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Q5.1: NCERT Physics - Part 1 WORK, ENERGY AND POWER 5.1 Solution: The sign of work done by a force on a body is important to…
⚠️ Common Exam Traps & Conceptual Pitfalls
Students often equate uniform motion with zero work and assign a negative sign to the applied force, forgetting that work is evaluated for each individual force as $W=\vec F\cdot\vec s$. Even when the net work is zero (constant speed), the driving force does positive work while friction does an equal negative amount; the sign must be decided from the direction of that specific force relative to the displacement.
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.
Step-by-Step Verified Solution
**Answer:** (a) Positive (b) Negative (c) Negative (d) Positive (e) Negative
**Step-by-Step Explanation:**
(a) The man applies force upward and the bucket moves upward. Force and displacement are in the same direction, so work is positive.
(b) Gravitational force acts downward while displacement is upward. Force and displacement are opposite, so work is negative.
(c) Friction acts opposite to the direction of motion (up the incline), so work done by friction is negative.
(d) The applied force is in the direction of motion, so work is positive. Friction does negative work, but the question asks about the applied force.
(e) The resistive force of air opposes the motion of the pendulum, so work done is negative.
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