中四physics mechanics既一條問題.....

to calculate the result force, let the angle between between two force is θ

( 5 + 5 ) cos(θ/2) = 5
cos(θ/2) = 0.5
θ/2 = 60
θ= 120
remark: each of the force can be resolved into 2 component, one is Fcos(θ/2) and another is Fsin(θ/2). for the direction cos(θ/2) , two component forces are in the same direction and can then be added up dirctly, for the direction sin(θ/2) the 2 component forces are in opposite direction and they cancel each other.

the angle is 90 degrees
a force can't influence other forces in 90 degree