medium difficulty5 min

Friction as a Force

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

In one line: Friction resists slipping at the contact surface.

Practice This Concept

Understand it

Friction acts along the surfaces in contact and usually opposes sliding or attempted sliding. Static friction acts before sliding begins, while kinetic friction acts when surfaces are sliding.

Example

When a book is pushed on a table, friction acts opposite to the direction in which the book tends to slide.

Rule to remember

At this level, focus on direction and type: static friction prevents slipping, kinetic friction acts during sliding.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

1If a box tends to move right on a rough floor, friction acts left.
2If the push is small and the box does not move, static friction balances the push.

Method you can reuse

Identify contact surfaces, find intended or actual sliding direction, draw friction opposite to relative motion or tendency.

Sketch it in your answer

Draw a block on a rough surface with applied force in one direction and friction arrow opposite along the surface.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

Why does a pushed book eventually stop on a table?

Think first, then show answer

The book stops because friction between the book and table acts opposite to its motion and reduces its speed.

Common wrong answer

Saying there is no friction when a pushed box does not move is wrong; static friction may be balancing the push.

Don't mix this up

Students sometimes say friction always acts opposite to motion, but more accurately it opposes relative motion or tendency of motion at the contact surface.

Exam tip

Mention the two surfaces and the direction of possible sliding when explaining friction.

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

How CBSE asks it

Questions ask direction of friction, difference between static and kinetic friction, and useful or harmful effects in daily life.

1
Enough for 1 mark
Friction is a contact force that opposes relative motion or the tendency of relative motion between two surfaces in contact.
2
Add for 2 marks
Example: When a book is pushed on a table, friction acts opposite to the direction in which the book tends to slide.
3
Add for 3 marks
Friction acts along the surfaces in contact and usually opposes sliding or attempted sliding.

MCQ Quiz

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