Friction is the force that opposes relative motion between two surfaces in contact. Friction acts along the surfaces, opposite to the direction of movement.

A block is pressed onto a surface with a normal force of 200 N, and the coefficient of friction is 0.3. Calculate the friction force.

The friction force is 60 N. Friction force = 0.3 × 200 = 60 N.

Where is friction useful in mechanical systems, and where is it a problem?

Friction is essential in brakes, clutches and belt drives. Friction wastes energy and causes wear in bearings, gears and slides.

How does a cantilever brake work?

Two pivoted arms, one on each side of a bicycle rim, carry brake blocks. A cable pull swings the arms together so the blocks squeeze the rim.

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Mechanical and Pneumatic Systems

Friction, brakes and clutches

3 things to know
A2 1

Overview

Static and dynamic friction and the coefficient of friction, cantilever, band, disc and drum brakes with a worked braking torque, and single plate, diaphragm, cone and centrifugal clutches.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

What is friction?

Key term

What is the difference between static and dynamic friction?

Key term

Define the coefficient of friction.

Questions from this subtopic

Each one with the model answer underneath.

1
What is friction?
Model answer
Friction is the force that opposes relative motion between two surfaces in contact. Friction acts along the surfaces, opposite to the direction of movement.
2
What is the difference between static and dynamic friction?
Model answer
Static friction must be overcome to start a stationary object moving. Dynamic friction resists motion once sliding has started and is usually smaller.
3
Define the coefficient of friction.
Model answer
The coefficient of friction is the friction force divided by the normal force pressing the surfaces together. The value depends on the pair of materials and has no units.
4
A block is pressed onto a surface with a normal force of 200 N, and the coefficient of friction is 0.3. Calculate the friction force.
Model answer
The friction force is 60 N. Friction force = 0.3 × 200 = 60 N.
5
Where is friction useful in mechanical systems, and where is it a problem?
Model answer
Friction is essential in brakes, clutches and belt drives. Friction wastes energy and causes wear in bearings, gears and slides.
6
How does a cantilever brake work?
Model answer
Two pivoted arms, one on each side of a bicycle rim, carry brake blocks. A cable pull swings the arms together so the blocks squeeze the rim.

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