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

1. Apr 9, 2012

### Mdhiggenz

1. The problem statement, all variables and given/known data

The position of a 2.75x10^5 N training helicopter under test is given by...?
vector r = (0.020m/s^3)t^3ihat+(2.2 m/s)tjhat-(0.060m/s^2)t^2 khat

Find the net force on the helicopter at t=5.0s

Express your answer in terms of ihat, jhat , and khat . Express your coefficient using two significant figures.

2. Relevant equations

3. The attempt at a solution

I dont understand why me must take the second derivative, already solved it just want some clarification on to why we get the second derivative.

Thank you!

2. Apr 9, 2012

### cepheid

Staff Emeritus

By definition, the second derivative of position with respect to time is the acceleration. This should be pretty intuitive:

velocity is the rate of change of position with time.

acceleration is the rate of change of velocity with time.

Hence, velocity is the derivative of position, and acceleration is the derivative of velocity.

3. Apr 9, 2012

### Mdhiggenz

I think I understand, since force = sigmaF = Ma

we have the force, and we are solving for the acceleration in order to multiply it by the mass to get the net?

4. Apr 9, 2012

### cepheid

Staff Emeritus
Yes, exactly. If you know the acceleration, and the mass, then you know the net force. That is because of Newton's 2nd Law.