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Q5.1(a):
The sign of work done by a force on a body is important to understand. State carefully if the following quantity is positive or negative: work done by a man in lifting a bucket out of a well by means of a rope tied to the bucket.

Solution :

Step 1: Definition of Work Done

In classical mechanics, the work done ($W$) by a constant force ($\vec{F}$) on an object undergoing a displacement ($\vec{d}$) is defined as the scalar product of the two vectors:

$W = \vec{F} \cdot \vec{d} = |\vec{F}| |\vec{d}| \cos(\theta)$

[Per the definition of Work in Newtonian Mechanics]

Step 2: Analysis of Vectors Involved

To determine the sign of the work done by the man, we must identify the direction of the force exerted by the man relative to the direction of the bucket's displacement:

  • Force ($\vec{F}$): The man exerts an upward force on the rope to pull the bucket out of the well against the force of gravity. Thus, the direction of the force is upward.
  • Displacement ($\vec{d}$): As the bucket is being lifted out of the well, it moves vertically from a lower position to a higher position. Thus, the direction of the displacement is upward.

Step 3: Determination of the Angle ($\theta$)

Since both the force applied by the man and the displacement of the bucket are directed upward, the vectors $\vec{F}$ and $\vec{d}$ are parallel to each other.

Therefore, the angle $\theta$ between the force vector and the displacement vector is $0^{\circ}$.

Applying the cosine function: $\cos(0^{\circ}) = 1$ [By trigonometric definition].

Step 4: Calculation of Work Sign

Substituting the values into the work equation:

$W = F \cdot d \cdot \cos(0^{\circ})$

$W = F \cdot d \cdot (1)$

$W = Fd$

Since both $F$ (magnitude of force) and $d$ (magnitude of displacement) are positive quantities, the product $W$ must be positive.

Visual Representation of Force and Displacement

$\vec{F}$ $\vec{d}$

Final Solution: The work done by the man is positive, as the force applied is in the same direction as the displacement of the bucket.


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