Forces – effects on motion | AceBGCSE Physics

Motion and the Role of Force

Motion refers to a change in position of a body with time.

A force is a push or pull that can affect how an object moves.

A force does not always cause motion, but when a force produces an effect, it may change the motion of a body in specific, identifiable ways.

Ways a Force Can Change Motion

A force can change the motion of a body by:

Starting motion

Stopping motion

Changing speed

Changing direction

Changing both speed and direction

Each effect is described below.

Starting Motion

A force can cause a stationary object to begin moving.

Examples:

Pushing a stationary trolley,

Kicking a football at rest,

Pulling a box across the floor.

Explanation:

The applied force overcomes inertia and any opposing forces such as friction.

[Insert diagram showing a stationary object being pushed and beginning to move]

Stopping Motion

A force can bring a moving object to rest.

Examples:

Applying brakes to stop a car,

Catching a moving ball,

Friction stopping a sliding object.

Explanation:

The force acts opposite to the direction of motion, reducing speed to zero.

[Insert diagram showing braking force acting on a moving vehicle]

Changing Speed (Acceleration or Deceleration)

A force can change the speed of a moving object by:

increasing speed (acceleration),

decreasing speed (deceleration).

Examples:

Pressing the accelerator pedal in a car,

A parachute slowing down a skydiver,

Pushing a swing to make it move faster.

Explanation:

A force acting in the direction of motion increases speed.

A force acting opposite to motion decreases speed.

Changing Direction of Motion

A force can change the direction of motion without changing speed.

Examples:

A stone tied to a string moving in a circle,

A car turning a corner,

The Earth’s gravitational force changing the direction of a satellite.

[Insert diagram showing circular motion with force directed towards the centre]

Explanation:

A sideways force causes the path to curve instead of remaining straight.

Changing Both Speed and Direction

In many real situations, a force changes both speed and direction simultaneously.

Examples:

A football being kicked at an angle,

A thrown object moving under gravity,

A car speeding up while turning.

This results in accelerated motion.

Summary of Effects of Force on Motion

Effect of Force	Description	Example
Start motion	Causes a stationary object to move	Kicking a ball
Stop motion	Brings a moving object to rest	Braking a car
Change speed	Increases or decreases speed	Car accelerating
Change direction	Alters direction of motion	Circular motion
Change speed & direction	Produces acceleration	Thrown object

Common Examination Errors (Examiner Insight)

Students often:

confuse change in motion with change in shape,

mention force without describing its effect,

give examples without explanation,

forget that changing direction is also a change in motion.

Clear descriptions linked to examples earn full marks.

Exam-Style Questions (Original)

Question 1

State two ways in which a force can change the motion of a body.

Question 2

Explain how a force can change the speed of a moving object.

Question 3

Describe how a force can change the direction of motion of a body without changing its speed.

Worked Solutions (Beyond Excellent)

Solution 1

A force can start motion and stop motion.

(It can also change speed or direction.)

Solution 2

If a force acts in the direction of motion, it increases the speed of the object. If it acts opposite to the direction of motion, it decreases the speed.

Solution 3

A force acting at right angles to the direction of motion causes the path of the object to curve, changing its direction while keeping the speed constant, as in circular motion.

End-of-Objective

A learner who has mastered this objective can:

describe all main effects of force on motion,

explain changes in speed and direction clearly,

use appropriate real-life examples,

apply correct scientific terminology.

Meaning of the Equation

The equation:

F = ma

states that:

The resultant force acting on a body is equal to the product of its mass and acceleration.

Where:

F = resultant force (newtons, N)

m = mass (kilograms, kg)

a = acceleration (m s²)

This equation is a mathematical statement of Newton’s Second Law of Motion.

Physical Interpretation

The equation shows that:

a larger force produces a larger acceleration (for the same mass),

a larger mass requires a larger force to produce the same acceleration,

acceleration occurs only when a resultant (unbalanced) force acts.

This explains everyday experiences such as:

pushing a light object is easier than pushing a heavy one,

a stronger push causes faster acceleration.

Resultant Force (Exam-Critical Clarification)

In $F = ma$ , the force F is the resultant force, not just any single force.

If forces are balanced → resultant force = 0 → no acceleration.

If forces are unbalanced → resultant force ≠ 0 → acceleration occurs.

[Insert free-body diagram showing forces acting on a block and the resultant force direction]

Units Consistency (High-Mark Skill)

Always use SI units:

Force in newtons (N)

Mass in kilograms (kg)

Acceleration in m s²

Incorrect units are a frequent cause of lost marks.

Rearranging the Equation

From:

F = ma

We can obtain:

a = \frac{F}{m}

m = \frac{F}{a}

These forms allow calculation of any unknown quantity when the other two are known.

[Insert formula triangle with F at the top, m and a at the base]

Step-by-Step Calculation Strategy

Identify the known quantities.

Write down the correct form of the equation.

Rearrange if necessary.

Substitute values with correct units.

Calculate carefully.

State the final answer with unit.

Clear working earns method marks.

Common Examination Errors (Examiner Insight)

Students often:

forget that F is the resultant force,
F

use grams instead of kilograms,

confuse acceleration with velocity,

omit units in final answers.

Accuracy and clarity are essential.

Exam-Style Questions (Original)

Question 1

State the equation that relates force, mass, and acceleration.

Question 2

A force of 10 N acts on a mass of 2 kg.

Calculate the acceleration produced.

Question 3

A body of mass 5 kg accelerates at 4 m s².

Calculate the resultant force acting on the body.

Question 4

A force of 12 N produces an acceleration of 3 m s².

Calculate the mass of the body.

Worked Solutions (Beyond Excellent)

Solution 1

F = ma

Solution 2

a = \frac{F}{m} = \frac{10}{2} = 5\ \text{m s}^{2}

Solution 3

F = ma = 5 \times 4 = 20\ \text{N}

Solution 4

m = \frac{F}{a} = \frac{12}{3} = 4\ \text{kg}

End-of-Objective

A learner who has mastered this objective can:

state and use the equation:
$F = ma$

rearrange the formula correctly,

solve simple numerical problems accurately,

interpret results in terms of force, mass, and motion.

Meaning of Friction

Friction is defined as:

A force that opposes the relative motion between two surfaces in contact.

Friction acts:

parallel to the surfaces in contact,

opposite to the direction of motion or attempted motion.

Direction of Frictional Force

If an object is moving to the right, friction acts to the left.

If an object tends to move but is prevented, friction acts in the direction opposing the tendency of motion.

[Insert diagram showing a block moving on a rough surface with friction acting opposite to motion]

Effects of Friction on Motion

Friction affects motion in several important ways:

(a) Friction Can Reduce Speed

A moving object slows down when friction acts on it.

Example: a ball rolling on the ground gradually slows and stops.

Explanation:

Friction acts opposite to motion, producing deceleration.

(b) Friction Can Stop Motion

Continuous friction eventually brings a moving object to rest.

Example: brakes stopping a bicycle or car.

Explanation:

Frictional force reduces the velocity to zero.

(c) Friction Can Prevent Motion

Friction can stop an object from starting to move.

Example: pushing a heavy box that does not move.

This occurs because static friction balances the applied force.

(d) Friction Can Require Extra Force to Maintain Motion

To keep an object moving at constant speed, a force equal to friction must be applied.

Example: pushing a box at steady speed across the floor.

Simple Demonstrations of Friction (Classroom-Based)

Friction can be demonstrated by:

Sliding a block on a smooth surface and then on a rough surface,

Rolling a toy car on different surfaces,

Using a spring balance to pull a block across a table.

[Insert diagram showing comparison of motion on smooth and rough surfaces]

Observation:

Motion slows faster on rough surfaces due to greater friction.

Types of Friction (Qualitative Awareness)

At BGCSE level, friction may be described as:

Static friction – prevents motion from starting,

Kinetic (sliding) friction – opposes motion once it has started,

Rolling friction – smaller friction when objects roll.

These types help explain different motion behaviours.

Friction as a Useful and Harmful Force

Useful effects:

Walking without slipping,

Braking vehicles,

Writing with chalk or pen.

Harmful effects:

Wear and tear of moving parts,

Energy loss as heat,

Reduced efficiency of machines.

Common Examination Errors (Examiner Insight)

Students often:

say friction helps motion without explanation,

forget that friction always opposes motion,

confuse friction with air resistance,

describe friction without relating it to motion.

Clear cause-and-effect explanation earns full marks.

Exam-Style Questions (Original)

Question 1

Define friction.

Question 2

Describe one way in which friction affects the motion of a moving body.

Question 3

Explain why a ball rolling on a rough surface stops sooner than on a smooth surface.

Question 4

A box is pushed across a floor at constant speed.

What can you say about the frictional force acting on the box?

Worked Solutions (Beyond Excellent)

Solution 1

Friction is a force that opposes the relative motion between two surfaces in contact.

Solution 2

Friction reduces the speed of a moving body by acting in the opposite direction to its motion, causing deceleration.

Solution 3

A rough surface produces a larger frictional force, which causes greater deceleration. As a result, the ball loses speed more quickly and stops sooner.

Solution 4

The frictional force is equal in magnitude to the applied force but acts in the opposite direction, resulting in zero resultant force and constant speed.

End-of-Objective

A learner who has mastered this objective can:

define friction correctly,

describe and demonstrate its effects on motion,

explain motion changes using frictional forces,

apply the concept to everyday situations and exam problems.

Friction in Force Calculations

When friction is present:

more than one force acts on the object,

friction always acts opposite to the direction of motion or attempted motion,

the resultant force determines how the object moves.

Calculations must therefore consider all forces acting on the body.

Resultant Force with Friction

The resultant force is the net force acting on a body.

For motion along a straight line:

\text{Resultant force} = \text{Driving force} - \text{Frictional force}

If:

driving force > friction → object accelerates,

driving force = friction → object moves at constant speed,

driving force < friction → object slows down or remains at rest.

[Insert free-body diagram showing a block pulled to the right with friction acting to the left]

Using Newton’s Second Law with Friction

Once the resultant force is known, motion can be analysed using:

$F=ma$

Where:

F is the resultant force,

m is the mass,

a is the acceleration (or deceleration).

Step-by-Step Problem-Solving Strategy

Draw or imagine a force diagram.

Identify:
- driving force,
- frictional force.

Calculate the resultant force.

Apply F=ma.

Solve for the required quantity.

Include correct units.

This structured approach is essential in exams.

Worked Numerical Examples (Teaching Core)

Example 1: Object Accelerating with Friction

A box of mass 5 kg is pulled along a horizontal surface by a force of 20 N.

The frictional force opposing motion is 8 N.

Calculate the acceleration of the box.

Solution

Resultant force:

F = 20 - 8 = 12\ \text{N}

a = \frac{F}{m} = \frac{12}{5} = 2.4\ \text{m s}^{2}

Example 2: Constant Speed Motion

A trolley is pushed with a force of 15 N and moves at constant speed.

Find the frictional force acting on the trolley.

Solution

Constant speed means:

\text{Resultant force} = 0

Therefore:

\text{Friction} = 15\ \text{N}

Example 3: Deceleration Due to Friction

A block of mass 4 kg slides on a floor with a frictional force of 6 N acting on it.

Calculate the deceleration.

Solution

Resultant force = friction = 6 N (opposing motion)

a = \frac{F}{m} = \frac{6}{4} = 1.5\ \text{m s}^{2}

The block decelerates at 1.5 m s².

Key Concepts to Remember

Friction reduces the resultant force.

Acceleration depends on the net force, not the applied force alone.

Constant speed means zero resultant force.

Deceleration is acceleration acting opposite to motion.

Common Examination Errors (Examiner Insight)

Students often:

use applied force instead of resultant force,

forget to subtract friction,

confuse deceleration with negative velocity,

omit units in final answers.

Clear force identification secures method and accuracy marks.

Exam-Style Questions (Original)

Question 1

A force of 18 N pulls a 3 kg object across a floor.

If friction is 6 N, calculate the acceleration.

Question 2

A box moves at constant speed when pushed with a force of 12 N.

Find the frictional force acting on the box.

Question 3

Explain why a larger force is required to move an object on a rough surface than on a smooth surface.

Worked Solutions (Beyond Excellent)

Solution 1

Resultant force:

F = 18 - 6 = 12\ \text{N}

a = \frac{12}{3} = 4\ \text{m s}^{2}

Solution 2

Constant speed implies zero resultant force.

Therefore, friction = 12 N.

Solution 3

A rough surface produces greater friction, which opposes motion more strongly. A larger applied force is therefore required to overcome friction and move the object.

End-of-Objective

A learner who has mastered this objective can:

identify forces acting on a body with friction,

calculate resultant force correctly,

apply F=ma accurately,

solve motion problems involving friction with confidence.

Curved Motion and the Need for a Force

An object moving along a curved path is constantly changing its direction of motion.

Since velocity includes direction, a change in direction means the object is accelerating, even if its speed remains constant.

Therefore:

A force is required to keep an object moving along a curved path.

Meaning of Centripetal Force

Centripetal force is defined as:

The force that acts towards the centre of a circular or curved path, causing an object to follow that path.

Key features:

Always directed towards the centre of the curve,

Acts at right angles to the direction of motion,

Changes the direction of velocity, not necessarily the speed.

Direction of Centripetal Force

For an object moving in a circle:

Velocity is tangential to the path,

Centripetal force is radial, pointing towards the centre.

[Insert diagram showing an object in circular motion with tangential velocity and inward centripetal force]

This inward force continually bends the path of the object.

Effect of Centripetal Force on Motion

Centripetal force causes:

(a) Continuous Change in Direction

The object does not move in a straight line.

Its direction changes at every point along the path.

(b) Curved or Circular Motion

Without centripetal force, the object would move in a straight line (Newton’s First Law).

With centripetal force, the path becomes circular or curved.

(c) Acceleration Without Speed Change

Speed may remain constant,

Velocity changes due to changing direction,

This is called centripetal acceleration.

What Happens If Centripetal Force Is Removed?

If the centripetal force suddenly stops:

the object moves off in a straight line,

the direction is tangential to the curved path at that point.

This shows that centripetal force is essential for curved motion.

[Insert diagram showing object leaving circular path tangentially when force is removed]

Examples of Centripetal Force in Everyday Life

The centripetal force may be provided by different physical forces:

Situation	Source of Centripetal Force
Stone on a string	Tension in the string
Car turning a corner	Friction between tyres and road
Satellite orbiting Earth	Gravitational force
Object in circular track	Normal reaction

[Insert diagram showing a car turning a corner with friction providing centripetal force]

Qualitative Relationship (No Advanced Mathematics)

At this level, it is sufficient to know that:

greater speed requires a larger centripetal force,

sharper curves require greater inward force,

heavier objects need more force to follow the same curved path.

Common Examination Errors (Examiner Insight)

Students often:

think centripetal force pushes objects outward,

confuse centripetal force with centrifugal effects,

say centripetal force increases speed,

forget that the force acts towards the centre.

Clear reference to direction and effect on velocity earns full marks.

Exam-Style Questions (Original)

Question 1

Define centripetal force.

Question 2

Describe the effect of centripetal force on the motion of an object moving in a circular path.

Question 3

Explain what happens to an object moving in a circular path if the centripetal force is removed.

Question 4

State the force that provides the centripetal force when a car turns a corner on a flat road.

Worked Solutions (Beyond Excellent)

Solution 1

Centripetal force is the force acting towards the centre of a circular or curved path, causing an object to move along that path.

Solution 2

The centripetal force continuously changes the direction of motion of the object, causing it to follow a curved or circular path without necessarily changing its speed.

Solution 3

The object moves off in a straight line tangential to the circular path because there is no longer an inward force to change its direction.

Solution 4

The centripetal force is provided by friction between the tyres and the road.

End-of-Objective

A learner who has mastered this objective can:

define centripetal force accurately,

describe its direction and effect on motion,

explain curved motion using force and velocity concepts,

apply the idea to real-life examples and exam questions.

Introduction to Newton’s Laws of Motion

The laws of motion were formulated by Isaac Newton and form the foundation of classical mechanics.

They describe:

why objects move,

how forces affect motion,

the relationship between force and acceleration.

These laws apply to everyday situations as well as scientific and engineering problems.

Newton’s First Law of Motion (Law of Inertia)

Statement

A body remains at rest or continues to move with constant velocity in a straight line unless acted upon by a resultant external force.

Explanation

This law means:

objects do not change their motion by themselves,

motion only changes when an unbalanced (resultant) force acts.

The tendency of an object to resist changes in motion is called inertia.

Examples:

A book on a table remains at rest until pushed.

A puck on a smooth surface continues moving at constant speed.

[Insert diagram showing a block at rest and another moving at constant speed with no resultant force]

Everyday Application

Seat belts protect passengers when cars stop suddenly.

Objects slide forward when a moving bus stops abruptly.

Newton’s Second Law of Motion

Statement

The acceleration of a body is directly proportional to the resultant force acting on it and inversely proportional to its mass, and takes place in the direction of the force.

This leads to the equation:

F=ma

Explanation

The law shows that:

larger force → larger acceleration,

larger mass → smaller acceleration for the same force,

acceleration occurs only if forces are unbalanced.

[Insert diagram showing a block with forces acting and acceleration in the direction of the resultant force]

Using the Law in Calculations

Key reminders:

F is the resultant force,

use SI units (N, kg, m s²),

friction must be included when present.

Newton’s Third Law of Motion

Statement

For every action, there is an equal and opposite reaction.

Explanation

This law means:

forces occur in pairs,

the two forces act on different bodies,

the forces are equal in size but opposite in direction.

Examples:

Walking: foot pushes ground backward, ground pushes foot forward.

Gun recoil: bullet moves forward, gun moves backward.

[Insert diagram showing action–reaction force pairs between two interacting bodies]

Important Clarification (Exam-Critical)

Action and reaction forces:

do not cancel out,

act on different objects,

therefore do not violate Newton’s First or Second Law.

Summary of Newton’s Laws (High-Value Revision)

Law	Key Idea	Main Use
First	Inertia	Explains constant motion
Second	F = ma	Calculations of motion
Third	Action–reaction	Explains interactions

Common Examination Errors (Examiner Insight)

Students often:

forget the term resultant force,

confuse Newton’s Second Law with equations of motion,

think action–reaction forces act on the same object,

omit direction when explaining laws.

Clear statements + correct application = full marks.

Exam-Style Questions (Original)

Question 1

State Newton’s First Law of Motion.

Question 2

Write down the equation associated with Newton’s Second Law of Motion.

Question 3

A force of 15 N acts on a mass of 3 kg.

Calculate the acceleration produced.

Question 4

Explain Newton’s Third Law using the example of a person walking.

Worked Solutions (Beyond Excellent)

Solution 1

/A body remains at rest or continues to move with constant velocity in a straight line unless acted upon by a resultant external force.

Solution 2

$F=ma$

Solution 3

Given:

F = 15 N

m = 3 kg

a = \frac{F}{m} = \frac{15}{3} = 5\ \text{m s}^{2}

Solution 4

When walking, the foot pushes the ground backward (action). The ground pushes the foot forward with an equal and opposite force (reaction), causing the person to move forward.

End-of-Objective

A learner who has mastered this objective can:

state all three of Newton’s laws accurately,

explain their physical meaning,

apply F=ma in calculations,
F = ma

analyse everyday motion using action–reaction pairs.