A rocket on its launch pad has a mass of 20,000kg. The engine fires at t=0s and produces a constant force of 500,000 N straight up. The engines fire for 1 minute during which the entire 10,000kg of fuel on poard is consumed and expelled from the rocket at a constant rate. Ignore air resistance and assume that the force of gravity is constant. We will find the velocity of the rocket as its engines stop firing.
a. Note the initial and final mass of the rocket. Write down an equation for m(t), the mass as a function of time. You will introduce a constant k which represents the rate at which fuell is burned. Make sure you have the correct units for k.
b. Draw a free-body diagram, and write down the equation (or equations) that govern the motion. Note that the mass has to be m(t).
c. From your answer to b, write down an expression for the velocity. Your answer may be left in the form of a definite integral. You do not have to evaluate the integral.
F_net = ma
x_f = x_i + v_i*t + 1/2 at^2
The Attempt at a Solution
I got a linear equation for part a, m(t) = 20,000 - 166.6t, k being 166.6. For b, I plugged that in for m in the first equation above, then solved for a and plugged that into the second equation, but I'm not sure if that's right. I ended up with x(t) = (250,000t^2)/(20,000 - 166.6t). c has me really stumped though. I'd really appreciate a quick answer, as this is due tomorrow morning. Thanks!