﻿ The casing of a rocket in flight burns up due to friction. At whose expense is the heat energy required for burning obtained– the rocket or the atmosphere? from Physics Work, Energy and Power Class 11 CBSE
The casing of a rocket in flight burns up due to friction. At whose expense is the heat energy required for burning obtained– the rocket or the atmosphere?

The heat energy required to burn the casing of rocket is at the cost of kinetic energy of the rocket.
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The sign of work done by a force on a body is important to understand. State carefully if the following quantities are positive or negative:
b) Work done by gravitational force when a man in lifting a bucket out of a well by means of a rope tied to the bucket.

Negative

In the given case, the direction of force (vertically downward) and displacement (vertically upward) are opposite to each other. Hence, the sign of work done is negative.
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A body of mass 2 kg initially at rest moves under the action of an applied horizontal forces of 7N on a table with co-efficient of kinetic friction 0.1. Compute:
(a) the work done by the applied force in 10 sec.,
(b) the work done by the friction force in 10 sec.,
(c) the work done by the net force on the body in 10 sec.,
(d) the change in kinetic energy of the body in 10 sec and interpret your results.

It is given that
m = 2 kg,                    F = 7N
,                      t = 10 sec

The force of friction between block and surface is,

The net accelerating force on body is,
F - f = 7 - 2 = 5N
∴     Acceleration,
Now the displacement undergone by the body in 10 sec is,

(a) Work done by applied force in 10 sec.,

(b) Work done by friction force is,

(c) Work done by net force is,

(d) Change in kinetic energy of body is equal to the work done by net force = 625 J.

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The sign of work done by a force on a body is important to understand. State carefully if the following quantities are positive or negative:

(d) work done by an applied force on a body moving on a rough horizontal plane with uniform velocity

Positive quantity.

In this case, the body is moving on a rough horizontal plane. Frictional force opposes the motion of the body. Therefore, in order to maintain a uniform velocity, a uniform force must be applied to the body. Since the applied force acts in the direction of motion of the body, the work done is positive.
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The sign of work done by a force on a body is important to understand. State carefully if the following quantities are positive or negative:

(a) Work done by a man in lifting a bucket out of a well by means of a rope tied to the bucket.

Positive

In the given case, force and displacement are in the same direction. Hence, the sign of work done is positive. In this case, the work is done on the bucket.
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The sign of work done by a force on a body is important to understand. State carefully if the following quantities are positive or negative:

(c) work done by friction on a body sliding down an inclined plane

Negative quantity

The direction of the frictional force is opposite to the direction of motion. Therefore, the work done by the frictional force is negative in this case.
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