Facts that Matter
- Friction: The force which opposes the relative motion between two surfaces in contact is called friction. The force of friction always opposes the applied force that may be push or pull.
Fig. 12.1 Friction opposes relative motion between the surfaces of the book and the table.
Friction is caused by the irregularities on the two surfaces in contact. Irregularities on the two surfaces lock into one another (interlocking) causing friction.
- Spring Balance: It is a device used for measuring the force acting on an object. It consists of a coiled spring, a pointer moving on a graduated scale. When a force is applied, stretching of spring takes place. The reading on the scale indicated by the pointer gives the magnitude of the force.
- Static Friction : The force required to overcome friction at the instant an object starts moving from rest is a measure of static friction.
- Sliding Friction: The force required to keep an object moving with the same speed is a measure of sliding friction. The sliding friction is slightly smaller than the static friction.
- Rolling Friction: When a body rolls over the surface of another body, the resistance to its movement is called rolling friction. Rolling reduces friction.
- Factors Affecting Friction: ä Friction depends on the nature of surfaces in contact. ä Friction is more between rough surfaces and less in smooth surfaces.
- Friction depends on how hard the two surfaces press together.
- Friction is independent of the area of contact.
- Friction is essential in following ways:
- We could not hold articles such as glass tumbler and other things without friction.
It becomes very difficult to hold a greasy glass.
- We could not write with a pen or pencil if there is no friction.
When we write with chalk on the blackboard, its rough surface rubs off some chalk particles which stick to the blackboard. It helps us to read what is written on the blackboard with a chalk.
- Friction helps objects to move, stop or to change the direction of motion. We cannot walk without friction.
- Without friction no building could be constructed.
- Friction is an evil:
- It wears out materials. For example, soles of shoes, ball bearings, steps of a stair, parts of machines etc.
- Friction produces heat. When a machine is operated, heat generated causes much wastage of energy.
- We deliberately increase friction in some cases to get the desired results. For example:
- Soles of shoes are grooved. It is done to provide the shoes better grip on the floor, so that we can safely walk on it.
- The treated tyres of cars, trucks, buses, bulldozers provide better grip with the ground.
- Bicycles, scooters and other automobiles are provided with the brake system. When we press the brake lever, brake pads arrest the motion of the rim due to friction. The wheel stops moving
- Sometimes we want to minimise the friction. Friction can be reduced by applying fine powder as in case of carrom. Oil, grease or graphite is applied between the moving parts of a machine to reduce friction. These things avoid interlocking of irregularities that lead to reduction of friction to a great extent.
Sometimes an air cushion between the moving parts is used to reduce friction.
- Lubricants: The substances which reduce friction are called lubricants. Lubricants form a thin layer on the moving surfaces. So, they do not directly rub against each other.
- The wheel is said to be one of the greatest inventions of mankind.
- Fluids: I n Science, the common name of gases and liquids is fluid.
- Fluid Friction: The force exerted by fluids on objects in motion through them is called fluid friction. The frictional force exerted by fluids is also called drag
- The frictional force on an object in a fluid depends on ä its s peed with respect to the fluid,
- the shape of the object, and
- the nature of the fluid.
- To overcome drag, objects are provided with special shape called streamlined. For example, shape of an aeroplane, a bird and a fish.