Class 8 Science: Friction

Friction is not simply “bad”: it lets shoes grip the floor, brakes slow a bicycle and a pencil leave a mark. The useful question is whether friction is helping or wasting energy in the situation.

Forces on a box sliding right: push right, friction left, normal upward and weight downward — Original force model: draw directions before deciding which force changes the motion.

What to understand

Describe the central idea in your own words.
Use a model, observation or relationship to explain a changed example.
Notice and correct the likely misconception in the worked reasoning.

Forces at a contact

Friction acts at a contact between surfaces and opposes their relative motion or attempted motion. Roughness, pressing surfaces together and the material pair can affect its size. Lubricants can reduce direct rubbing, while treads and rough soles can increase grip.

When a box starts moving, static friction has been overcome. Once sliding begins, kinetic friction often behaves differently. Do not confuse the direction of motion with the direction of friction: friction points opposite the relative sliding.

Worked reasoning: For a book sliding right across a desk, draw an arrow left for friction. If the book is pushed harder, friction may increase because the surfaces press together more, but acceleration also depends on the net force.

Try, explain and check

Diagram

Draw the push, friction and normal contact force on a box.

Show a reasoning prompt

State the evidence, relationship or model that supports your answer, then check whether the units and direction make sense.

Application

Why are bicycle brake pads made to create friction?

Show a reasoning prompt

State the evidence, relationship or model that supports your answer, then check whether the units and direction make sense.

Misconception

Friction does not always stop motion; it can make motion possible by preventing slipping.

Show a reasoning prompt

State the evidence, relationship or model that supports your answer, then check whether the units and direction make sense.

Next step

Write a three-sentence revision summary, then continue to the related resource. For extra practice, visit the resource library and choose a worksheet that matches the skill you need.

About this resource
Prepared as part of the BIS Quiz learning resource. It is supplementary study material; use your teacher's instructions and prescribed materials when they differ. Report a factual error or unclear explanation.