Question 6.1 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.
(b) work done by gravitational force in the above case,
(c) work done by friction on a body sliding down an inclined plane,
(d) work done by an applied force on a body moving on a rough horizontal plane with uniform velocity,
(e) work done by the resistive force of air on a vibrating pendulum in bringing it to rest.
Chapter Work, Energy And Power Page 134
Answers
Answered by
24
Hi________
hope u know that
work=force*distance
when we work is done on system then
w=+ve
when system work on we then
then
w=-ve.
(a) 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.
(b) 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.
(c) Negative
Since the direction of frictional force is opposite to the direction of motion, the work done by frictional force is negative in this case.
(d) Positive
Here 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.
(e) Negative
The resistive force of air acts in the direction opposite to the direction of motion of the pendulum. Hence, the work done is negative in this case.
hope it help u
hope u know that
work=force*distance
when we work is done on system then
w=+ve
when system work on we then
then
w=-ve.
(a) 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.
(b) 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.
(c) Negative
Since the direction of frictional force is opposite to the direction of motion, the work done by frictional force is negative in this case.
(d) Positive
Here 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.
(e) Negative
The resistive force of air acts in the direction opposite to the direction of motion of the pendulum. Hence, the work done is negative in this case.
hope it help u
Answered by
14
(a) A bucket is lefted by man into a well .
bucket applied vertically upward force and bucket also moves vertically upward .e.g angle between force and displacement is 0°.
W = F.Scos0° = FS → positive .
_____________________________________________
(b) Gravitational force acts vertically downward and the bucket moves vertically upwards .e.g angle between the gravitational force and displacement is 180° .
W = F.Scos180° = -FS → negative
________________________________________________
(c) We know,
Frictional force apposes the relative motion , therefore friction on a body sliding down an inclined plane is in opposite direction of motion.
so, W = F.Scos180° = - F.S →negative
________________________________________________
(d) the body moves along the the direction of applied force e.g angle between force and displacement = 0°
W = F.S cos0° = F.S → positive
___________________________________________________
(e) we know ,
resistive force opposes the motion , therefore angle between resistive force and displacement is 180°.
W = F.Scos180° = -F.S →negative
__________________________________________________
bucket applied vertically upward force and bucket also moves vertically upward .e.g angle between force and displacement is 0°.
W = F.Scos0° = FS → positive .
_____________________________________________
(b) Gravitational force acts vertically downward and the bucket moves vertically upwards .e.g angle between the gravitational force and displacement is 180° .
W = F.Scos180° = -FS → negative
________________________________________________
(c) We know,
Frictional force apposes the relative motion , therefore friction on a body sliding down an inclined plane is in opposite direction of motion.
so, W = F.Scos180° = - F.S →negative
________________________________________________
(d) the body moves along the the direction of applied force e.g angle between force and displacement = 0°
W = F.S cos0° = F.S → positive
___________________________________________________
(e) we know ,
resistive force opposes the motion , therefore angle between resistive force and displacement is 180°.
W = F.Scos180° = -F.S →negative
__________________________________________________
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