Q: An object of mass 2 Kg is moved from infinity to a point P. Initially that object was at rest but on reaching P its speed is 2m/s. The workdone in moving that object is -4J. Then potential at P is ………..J/Kg.

(a) 8

(b) -8

(c) 4

(d) -4

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Ans: (b)

Sol:W = ΔK.E + ΔP.E

$ \displaystyle W = \frac{1}{2}m v^2 -0 + U_P – U_{\infty} $

$ \displaystyle -4 = \frac{1}{2}\times 2 \times 2^2 – 0 + U_P – 0 $

-4 = 4 + U_{P}

U_{P} = -8