2. A large rocket has a mass of 2x10^6 kg at takeoff, and its engines produce a thrust of 2.50x10^7N. Neglect air resistance.

Question

a. Draw the free-body diagram for the rocket

b. Find its initial acceleration if it takes off vertically.

c.  How long does it take to reach a velocity of 120 km/h straight up, assuming constant mass and thrust?

d. In reality, the mass of a rocket decreases significantly as its fuel is consumed. Describe qualitatively how this affects the acceleration and time for this motion.

Answer 1

This is the answer. I scanned it and have posted it as a picture

Answer 2

a) Always include the weight in a force diagram on earth. Thrust is upward and weight (mg) is downward.

The net force upwards is Thrust - mg

b) Second law states that a = Fnet/m = (Thrust - mg)/m

c) time to reach a velocity of 120 km/hr (convert to m/s) given the a in (b) and the initial velocity of 0 is now purely a kinematics problem. Use the definition of constant a:

a = (v - v0)/t or v/t in this case.

d) Look at the expression in (b). What happens to the acceleration if the thrust is constant and the mass decreases with time?