Work And Energy Class 9 Pdf Download

Work, Energy and Power are fundamental concepts of Physics. Work is said to be done when a force (push or pull) applied to an object causes a displacement of the object. We define the capacity to do the work as energy. Power is the work done per unit of time. This article discusses work, energy and power in detail.

For work to be done, a force must be exerted and there must be motion or displacement in the direction of the force. The work done by a force acting on an object is equal to the magnitude of the force multiplied by the distance moved in the direction of the force. Work has only magnitude and no direction. Hence, work is a scalar quantity.

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Where W is the work done, F is the force, d is the displacement, tag_hash_110 is the angle between force and displacement and F cos is the component of force in the direction of displacement.

An object is horizontally dragged across the surface by a 100 N force acting parallel to the surface. Find out the amount of work done by the force in moving the object through a distance of 8 m.

Energy is the ability to perform work. Energy can neither be created nor destroyed, and it can only be transformed from one form to another. The unit of Energy is the same as of Work, i.e. Joules. Energy is found in many things, and thus there are different types of energy.

All forms of energy are either kinetic or potential. The energy in motion is known as Kinetic Energy, whereas Potential Energy is the energy stored in an object and is measured by the amount of work done.

Power is a physical concept with several different meanings, depending on the context and the available information. We can define power as the rate of doing work, and it is the amount of energy consumed per unit of time.

(d) The object which is moving in a rough horizontal plane faces the frictional force, which is opposite to the direction of the motion. To maintain a uniform velocity, a uniform force is applied to the object. So, the motion of the object and the applied force are in the same direction. Thus, the work done is positive.

Que.2. A body of mass 2 kg initially at rest moves under the action of an applied horizontal force of 7 N on a table with the coefficient of kinetic friction = 0.1. Compute the

(a) work done by the applied force in 10 s.

(b) work done by friction in 10 s.

(d) change in kinetic energy of the body in 10 s.

Que. 3. Given in the figure are examples of some potential energy functions in one dimension. The total energy of the particle is indicated by a cross on the ordinate axis. In each case, specify the regions, if any, in which the particle cannot be found for the given energy. Also, indicate the minimum total energy the particle must have in each case. Think of simple physical contexts for which these potential energy shapes are relevant.

(a) For the region x = 0 and x = a, potential energy is zero. So, kinetic energy is positive. For x > a, the potential energy has a value greater than E. So, kinetic energy becomes zero. Thus, the particle will not exist in the region x > a.

The minimum total energy that the particle can have, in this case, is zero.

Ans. The force due to gravity is a conservative force. The work done on a closed path by the conservative force is zero. Hence, for every complete orbit of the comet, the work done by the gravitational force is zero.

gradually due to dissipation against atmospheric resistance, however small. Why, then, does its speed increase progressively as it comes closer and closer to the earth?

(d) In Fig. 6.13(i), the man walks 2 m carrying a mass of 15 kg on his hands. In Fig., he walks the same distance pulling the rope behind him. The rope goes over a pulley, and a mass of 15 kg hangs at its other end. In which case is the work done greater?

When a body is displaced in the direction of the force, positive work is done on the body by the conservative force, due to which the body moves to the centre of force. Thus, the separation between the two decreases and the potential energy of the body decreases.

(d) In an inelastic collision of two bodies, the quantities which do not change after

the collision are the total kinetic energy/total linear momentum/total energy of

the system of two bodies

(d) If the potential energy of two billiard balls depends only on the separation distance between their centres, is the collision elastic or inelastic? (Note, we are talking here of potential energy corresponding to the force during a collision, not gravitational potential energy).

Que. 13. A raindrop of radius 2 mm falls from a height of 500 m above the ground. It falls with decreasing acceleration (due to the viscous resistance of the air) until, at half its original height, it attains its maximum (terminal) speed and moves with uniform speed thereafter. What is the work done by the gravitational force on the drop in the first and second half of its journey? What is the work done by the resistive force in the entire journey if its speed on reaching the ground is 10 ms-1?

Q.18. The bob of a pendulum is released from a horizontal position. If the length of the pendulum is 1.5 m, what is the speed with which the bob arrives at the lowermost point, given that it dissipated 5% of its initial energy against air resistance?

Q.21. The windmill sweeps a circle of area A with its blades. If the velocity of the wind is perpendicular to the circle, find the air passing through it in time t and also the kinetic energy of the air. 25 % of the wind energy is converted into electrical energy, and v = 36 km/h, A = 30 m2 and the density of the air is 1.2 kg m-3. What is the electrical power produced?

Q. 22. A person trying to lose weight (dieter) lifts a 10 kg mass one thousand times to a height of 0.5 m each time. Assume that the potential energy lost each time she lowers the mass is dissipated. (a) How much work does she do against the gravitational force? (b) Fat supplies 3.8 107J of energy per kilogram which is converted to mechanical energy with a 20% efficiency rate. How much fat will the dieter use up?

Q.23. A family uses 8 kW of power. (a) Direct solar energy is incident on the horizontal surface at an average rate of 200 W per square meter. If 20% of this energy can be converted to useful electrical energy, how large an area is needed to supply 8 kW?

(b) Compare this area to that of the roof of a typical house.

The bolt does not rebound. So, the whole of the potential energy gets converted to heat energy. The heat produced will remain the same even if the lift is stationary since the value of acceleration due to gravity is the same in all inertial systems.

The potential energy of two masses in a system is inversely proportional to the distance between them. The potential energy of the system of two balls will decrease as they get closer to each other. When the balls touch each other, the potential energy becomes zero, i.e., at r = 2R. The potential energy curve in (i), (ii), (iii), (iv) and (vi) do not satisfy these conditions. So, there is no elastic collision.

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Fill in each blank with the appropriate term from the brackets.

(a) A bucketful of water is to be drawn from a well. ....... will be done when a ......... is applied to do this, because there will be a ........... of water.

(displacement, work, force)

(b) If a ball is dropped on the sloping roof of a house, it acquires ...... and falls on the ground. That is, transformation of ........ energy into .......... energy takes place.

(kinetic, potential, motion)

(c) You might have seen some beautiful fireworks during Diwali. It is an example of transformation of ....... energy into....... energy.

(light, atom, chemical, solar)

(d) The solar cooker is an application of the ...... energy of the sun, while solar cells, solar lamps are applications of the ...... energy of the sun.

(light, chemical, heat)

(e) One labourer carried four pans of road metal through a distance of 100 metres. If he carries two pans of road metal through a 200 metre distance ......... work will be done.

(equal, more, less)

(f) The capacity that an object has for doing work is called ....... .

(energy, displacement, force)

(a) A bucketful of water is to be drawn from a well. Work will be done when a force is applied to do this, because there will be a displacement of water.

(b) If a ball is dropped on the sloping roof of a house, it acquires motion and falls on the ground. That is, transformation of potential energy into kinetic energy takes place.

(c) You might have seen some beautiful fireworks during Diwali. It is an example of transformation of chemical energy into light energy.

(d) The solar cooker is an application of the heat energy of the sun, while solar cells, solar lamps are applications of the light energy of the sun.

(e) One labourer carried four pans of road metal through a distance of 100 metres. If he carries two pans of road metal through a 200 metre distance equal work will be done.