Assumptions in Newton's Second Law

[kq6up]

With my understanding of Newton's second -- if I am going to use it to solve a problem, I would need to write ALL velocities in the system with respect to the lab frame (inertial rest frame) -- correct?

Thanks,
Chris

 

[A.T.]

With my understanding of Newton's second -- if I am going to use it to solve a problem, I would need to write ALL velocities in the system with respect to the lab frame (inertial rest frame) -- correct?

Velocity doesn't even appear in Newton's second law.

 

[kq6up]

It does in the rocket problem. $\dot{p}=\dot{m}v+m\dot{v}=F_{thrust}$. Also, if you are calculating the the change in momentum over some time interval $F=m\frac{\Delta v}{\Delta t}$.

Chris

 

[A.T.]

It does in the rocket problem. $\dot{p}=\dot{m}v+m\dot{v}=F_{thrust}$. Also, if you are calculating the the change in momentum over some time interval $F=m\frac{\Delta v}{\Delta t}$.

What, you have there is acceleration and average acceleration, which of course can be expressed in terms of velocity.