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Physics Part I

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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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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, 

Mass of the body, m = 2 kg

Force acting on the body, F = 7N

Coefficient of friction, 
space straight mu equals 0.1 

Time taken, t = 10 sec



The force of friction between block and surface is,

                       f equals mu space m g space equals space 2 straight N

 The net accelerating force on body is,

                   F - f = 7 - 2 = 5N

Therefore,

Acceleration, a equals F over m equals 2.5 m divided by s squared

Now the displacement undergone by the body in 10 sec is,
                    straight S equals ut plus 1 half at squared

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

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(b) Work done by friction force is,

     space space straight W subscript friction space equals space straight f times straight s space equals space 2 cross times 125 space equals space 250 space straight J

(c) Work done by net force is,

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




743 Views

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.
288 Views

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. 
290 Views

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. 
272 Views

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.
272 Views