Look at the activities listed below. Reason out whether or not work is done in the light of your understanding of the term ‘work’.
• Suma is swimming in a pond.
• A donkey is carrying a load on its back.
• A wind-mill is lifting water from a well.
• A green plant is carrying out photosynthesis.
• An engine is pulling a train.
• Food grains are getting dried in the sun.
• A sailboat is moving due to wind energy.
Work is done whenever the given two conditions are satisfied:
(i) Force is applied.
(ii) Displacement takes place.
(iii) The angle between the force and displacement is not 90 degrees .
(a) When Suma is swimming in a swimming pool, she applies force in forward direction and moves in the forward direction. So, displacement is in the forward direction. Both are in the same direction. Therefore, work is done.
(b) When a donkey is carrying a load on his back, it is applied a force in the upward direction. But, displacement of the load is in the forward direction. Since, displacement is perpendicular to force, the work done is zero.
(c) When a wind-mill is lifting water from a well, it is applying a force in the upward direction and it is moving water in an upward direction. Hence, work is done by the wind-mill in lifting water from the well.
(d) In this case, there is no force involved when a green plant is carrying photosynthesis.
Therefore, the work done is zero.
(e) When an engine is pulling a train, it is applying a force in the forward direction. This allows the train to move in the forward direction. Hence, there is a displacement in the train in the same direction. Therefore, work is done by the engine on the train.
(f) When food grains are getting dried in the Sun, there is no force involved. Hence, the work done is zero during the process of food grains getting dried in the Sun.
(g) When a sailboat is moving due to wind energy, force is applied by the boat in the forward direction. Therefore, there is a displacement in the boat in the direction of force. Hence, work is done by wind on the boat.
An object of mass 40 kg is raised to a height of 5 m above the ground. What is its potential energy? If the object is allowed to fall, find its kinetic energy when it is half-way down.
Soni says that the acceleration in an object could be zero even when several forces are acting on it. Do you agree with her? Why?
Illustrate the law of conservation of energy by discussing the energy changes which occur when we draw a pendulum bob to one side and allow it to oscillate. Why does the bob eventually come to rest? What happens to its energy eventually? Is it a violation of the law of conservation of energy?
A freely falling object eventually stops on reaching the ground. What happenes to its kinetic energy?
Define 1 J of work.
In each of the following a force, F is acting on an object of mass, m. The direction of displacement is from west to east shown by the longer arrow. Observe the diagrams carefully and state whether the work done by the force is negative, positive or zero.
Does the transfer of energy take place when you push a huge rock with all your might and fail to move it? Where is the energy you spend going?
The potential energy of a freely falling object decreases progressively. Does this violate the law of conservation of energy? Why?
The kinetic energy of an object of mass, m moving with a velocity of 5 m s-1 is 25 J. What will be its kinetic energy when its velocity is doubled? What will be its kinetic energy when its velocity is increased three times?
Certain force acting on a 20 kg mass changes its velocity from 5 m s-1 to 2 m s-1. Calculate the work done by the force.
Which of the following has more inertia: (a) a rubber ball and a stone of the same size? (b) a bicycle and a train? (c) a five-rupees coin and a one-rupee coin?
State the universal law of gravitation.
Which of the following are matter?
Chair, air, love, smell, hate, almonds, thought, cold, cold-drink, smell of perfume.
What is meant by a pure substance?
How does the sound produced by a vibrating object in a medium reach your ear?
In a reaction, 5.3 g of sodium carbonate reacted with 6 g of ethanoic acid. The products were 2.2 g of carbon dioxide, 0.9 g water and 8.2 g of sodium observations are in agreement with the law of conservation of mass.
sodium carbonate + ethanoic acid → sodium ethanoate + carbon dioxide + water
What are canal rays?
State any two conditions essential for good health.
How is our atmosphere different from the atmospheres on Venus and Mars?
Who discovered cells, and how?
What can you say about the motion of an object if its speed-time graph is a straight line parallel to the time axis?
Explain the working and application of a sonar.
Why is the cell called the structural and functional unit of life?
Name the technique to separate
(i) butter from curd,
(ii) salt from sea-water,
(iii) camphor from salt.
What is genetic manipulation? How is it useful in agricultural practices?
A train is travelling at a speed of 90 km h-1. Brakes are applied so as to produce a uniform acceleration of – 0.5 m s-2. Find how far the train will go before it is brought to rest.
What type of clothes should we wear in summer?
According to the third law of motion when we push on an object, the object pushes back on us with an equal and opposite force. If the object is a massive truck parked along the roadside, it will probably not move. A student justifies this by answering that the two opposite and equal forces cancel each other. Comment on this logic and explain why the truck does not move.
How does the force of gravitation between two objects change when the distance between them is reduced to half ?
Why will a sheet of paper fall slower than one that is crumpled into a ball?