1. Meaning of Friction
Friction is a force that opposes the relative motion or tendency of motion between two surfaces in contact.
For example, when you push a box across the floor, friction acts in the opposite direction to its movement.
2. Types of Friction
1. Static friction: Acts between surfaces at rest and prevents motion from starting.
2. Limiting friction: The maximum static friction acting just before an object begins to move.
3. Sliding friction: Acts when one surface slides over another.
4. Rolling friction: Acts when an object rolls over a surface. It is usually smaller than sliding friction.
5. Fluid friction: Resistance experienced by objects moving through liquids or gases.
3. Laws and Factors Affecting Friction
Friction depends mainly on:
- The nature of the surfaces in contact.
- The force pressing the surfaces together (normal reaction).
Generally:
- Rough surfaces produce greater friction.
- Smooth surfaces produce less friction.
- Increasing the normal reaction increases friction.
For dry surfaces, friction is approximately independent of the apparent area of contact.
4. Coefficient of Friction
The coefficient of friction (μ) is the ratio of frictional force to normal reaction. This simply tells us how much friction exists between two surfaces compared with the force pressing them together.
-\mu = \frac{F}{R}
Where:
- μ = coefficient of friction
- F = frictional force
- R = normal reaction
Therefore:
-F = \mu R
Important: The coefficient of friction has no unit.
5. Limiting Friction
Limiting friction is the maximum frictional force just before motion begins.
-F\_{\text{max}} = \mu R
For an object on a horizontal surface:
-R = mg
Therefore:
-F\_{\text{max}} = \mu mg
6. Advantages of Friction
- Enables us to walk without slipping.
- Allows vehicles to move and stop.
- Makes writing with a pen or pencil possible.
- Helps us grip and hold objects.
- Enables nails and screws to remain in place.
7. Disadvantages of Friction
- Causes wear and tear of machine parts.
- Produces unwanted heat.
- Reduces the efficiency of machines.
- Opposes the movement of vehicles.
- Wastes useful mechanical energy.
8. Methods of Reducing Friction
- Applying lubricants such as oil and grease.
- Using ball bearings.
- Polishing surfaces.
- Using wheels and rollers.
- Streamlining vehicles to reduce air resistance.
9. Methods of Increasing Friction
- Making surfaces rougher.
- Providing grooves on vehicle tyres.
- Using spikes on football boots.
- Applying sand to slippery surfaces.
- Using materials with better grip.
10. Important WASSCE Calculations
Example 1: A block experiences a frictional force of 15 N when the normal reaction is 50 N. Find the coefficient of friction.
Solution:
-\mu = \frac{F}{R} = \frac{15}{50}
-\boxed{\mu = 0.3}
Example 2: A 10 kg object rests on a horizontal surface. If the coefficient of friction is 0.2, calculate the limiting friction. Take g = 10 m/s².
Solution:
-R = mg = 10 \times 10 = 100N
-F = \mu R = 0.2 \times 100
-\boxed{F = 20N}
Key Points to Remember for WASSCE
1. Friction always opposes relative motion or its tendency.
2. Limiting friction is the maximum static friction.
3. Rolling friction is generally less than sliding friction.
4. Coefficient of friction has no unit.
5. Lubricants reduce friction.
6. Friction produces heat.
7. Normal reaction acts perpendicular to the contact surface.
8. At constant velocity, the resultant force is zero.
9. On a horizontal surface, with no other vertical forces, R = mg.
10. Limiting friction = μR.
HINT: As a registered and paid user, you get detailed answers to the Objective Tests with our quiz engine.