m દળ અને l લંબાઇના સાદા લોલકને દોરી સમક્ષિ

P.E. of bob at point $ A = mgl $ This amount of energy will be converted into kinetic energy K.E. of bob at point $B = mgl $ and as the collision between bob and block (of same mass) is elastic so after collision bob will come to rest and total Kinetic energy will be transferred to block.

So kinetic energy of block $ = mgl$