Physics - Dynamics (2)

hei man

there is something wrong with the above answer
if Tcos(15) cancel mg
there will be no vertical acceleration and the bob would not go down and only accelerate horizontally

the correct way to do it
is to decomposed mg to mgsin15 (in the direction shown in the above figure) and mgcos15 ( in opposite direction of T)

while at this moment since we can think that the ball is actually moving with 0 speed
so the required (radial direction) centripetal force =mv^2/r=0
so the T must cancel out mgcos15
T=mgcos15

while in tangential direction
the net force is mgsin15
the acceleration is g sin15 only
since the only acceleration is in tangential direction
the magnitude net acceleration is g sin15

We always do circular motion questions in the way that we docomposed all the forces in tangential direction and radial direction.Then we can consider the tangential and radial acceleration.