It seems like σ(t) is the positive function of the particle. Compute its acceleration by differentiating this function twice.
σ(t) = {t ², sin(t ), cos(t )}
σ'(t) = {2t, cos(t ), -sin(t )}
σ''(t) = {2, -sin(t ), -cos(t )}
When t = 0, the acceleration on the particle is
σ'' (0) = {2, -sin(0), -cos(0)} = {2, 0, -1}
Then the force acting on the particle at time t = 0 is
F = {2m, 0, -m}
by Newton's second law.
