a) 
b) 5843 N
Explanation:
a)
The position of the UFO at time t is given by the vector:

Therefore it has 3 components:

We start by finding the velocity of the UFO, which is given by the derivative of the position:

And then, by differentiating again, we find the acceleration:

The weight of the UFO is W = 12,500 N, so its mass is:

Therefore, the components of the force on the UFO are given by Newton's second law:

So, Substituting t = 2 s, we find:

So the net force on the UFO at t = 2 s is

b)
The magnitude of a 3-dimensional vector is given by

where
 are the three components of the vector
 are the three components of the vector
In this problem, the three components of the net force are:

Therefore, substituting into the equation, we find the magnitude of the net force:
