Question

A person brings a mass of $1kg$ from infinity to a point $A$. Initially the mass was at rest but it moves at a speed of $2m/s$ as it reaches $A$. The work done by a person on the mass is $3J$. The potential at $A$is:

• A. $-3J/kg$
  
• B. $-2J/kg$
  
• C. $-5J/kg$
  
• D. none of these

Answer 1

Answer: (C)

$-5J/kg$

$M=1kg$

$v={2ms}^{-1}$

$W=-3J$

$\therefore KE=\frac{1}{2}{mV}^2=\frac{1}{2}\times 1\times 2\times 2=2J$

Now, $W=PE+KE$

or $PE=W-KE$

$=(-3-2)J=-5J$

$\therefore$ Potential per unit mass =$\frac{PE}{mass}=\frac{-5}{1}$ $=-5J/kg$