SIR PLZ CLEAR MY DOUBT FOR THIS QUESTION I HAVE ASKED THIS QUESTION ALREADY BUT THE ANSWER GIVEN TO ME - Physics - Basic Mathematics

Question

SIR PLZ CLEAR MY DOUBT FOR THIS QUESTION I HAVE ASKED THIS QUESTION
ALREADY BUT THE ANSWER GIVEN TO ME WAS NOT HELP FULL A particle was
projected with initial velocity 20m/s at 60 degree to horizontal
After what time its velocity will be perpendicular to initial
direction of motion CHAPTER NAME - VECTOR

Rajnish Kumar · Accepted Answer

Dear student
 strategy :  first write the initial velocity in vector form u→                   : then write the final velcity after time t when particle makes makes angle 900 with u→ let v→                       :last make dot product of u→
 v→=0 which gives you t as below u→=20cos600i^+20sin600j^=10i^+103j^v→=u→+a→t=10i^+103j^+10t(-j)=10i^+(103-10t)j^(10i^+103j^)
{10i^+(103-10t)j^}=0100+300=1003tt=43 secRegards