Chapter 9: Friction
The Force of Friction
Friction is a force that opposes the motion between two surfaces in contact. It always acts in the direction opposite to the direction of motion. If you push a book on a table, friction acts in the opposite direction of your push.
Factors Affecting Friction
Friction is caused by the irregularities (hills and valleys) on the two surfaces in contact. Even surfaces that appear smooth have microscopic irregularities.
- Nature of the Surfaces: Rough surfaces have larger irregularities and therefore produce greater friction. Smooth surfaces produce less friction.
- Force Pressing the Surfaces Together: The harder the surfaces are pressed together, the greater the interlocking of irregularities, and thus, the greater the friction. This is why it's harder to push a heavy box than a light one.
Types of Friction
Friction can be categorized based on the state of motion of the object.
- Static Friction: The friction that comes into play when we try to move an object at rest. It is a self-adjusting force that increases as the applied force increases, up to a certain limit.
- Sliding Friction: The friction that comes into play when an object is sliding over another surface. Sliding friction is slightly smaller than the maximum static friction.
- Rolling Friction: The friction that comes into play when an object (like a wheel or a ball) rolls over a surface. Rolling friction is much smaller than sliding friction, which is why it is easier to roll an object than to slide it.
Friction: A Necessary Evil
Friction is considered a "necessary evil" because it has both advantages and disadvantages.
Advantages (Necessary)
- Allows us to walk without slipping.
- Enables cars to move and stop (brakes).
- Makes it possible to write with a pen or chalk.
- Allows us to light a matchstick.
Disadvantages (Evil)
- Causes wear and tear of machine parts and shoe soles.
- Produces heat, which can damage machinery.
- Reduces the efficiency of machines by wasting energy.
- Slows down or stops moving objects.
Increasing and Reducing Friction
We deliberately increase or decrease friction based on the situation.
Methods to Increase Friction:
- Treads on Tyres: The grooved patterns on tyres increase friction for a better grip on the road.
- Spikes on Shoes: Athletes use shoes with spikes to increase friction and prevent slipping.
- Brake Pads: In vehicles, brake pads are used to create high friction to stop the wheels.
Methods to Reduce Friction:
- Lubricants: Substances like oil, grease, or graphite are applied between moving parts to form a thin layer, reducing friction.
- Polishing: Making surfaces smoother reduces irregularities and friction.
- Ball Bearings: Using ball bearings between moving parts converts sliding friction into much smaller rolling friction.
Fluid Friction (Drag)
Gases and liquids are collectively known as fluids. The frictional force exerted by fluids is called drag.
Fluid friction depends on:
- The speed of the object with respect to the fluid.
- The shape of the object.
- The nature of the fluid.
To minimize drag, objects moving through fluids are given special shapes. This is called streamlining. Notice the streamlined shapes of fish, birds, aeroplanes, and racing cars, which are designed to reduce fluid friction and allow for easier movement.
Frequently Asked Questions (FAQ)
Why is it easier to roll a suitcase than to slide it?
It is easier because rolling friction is significantly less than sliding friction. The wheels reduce the area of contact and the interlocking between surfaces.
What is drag?
Drag is the frictional force that acts on objects moving through fluids (liquids or gases). It opposes the motion of the object.
How do lubricants work?
Lubricants form a thin layer between two surfaces, which prevents the irregularities on the surfaces from interlocking directly. This reduces friction and allows for smoother movement.
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