JEE Main 2014
Physics in day to day Life – Part I
When a chalk stick is immersed in water, bubbles are emitted
A chalk stick is made up of compacted fine particles of calcium carbonate. This is a porous material containing a large number of interconnected pores of varying sizes. When the chalk is dry, these pores are filled with air.
When a piece of chalk is dipped into water, water is sucked into the fine pores. The air displaced from these fine pores gets released from the larger pores which open to the chalk surface. Thus, air comes out as bubbles
Tents are coated with a thin layer of aluminium hydroxide.
Tents and umbrella cloths are made of tightly woven fabric so that water with its high surface tension cannot easily penetrate. But falling raindrops can overcome the surface tension and wet the fabric which will then start seeping water to its other side.
Aluminium hydroxide is water insoluble and water repellent. Thus, it drains off the raindrops before they can wet the fabric. Hence, tents are coated with a thin layer of aluminium hydroxide.
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SCIENCE IN EVERYDAY LIFE PART- 2
The holding belts of a schoolbag are wide
Pressure is thrust per unit area. For a given thrust (force perpendicular to the surface), the greater the area on which the force acts, the less is the pressure due to the force. The holding belts of a school bag are made wide so that the pressure due to the weight of the schoolbag is reduced by increasing the area. It is then easier to bear the load of the bag.
A sheet of metal that sinks in water can float if shaped like a pan.
A sheet of metal sinks in water because its density is more than that of water. If the sheet is shaped like a pan, it can displace a large amount of water such that the upthrust on the pan due to water balances the weight of the pan. Hence, it can float on water.
A ship entering fresh water from sea water sinks more.
The buoyant force due to liquid is proportional to the density of the liquid. Hence, the volume of the liquid displaced by a floating body is inversely proportional to the density of the liquid. The density of freshwater is less than that of seawater. Hence, a ship displaces a greater volume of fresh water relative to that of seawater. Therefore, a ship entering fresh water from sea water sinks more.
A pointed nail can be easily hammered into wood than a blunt nail.
Pressure varies inversely as the area of the surface on which the force is applied, if the force remains constant. The tip of a pointed nail has extremely small area while that of a blunt nail has a comparatively large area. Therefore, a given force creates a large pressure on the pointed nail and it can be easily hammered into the wood, while a very less pressures is produced on the blunt nail by the same force and it cannot be easily hammered into the wood.
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INTERESTING Q & A
Why do we fall sideways when we are sitting in a bus and it takes a sharp turn?
When we sit in a bus and the bus is in motion, we are in a state of motion in the same direction. When the bus takes a sharp turn, our body tends to maintain the state of motion in the straight line due to inertia. The portion of our body in firm contact with the seat acquires the motion along the curved path, but the upper portion of our body, in loose contact with the seat, tends to move in the initial direction of motion. Hence, we fall sideways.
What happens when you shake a wet piece of cloth? Explain your observation.
When we shake a wet piece of cloth, water droplets come out. Initially, the wet piece of cloth is at rest. When the cloth is shaken, it is accelerated, but the water droplets in it, due to inertia, tend to maintain the state of rest. Hence, the droplets come out.
If brakes are suddenly applied to a moving car, the passengers in the car are pushed in the forward direction. Explain why.
The passengers in a moving car have the same velocity as that of the car. When brakes are suddenly applied to the car, it stops suddenly and the lower parts of the passengers’ bodies in contact with the seats, come to rest. The upper parts of their bodies, however, continue to be in a state of motion due to inertia. Hence, the passengers are pushed in the forward direction.
We get a backward push when the bus starts at one. Explain why.
When we sit or stand in a stationary bus, we are in a state of rest. When the bus starts suddenly, the lower parts of our bodies in contact with the bus acquire the motion of the bus. The upper parts of our bodies, however, continue to be in the state of rest due to inertia. Hence, we get a backward push when the bus starts at once.
A person alighting from a moving train is likely to fall in the direction of motion of the train. Explain why.
A person in a moving train has the same velocity as that of the train. After alighting from the moving train his feet come to rest on the platform. However, due to inertia, the upper part of his body continues to be in a state of motion in the direction of motion of the train. Hence, he is likely to fall in the direction of motion of the train.
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POINTS TO PONDER
Distance and displacement are different concepts.
Distance is the length of the actual path followed by a body between the points under consideration. It can be greater than the length of the straight line joining the two points. i.e. the initial and the final points. It does not involve the concept of direction.
Displacement is the shortest distance from the initial point to the final point of movement of a body. It involves the concept of direction. It, therefore, follows that distance and displacement are different concepts.
An object at rest can be considered to have uniform motion.
An object is said to be in uniform motion when it has constant speed. For an object at rest, its speed is zero all the time. Hence, an object at rest can be considered to have uniform motion.
When a body falls freely to the ground, its motion has a uniform acceleration
When a body falls freely, the only force acting on the body is that due to gravity. Near the earth’s surface, this force on a given body is (practically) uniform. Hence, a body falling freely to the ground has (almost) a uniform acceleration.
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DO U KNOW THIS?
The weather turns bitterly cold after a hailstorm.
Ice has a very high specific latent heat of fusion, 3.36 × 105 J/kg. A huge amount of heat is absorbed from the surrounding when ice starts melting after a hailstorm. As a result, the weather becomes bitterly cold.
The weather gets pleasant in cold countries when water starts freezing.
Every kilogram of water releases 3.36 × 105J of heat energy in the surroundings when it freezes. Therefore, the weather gets pleasant (comfortably warm) in the cold countries when water starts freezing.
Soft drinks are cooled by adding ice cubes rather than ice-cold water.
Ice has a large specific latent heat of fusion. Hence, melting ice cubes at 0 ºC absorb from the soft drink a large amount of heat equal to their latent heat. The mixture subsequently attains a lower equilibrium temperature than that could be attained if ice-cold water was used instead. Therefore, they are more effective in cooling a soft drink than ice-cold water.
Ice at 0 ºC feels colder than water at 0 ºC.
Ice absorbs a large amount of heat from the part of skin (finger or mouth) in contact with it when it melts. This occurs due to the large specific latent heat of fusion of ice. Therefore, ice at 0 ºC feels colder than water at 0 ºC.
A steam burn is more severe than that caused by boiling water.
Steam has a large specific latent heat of vaporisation (2260 kJ/kg). Thus, when steam condenses to form water at 100 ºC, additional heat is released compared to boiling water at 100 ºC. Hence, steam produces more severe burns than boiling water.
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Free Seminar: Preparing for JEE / NEET 2014
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