Question

A block of mass $\mathrm{M}$ is hanging over a smooth and light pulley through a light string. The other of the string is pulled by a constant force $\mathrm{F}$. The kinetic energy of the block increases by $20 \mathrm{J}$ in 1s
A The tension in the string is $\mathrm{Mg}$
B) The tension in the string is $\mathrm{F}$
C) The work done by the tension on the block is $20 \mathrm{J}$ in the above $1 \mathrm{s}$.
D) The work done by the force of gravity is $-20 \mathrm{J}$ in the above $1 \mathrm{s}$

Answer 1

Hint: We know that in physics, the kinetic energy of an object is the energy that it possesses due to its motion. It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its speed changes. On the other hand, potential energy is energy that is stored or conserved in an object or substance. This stored energy is based on the position, arrangement or state of the object or substance. We can think of it as energy that has the 'potential' to do work. Based on this concept we have to solve this question.

Complete step by step answer
We should know that tension is nothing but the drawing force acting on the body when it is hung from objects like chain, cable, string etc. It is represented by T. The resultant forces at the ends must be equal in magnitude and opposite in direction, along the line of the joints of the member. These forces are called axial forces. The member is said to be in compression if T is negative (that is, the forces at each end are toward each other) or in tension if T is positive.
Tension in the string would be equal to the force exerted on the other end of the string. Thus $\mathrm{T}=\mathrm{F}$.
Net work done on the body is equal to the gain in kinetic energy of the block.
Thus increase in kinetic energy in $1 \mathrm{s}=$ Work done by tension on the block in $1 \mathrm{s}+$ Work done by gravity on the block in $1 \mathrm{s}$.
$\Rightarrow 20\text{J}={{\text{W}}_{\text{T}}}+{{\text{W}}_{\text{G}}}$
Thus $\mathrm{W}_{\mathrm{T}} \neq 20 \mathrm{J}$
$\mathrm{W}_{\mathrm{G}} \neq 20 \mathrm{J}$

So, the correct answer is option B.

Note: We should know that kinetic energy is built up in an object by motion and can be defined as the energy that is needed to either slow it down or speed it up. Studies have also been done using kinetic energy and then converting it to other types of energy, which is then used to power everything from flashlights to radios and more. Energy is transferred from one object to another when a reaction takes place. Energy comes in many forms and can be transferred from one object to another as heat, light, or motion, to name a few.
On the other hand, potential energy becomes useful when it is converted to kinetic energy. Kinetic energy is used to perform work. Work occurs when force is applied to an object. Kinetic energy is useful because it allows work to be done. Potential energy is the energy by virtue of an object's position relative to other objects. Potential energy is often associated with restoring forces such as a spring or the force of gravity. This work is stored in the force field, which is said to be stored as potential energy.