Find the instantaneous direction of force in following case (magnetic field and velocity are indicated)

Explanation

As charge is positive, the direction of force is same as direction of `(vec(v) xx vec(B))`. So by Right hand thumb rule, curling the fingers of right hand from `vec(v) " to " vec(B)` thumb extends in direction of `-x`. Thus magnetic force at the instant shown is along `-hat(i)` direction.
We can also do the problem by writing v and B in vector form.
`vec(v) = v vec(j)`
`vec(B) = B (-hat(k))`
`vec(F) = q (vec(v) xx vec(B)) = q (v hat(j) xx B (-hat(k))`
`= -qvB [hat(j) xx hat(k)]`
`= - q v B hat(i)`
Thus magnetic force is in `(-hat(i))` direction