Give the sign of work done by a force in following cases 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 above case
Answers
(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.
Ans.
(a) It is clear that the direction of both the force and the displacement are the same and thus the work done on it is positive.
(b) It can be noted that the displacement of the object is in an upward direction whereas, the force due to gravity is in a downward direction. Hence, the work done is negative.
(c) It can be observed that the direction of motion of the object is opposite to the direction of the frictional force. So, the work done is negative.
(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 on the object. So, the motion of the object and the applied force are in the same direction. Thus, the work done is positive.
(e) It is noted that the direction of the bob and the resistive force of air which is acting on it are in opposite directions. Thus, the work done is negative.