HighMarks

Forces and friction — KCSE Physics

KCSE Physics · 109 practice questions · 6 syllabus objectives · 6 revision lessons

34 easy38 medium37 hard

Last updated · Aligned to the KNEC KCSE syllabus

What You'll Learn

Key learning outcomes for this topic, aligned to the KNEC KCSE syllabus.

Identify and describe the types of forces (gravitational, normal reaction, tension, friction) and distinguish mass from weight

Distinguish between static and kinetic friction and apply the formula f = μN to solve problems

Solve problems involving objects on inclined planes, taking into account the component forces

Define viscosity, explain terminal velocity and sketch relevant velocity–time graphs

State Newton’s laws of motion; define momentum and impulse; apply F = ma and conservation of momentum

Forces and friction

Revision Notes

Concise lesson notes for Forces and friction, written to the KCSE Physics marking standard. Read the first lesson free below.

Understanding Forces and Their Types

In physics, forces are interactions that can change the motion of an object. Here are the main types of forces:

  • Gravitational Force: This is the force of attraction between two masses. It pulls objects towards each other, like the Earth pulling us down.
  • Normal Reaction Force: This is the support force exerted by a surface perpendicular to the object resting on it. For example, when a book is on a table, the table exerts an upward normal force to support the book.
  • Tension Force: This force is transmitted through a string, rope, or cable when it is pulled tight by forces acting from opposite ends. For instance, when you pull a rope in a tug-of-war.
  • Frictional Force: This force opposes the motion of an object sliding or rolling on a surface. It is caused by the interactions between the surfaces in contact.

Additionally, it is crucial to distinguish between mass and weight:

  • Mass is the amount of matter in an object, measured in kilograms (kg).
  • Weight is the force of gravity acting on that mass, calculated as Weight = Mass × Gravitational Acceleration (9.81 m/s²).

Key points to remember

  • Gravitational force attracts objects towards each other.
  • Normal force supports objects resting on surfaces.
  • Tension is the force in a stretched string or rope.
  • Friction opposes motion between surfaces.
  • Mass is measured in kg; weight is mass times gravity.

Worked example

Identify the type of force acting on a book resting on a table.

  • The normal reaction force acts upward on the book, balancing its weight.

Read all 6 Forces and friction lessons free

Sign up free to unlock the full set of revision notes, all 109 practice questions with marking schemes, plus a personalised study plan that adapts to the topics you keep getting wrong.

More lessons in this topic

Lesson 2: Understanding Static and Kinetic Friction

Objective: Distinguish between static and kinetic friction and apply the formula f = μN to solve problems

Friction is a force that opposes motion. It comes in two main types: static friction and kinetic friction.

  • Static friction acts on an object that is not moving. It prevents the object from starting to move when a force is applied. The maximum static friction force can be expressed as:

    • ( f_s \leq \mu_s N ) where ( \mu_s ) is the coefficient of static friction and ( N ) is the normal force.

  • Kinetic friction, on the other hand, acts on an object that is already in motion. It is generally less than static friction, and can be expressed as:

    • ( f_k = \mu_k N ) where ( \mu_k ) is the coefficient of kinetic friction.

To solve problems involving friction, use the formula ( f = \mu N ) appropriately, depending on whether the object is at rest or in motion.

  • Static friction prevents motion; kinetic friction acts during motion.
  • Static friction is usually greater than kinetic friction.
  • Use \( f = \mu N \) for calculations of friction forces.

A box weighing 50 N is at rest on a surface with a static friction coefficient of 0.4. Calculate the maximum static friction force.

  • Maximum static friction, ( f_s = \mu_s N = 0.4 \times 50 = 20 , \text{N} ).
Lesson 3: Understanding Forces on Inclined Planes

Objective: Solve problems involving objects on inclined planes, taking into account the component forces

When dealing with objects on inclined planes, it is essential to resolve forces into components. The two main forces to consider are gravity and friction.

  1. Weight (W): The weight of the object acts vertically downwards. It can be calculated as W = mg, where m is mass and g is the acceleration due to gravity (9.8 m/s²).
  2. Component Forces:
    • The parallel component (F₁) acts down the slope and is calculated as F₁ = W sin(θ), where θ is the angle of inclination.
    • The perpendicular component (F₂) acts perpendicular to the slope and is calculated as F₂ = W cos(θ).
  3. Frictional Force (F_f): This opposes the motion and can be calculated using F_f = μN, where μ is the coefficient of friction and N is the normal force (equal to F₂).

By combining these forces, you can analyze the motion of the object on the incline, determining whether it will slide down or remain stationary.

  • Resolve weight into parallel and perpendicular components.
  • Use F₁ = W sin(θ) for parallel force.
  • Use F₂ = W cos(θ) for perpendicular force.
  • Calculate friction using F_f = μN.
  • Analyze net force to determine motion.

An object of mass 5 kg is on a 30° incline. Calculate the parallel and perpendicular components of the weight.

  • Weight, W = mg = 5 kg × 9.8 m/s² = 49 N.
  • F₁ = W sin(30°) = 49 N × 0.5 = 24.5 N.
  • F₂ = W cos(30°) = 49 N × √3/2 ≈ 42.4 N.
Lesson 4: Understanding Viscosity and Terminal Velocity

Objective: Define viscosity, explain terminal velocity and sketch relevant velocity–time graphs

Viscosity is the measure of a fluid's resistance to flow. It determines how easily a fluid can move. Higher viscosity means the fluid flows more slowly, while lower viscosity indicates it flows more freely.

Terminal velocity occurs when the force of gravity is balanced by the drag force acting on an object falling through a fluid. At this point, the object stops accelerating and moves at a constant speed.

To illustrate, consider a skydiver:

  1. Initially, they accelerate downwards due to gravity.
  2. As they fall, air resistance increases, opposing their motion.
  3. Eventually, the downward force of gravity equals the upward drag force, resulting in terminal velocity.

Velocity-Time Graphs:

  • The graph of an object falling through a fluid shows:
    • Initial increase in velocity (acceleration).
    • Plateau at terminal velocity (constant speed).

Sketching this graph involves plotting velocity on the y-axis and time on the x-axis.

  • The curve starts steep and then flattens out as terminal velocity is reached.
  • Viscosity measures a fluid's resistance to flow.
  • Higher viscosity means slower fluid flow.
  • Terminal velocity is reached when forces balance.
  • Velocity-time graph shows initial acceleration and plateau.
  • At terminal velocity, speed remains constant.

Define viscosity and explain terminal velocity.

  • Viscosity is the resistance of a fluid to flow.
  • Terminal velocity is the constant speed of a falling object when drag equals gravitational force.

Sample Questions

Read 3 questions and answers free. Sign up to access all 109 questions with full KNEC-style marking schemes and a personalised study plan.

1
easySHORT ANSWER4 marks

A sled of mass 10 kg is sliding down a frictional incline of 20° with a coefficient of kinetic friction of 0.1. (a) State the formula for calculating the frictional force acting on the sled. (2 marks) (b) Calculate the frictional force acting on the sled. (2 marks)

Answer & marking scheme

Part (a) — 2 marks
Frictional force (F_f) = μN (1 mk)
Where μ is the coefficient of kinetic friction and N is the normal force (1 mk)
Part (b) — 2 marks
Calculate normal force: N = mg cos(20°) (1 mk)
Then F_f = 0.1 × (10 kg × 9.8 m/s² × cos(20°)) ≈ 0.1 × 92.1 N = 9.21 N (1 mk)
2
easySHORT ANSWER4 marks

A block of mass 5 kg is placed on an inclined plane at an angle of 30°. (a) State the component of the gravitational force acting parallel to the incline. (2 marks) (b) Calculate this component. (2 marks)

Answer & marking scheme

Part (a) — 2 marks
The component is given by mg sin(θ) (1 mk)
Where m is the mass and θ is the angle of inclination (1 mk)
Part (b) — 2 marks
Using the formula: F = mg sin(30°) (1 mk)
Substituting: F = 5 kg × 9.8 m/s² × 0.5 = 24.5 N (1 mk)
3
easySHORT ANSWER4 marks

Explain the difference between weight and mass. (4 marks)

Answer & marking scheme

Part (a) — 2 marks
Mass is the amount of matter in an object (1 mk)
SI unit of mass is kilogram (kg) (1 mk)
Part (b) — 2 marks
Weight is the gravitational force acting on an object (1 mk)
SI unit of weight is newton (N) (1 mk)
4

Name two types of contact forces and give one example of each. (4 marks)

+106 More Questions

Sign up free to access all 109 questions with marking schemes, track your progress, and get personalised recommendations.

Frequently asked questions

What does the KCSE Physics topic "Forces and friction" cover?

Forces and friction covers Identify and describe the types of forces (gravitational, normal reaction, tension, friction) and distinguish mass from weight; Distinguish between static and kinetic friction and apply the formula f = μN to solve problems; Solve problems involving objects on inclined planes, taking into account the component forces, and more, all aligned to the official KNEC KCSE Physics syllabus.

How many practice questions are available for Forces and friction?

HighMarks has 109 Forces and friction practice questions for KCSE Physics, each with a full marking scheme. The first 3 are free; sign up to access the rest, plus all KCSE mock exams and past papers.

Are these aligned with the KNEC KCSE syllabus?

Yes. Every objective on this page is taken directly from the official KNEC KCSE Physics syllabus. Practice questions match the KCSE exam format and are graded against the standard KNEC marking scheme.

How should I revise Forces and friction for the KCSE exam?

Start with the revision notes on this page to refresh the core concepts, then work through the practice questions in increasing difficulty. Sign up for HighMarks to get a personalised study plan that adapts to the topics you keep getting wrong, plus mock exams, subject-wide practice, and detailed performance tracking. See pricing.

Why Practise Forces and friction?

KNEC Aligned

Questions match the KCSE syllabus objectives and exam format exactly.

Detailed Marking Schemes

Every answer shows exactly what examiners award marks for.

Track Your Mastery

See your score improve as you practise and identify remaining gaps.

Related topics in KCSE Physics

Continue revising adjacent topics from the Physics syllabus.

Physics
Speed, velocity and acceleration
Physics
Pressure in solids
Physics
Distance and displacement
Physics
Pressure in liquids
Physics
Errors, accuracy and precision
Physics
Atmospheric pressure
Previous topic
Newton's laws of motion
Next topic
Moments and equilibrium

More KCSE resources

Round out your Physics preparation with full mock exams, past papers, and every Physics topic in one place.

All Physics topics

Browse every Physics revision topic on HighMarks.

KCSE past papers

Years of past KCSE papers with worked answers.

KCSE mock exams

Full-length, timed mocks scored against KNEC-style rubrics.

KCSE practice questions

All KCSE practice questions across every subject.

Master Forces and friction for KCSE

Sign up free to unlock all 109 questions, track your progress, and get a personalised study plan for Physics.