7. A boy throws a table tennis ball of mass 20 g upwards with a velocity of u 0 = 10 m/s at an angle q 0 with

the vertical. The wind imparts a horizontal force of 0.08 N, so that the ball returns to the starting point.

Then, the angle q 0 must be such that, tan q 0 is

(A) 0.2 (B) 0.4 (C) 2.5 (D) 1.2

Dear Deepak

see the pic for the solution

All the best.

AKASH GOYAL

AskiitiansExpert-IITD

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for verticle motion

 Uy _{= U0}COS@   &   ay = -g

 applying  V_Y = U_Y + a_yt

    V_y at topmost point is zero so

 t= u_y/g

tim of flight (T)= 2t=2U_y/g .........1

for horizontal motion

U_x = U_OCOS_@

 S_x = U_xT + a_xT²

finally particle reaches to same point so displacement  is zero ..

 0 = U_xT + a_xT²/2    .........2

putting value of T in eq 2

U_y/U_x = g/a  .........3

ma_x = force =0.08

a_{x =} 4m/s²

now , U_Y/U_{X = Uo}cos@/U_osin@

                       _{=tan@=10/4=2.5}

option c is correct