What is the Net Force on a Helicopter at 5 Seconds?

[xmflea]

The position of a 2.75x10^5N helicopter under test is given by

r=(0.020m/s^2)t^3i + (2.2m/s)tj - (0.060m/s^2)t^2k

Find the net force of the helicopter at t=5.0s express the vector F in the form Fx, Fy, and Fz

so far i plugged in 5.0 to each component in the r equation which gave me different masses: 2.5, 11.0, and 1.5

then i divided each mass by 5.0 to get velocity, then i plugged everything into the average acceleration formula to get different accelerations, and then i finally used F=ma to plug into get different forces for Fx Fy and Fz
.. and it looks like I am just going in a huge circle. not sure how to do this.

 

[Kurdt]

How do you find acceleration when given position as a function of time?