with another object.

Effects of a force

Effects of a force a force can change an object's speed, direction, shape or size.
Engine thrust makes a car speed up and brake friction makes it slow down.
The Sun's gravity pulls a comet so it changes direction.
Two opposing pushes on a spring make it compress and change shape.

Section 2 of 31Resultant force

Definition

Definition the resultant force is the single force with the same effect as all the forces acting on an object.
Forces along one straight line are combined into one net force.

What it decides

What it decides the resultant gives the direction the object moves and the size of the net force.

Vector nature

Vector nature force is a vector, so directions must be shown by signs.
Choose one direction as positive and the opposite as negative.
It does not matter which you choose if you are consistent.

Section 3 of 31Balanced and unbalanced forces

Feature	Balanced forces	Unbalanced forces
Along a line	Equal and opposite	Do not cancel
Resultant force	Zero	Non-zero
Effect on motion	No change in velocity	Acceleration in the direction of the resultant
Example	Book resting on a table	Car braking

Book on a table
- the weight acts downwards and the table's contact force acts upwards.
- The two are equal in size and opposite, so the resultant force is zero.

Section 4 of 31Calculating a resultant along a line

Choose a positive direction, for example right is positive.

Give each force a sign according to its direction.

Add the forces together.

State the size and the direction of the answer.

Equal and opposite

5 N right and 5 N left give 5 + (−5) = 0 N.

Same direction

3 N and 7 N together give 10 N.

Opposite directions

7 N left and 3 N right give (−7) + 3 = −4 N, so 4 N to the left.

Section 5 of 31Newton's first law

1Statement

Statement an object stays at rest, or continues in a straight line at constant speed, unless a resultant force acts on it.
A stationary object stays stationary.
A moving object keeps its constant velocity.

2Converse

Converse if an object is at rest or has constant velocity, the resultant force on it is zero.

3Constant velocity

Constant velocity this means no change in speed and no change in direction.

Section 6 of 31Examples of the first law

1Mug on a table

Mug on a table it stays still until someone picks it up or knocks the table.

2Space debris

Space debris it keeps going in a straight line at constant speed until it meets a planet's gravity or hits an asteroid.

3Car at steady speed

Car at steady speed it continues until the driver brakes or accelerates.
Engine force balances friction and air resistance, so the resultant is zero.
More engine force speeds the car up, while more friction (brakes) slows it down.

Section 7 of 31More from the specification

Section 8 of 31Statement of the law

Statement of the law

Law

Law acceleration is proportional to the resultant force and inversely proportional to the mass.
It describes what happens when forces are unbalanced.

Effect of a resultant force

Effect of a resultant force the object accelerates in the direction of the resultant.
Acceleration can mean speeding up, slowing down or changing direction.

Two relationships

Two relationships a bigger resultant gives a bigger acceleration, and a bigger mass gives a smaller acceleration for the same force.

Section 9 of 31The equation

Equation

F = ma.
What the symbols mean: F = resultant force (N) · m = mass (kg) · a = acceleration (m s⁻²)
F is the resultant force in newtons (N).
m is the mass in kilograms (kg).
a is the acceleration in metres per second squared (m/s²).

Rearrangements

Rearrangements a = F ÷ m and m = F ÷ a.
A formula triangle helpscover the unknown and read the rest.

Direction

Direction acceleration is in the same direction as the resultant force.

Meaning of 1 N

Meaning of 1 N it is the force that gives a 1 kg mass an acceleration of 1 m/s².

Section 10 of 31Using the law in problems

Identify all the forces and find the resultant force.

Write F = ma and rearrange for the unknown.

Convert masses to kilograms.

Substitute and calculate with units.

State the direction of the acceleration.

Key point

Key point an object can have several forces but only one acceleration, so substitute the resultant force.

Link to motion

Link to motion a = (v − u) ÷ t can be combined with F = ma in two-step problems.

Section 11 of 31Mass, force and acceleration compared

Change	Effect on acceleration
Double the resultant force, same mass	Acceleration doubles
Halve the resultant force, same mass	Acceleration halves
Double the mass, same force	Acceleration halves
Halve the mass, same force	Acceleration doubles
Resultant force is zero	Acceleration is zero (Newton's first law)

Section 12 of 31Links between Newton's laws

1Zero resultant

F = ma with F = 0 gives a = 0, which is Newton's first law.

2Two-stage problems

Two-stage problems acceleration from a velocity change, then force from F = ma.

3Mass in the denominator

Mass in the denominator adding passengers raises the mass, so for the same engine force the acceleration is lower.

Section 13 of 31Unit relationships

Quantity	Unit	Equivalent
Force	newton (N)	kg m/s²
Acceleration	m/s²	N/kg
Mass	kilogram (kg)	N s²/m

Section 14 of 31More from the specification

Section 15 of 31Aim and variables

Aim

Aim to find the relationship between the load on a spring and its extension.

Variables

Variables each has a role in a fair test.
Independentthe load (force) applied.
Dependentthe extension of the spring.
Controlthe same spring and the same reference point for every reading.

Section 16 of 31Equipment

Equipment	Purpose
Clamp stand, boss and clamp	Holds the spring and supports the upward force
Spring	The elastic object being investigated
100 g masses and hanger	Apply the downward load
Ruler (1 mm resolution)	Measures the position of the spring
Pointer	Marks the bottom of the spring on the scale and reduces parallax

Section 17 of 31Method

Fix the ruler vertically to the clamp stand and align the pointer with the spring unloaded.

Record the original length l₀.

Add the 100 g hanger and record the mass and the new length.

Add another 100 g and record again, continuing until all masses are used.

Remove the masses and repeat so there are three runs, then average the lengths.

Calculate extension e = l − l₀ and load L = mg with g = 9.8 N/kg.

Section 18 of 31Analysis and errors

Analysis and errors

Graph

Graph plot load (y) against extension (x), starting both axes at the origin, and draw a best-fit line.
A straight line through the origin shows extension is proportional to load.
Past the limit of proportionality the line curves.

Systematic errors

Systematic errors read at eye level, from the same point on the spring, with the ruler vertical and zero aligned.

Random error

Random error wait for the mass to become still before reading.

Safety

Safety wear goggles, clamp the stand with a G clamp, keep feet away and place a mat below.

Common slip

Common slip extension is the total increase over the original length, not the increase from the previous reading.

Section 19 of 31Hooke's law and the spring constant

Hooke's law

Hooke's law the extension of an elastic object is directly proportional to the force applied, up to the limit of proportionality.
Doubling the force doubles the extension.

Equation

F = kx.
What the symbols mean: F = force (N) · k = spring constant (N m⁻¹) · x or e = extension (m)
F is force in N, x is extension in m and k is the spring constant in N/m.

Spring constant

Spring constant the force per unit extension, k = F ÷ x.
A stiff spring has a high k and a stretchy spring has a low k.
N/cm is acceptable if extensions are in cm.

Section 20 of 31Reading the load-extension graph

Feature	Meaning
Straight section through the origin	Hooke's law applies, load proportional to extension
Gradient of the straight section	Spring constant k (for load on the y-axis)
Point where the line starts to curve	Limit of proportionality
Curved section	Load and extension no longer proportional

Section 21 of 31More from the specification

Newton's First Law of Motion

A body remains at rest or continues moving at constant speed and velocity in a straight line unless acted upon by a non-zero resultant force.

Newton's Second Law Formula

When an unbalanced resultant force acts on a mass m, it produces acceleration according to the formula F = ma, measured in newtons (N).

Friction and Motion Resistance

Friction is a contact force acting in the direction opposite to relative motion, which can change kinetic energy into thermal energy.

Balanced vs Unbalanced Forces

Balanced forces acting on an object cancel out, maintaining uniform motion,
whereas an unbalanced resultant force alters speed, direction,
or shape.

Constant speed

Speed that does not change.

Circular motion

Motion along a circular path with a force towards the centre.

Resultant

Vector sum of two or more vectors.

Velocity

Rate of change of displacement.

Newton

Unit of force, kg m s^-2.

Acceleration

Rate of change of velocity.

Friction

Force parallel to the surface opposing relative motion or its tendency.

Balanced

Having equal and opposite moments.

Weight

Gravitational force on a mass.

Section 22 of 31Why circular motion is accelerated motion

Velocity changes

Velocity changes velocity is a vector, so a changing direction means a changing velocity.
An object in a circle at constant speed is still accelerating.

Example

Example the International Space Station orbits at about 7660 m/s but its direction constantly changes.

Section 23 of 31The resultant force in a circle

Perpendicular force

Perpendicular force a resultant force at 90° to the motion changes the direction but not the speed.
If it keeps acting at 90°, the object follows a circle.

Direction

Direction the resultant force always points towards the centre of the circle.
It is inwards, never outwards.

Examples

Examples a ball swung on a string, the Moon around the Earth and a planet around a star.
For the Moon the gravitational pull provides the inward force.

Section 24 of 31What decides the force needed

Change (others constant)	Effect on resultant force needed
Greater mass	Greater force
Greater speed	Greater force
Smaller radius	Greater force

Section 25 of 31Friction between solids

Definition

Definition solid friction is the force between two surfaces that opposes motion.
It acts opposite to the direction of motion.

Cause

Cause imperfections and bumps on the surfaces catch on each other.

Energy transfer

Energy transfer work done against friction transfers energy from the kinetic store to the internal (thermal) store.
The object and its surroundings get hotter.

Reducing it

Reducing it lubricate the surfaces or make them smoother.

Section 26 of 31Friction in fluids

Fluids

Fluids gases and liquids are fluids because their particles can move freely.

Drag

Drag friction on an object moving through a fluid comes from particle collisions with the object.
In a liquid it is called drag, and in air it is called air resistance.
Drag increases with speed, not with time.

Terminal velocity

Terminal velocity a falling object stops accelerating when drag equals weight.

Heating

Heating air resistance heats both the object and the air, as for a returning rocket module.

Reducing it

Reducing it streamline the shape of the object.

Section 27 of 31Applications of circular motion

Satellites

Satellites gravity provides the inward force that keeps them in orbit.

Cars on bends

Cars on bends friction between the tyres and the road provides the inward force.
On ice the friction is too small so the car continues in a straight line.

Hammer throw

Hammer throw the athlete's pull on the wire gives the inward force and the hammer flies off straight on release.

Section 28 of 31Useful and harmful friction

Useful friction	Harmful friction
Grip between shoes and the ground	Wear and heating of moving parts
Brakes slowing a vehicle	Wasted energy in engines
Holding a nail in wood	Air resistance slowing a cyclist

Section 29 of 31More from the specification

Section 30 of 31Key terms

Term	Meaning
Force	A push or pull on an object caused by an interaction.
Resultant force	The single force with the same effect as all the forces acting on an object.
Balanced forces	Forces that cancel, giving a zero resultant.
Unbalanced forces	Forces that do not cancel, giving a resultant.
Newton's first law	Without a resultant force an object stays at rest or moves at constant velocity.
Constant velocity	Constant speed in a constant direction.

Term	Meaning
Newton's second law	Acceleration is proportional to resultant force and inversely proportional to mass.
Newton (N)	The unit of force, the force that gives 1 kg an acceleration of 1 m/s².
Inversely proportional	One quantity falls as the other rises in the same ratio.
Resultant force	The net force on an object after combining all forces.
Acceleration	Rate of change of velocity.

Term	Meaning
Hooke's law	Extension is directly proportional to force up to the limit of proportionality.
Spring constant	Force per unit extension, measured in N/m.
Extension	The increase in length compared with the original length.
Limit of proportionality	The point beyond which force and extension are no longer proportional.
Load	The force applied to a spring.
Parallax error	An error in reading a scale caused by viewing at an angle.

Term	Meaning
Circular motion	Motion along a circular path with a force towards the centre.
Friction	A force between surfaces or with a fluid that opposes motion.
Drag	Frictional force on an object moving through a fluid.
Air resistance	Drag on an object moving through air.
Streamlining	Shaping an object to reduce drag.
Lubricant	A substance that reduces friction between surfaces.

Section 31 of 31Exam technique

Believing a moving object must have a resultant force — constant velocity needs zero resultant force.

Mixing up constant speed with constant velocity — velocity also needs a constant direction.

Adding opposing forces without signs — opposite directions subtract.

Giving only the size of a resultant — state the direction too.

Using the driving force instead of the resultant force in F = ma.

Leaving mass in grams — convert to kilograms first.

Saying a larger mass gives a larger acceleration — for the same force it gives a smaller one.

Forgetting that acceleration includes slowing down — a resultant opposite to the motion decelerates.

Dividing by the total length of the spring instead of the extension.

Leaving the extension in cm when k is wanted in N/m.

Saying only that extension increases with load — Hooke's law needs proportional, up to the limit.

Using the word elastic limit in place of limit of proportionality.

Drawing the circular-motion force pointing outwards — it points towards the centre.

Saying constant speed means no acceleration in a circle — the direction change is an acceleration.

Writing "resistance" for fluid friction — name it drag or air resistance.

Thinking drag grows with time — it grows with speed.

Last updated: 15 October 2026