Posted by **JC** on Thursday, November 6, 2008 at 6:26am.

Sorry for posting so many, but I just need to make sure I'm on the right track.

Just need someone to check my working and answers.

A rocket has a mass 3000kg. The fuel used in the rocket has an additonal mass of 500kg. At the launch, the rocket accelerates at 4m/s/s. Using 10m/s/s as gravity.

a) Calculate the net force on the rocket.

f=ma (Newton's 2nd Law)

=3500 x 4

=14000 N

b) What is the weight of the rocket at launch?

w=mg

=3500 x 10

=35000N

c) Determine the upward force exerted by the rocket motors at the instant of the launch

F(net)=W + F(up)

=35000 + (4 x 35000)

=175000N (upwards)

d) The rocket takes 3 minutes to burn all of it's fuel. If the rocket motors suply a constant force, would you expect the acceleration to remain constant during the first 3 minutes. Explain your answer.

No. If the rocket takes 3 mintues to burn all of it's fuel, during that time, the mass of the fuel decreases. Since the force remains the same thorughout the 3 mintues, the rocket will accelerate when there is less mass.

Determine

e) the upward force acting on the rocket after the fuel tank beocme empty.

F(up)=0N

f)the acceleration of the rocket after this instant.

10m/s/s due to gravity.

- Physics -
**drwls**, Thursday, November 6, 2008 at 7:50am
Check your decimal poing on (c). You went from 14,000 net force to 140,000

d) The acceleration rate increases as the mass (including fuel) decreases

e) is correct. So is (f). Upward thrust ends when the fuel is gone

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