give 4 examples of positive, negative and zero work done with diagram
ZERO WORK DONE
Work can be zero even when there is a force. The centripetal force in a uniform circular motion, for example, does zero work since the kinetic energy of the moving object doesn 't change. This is because the force is always perpendicular to the motion of the object; only the component of a force parallel to the velocity vector of an object can do work on that object. Likewise when a book sits on a table, the table does no work on the book despite exerting a force equivalent to mg upwards, because no energy is transferred into or out of the book.
If the table were moving upward at a constant velocity, then it will be doing work on the book, since the force of the table on the book will be acting through a distance. However, the force of gravity will be doing equal and opposite work on the book, and the net rate of work done on the book will still be zero, as evidenced by the fact that its kinetic energy remains constant throughout the process.
We explain it as follows:
1. When θ is acute (<90), cosθ is positive. Hence work done is positive.
example: when a body falls freely under the action of gravity.
cos0 = 1. therefore work done by the gravity on the body falling freely is +ve.
2. When θ is acute (>90), cosθ is -ve. Hence work done is -ve.
example: when a body is thrown up, its motion is opposed by the gravity. the angle θ b/w gravitational force and displacement is 180. cos 180 = -1, therefore work done by the gravity on a body moving upward is -ve.
when, θ = 90
cos90 = 0
work done is said to be zero.
example: when a body tied to one end of a string is rotating in a circle, work done by the centripetal force applied along the string is zero because
θ = 90
Hope this will help you to understand the concept.
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