JEE Main & Advanced
JEE Main Paper (Held On 25 April 2013)
question_answer
In an experiment, a small steel ball falls through a liquid at a constant speed of 10cm/s. If the steel ball is pulled upward with a force equal to twice its effective weight, how fast will it move upward?
JEE Main Online Paper ( Held On 25 April 2013 )
A) 5 cm/s
B) Zero
C) 10 cm/s
D) (d) 20 cm/s
Correct Answer:
C
Solution :
Weight of the body \[W=mg=\frac{4}{3}\pi {{r}^{3}}\rho g\] \[T=\frac{4}{3}\pi {{r}^{3}}\sigma g\]and\[F=6\pi \eta vt\] When the body attains terminal velocity net force acting on the body is zero. i.e., W-T-F=0 And terminal velocity\[v=\frac{2}{9}\frac{{{r}^{2}}(\rho -\sigma )g}{\eta }\] As in case of upward motion upward force is twice its effective weight, therefore, it will move with same speed 10 cm/s