Posted by **Dreskillz89** on Sunday, September 20, 2009 at 3:59pm.

A small 7.10kg rocket burns fuel that exerts a time-varying upward force on the rocket. This force obeys the equation F = A + Bt^2. Measurements show that at t = 0, the force is 121.0N, and at the end of the first 2.10s , it is 176.0N.

Part A

Find the net force on this rocket the instant after the fuel ignites.

Part B

Find the acceleration of this rocket the instant after the fuel ignites.

Part C

Find the net force on this rocket 4.10s after fuel ignition.

Part D

Find the acceleration of this rocket 4.10s after fuel ignition.

I found out that

A = 121.0 N

B = 12.5 N/s^2

Net Force = 51.42 N

Acceleration = 7.24 m/s^2

Part C and Part D is what I am having trouble with.

- Physics -
**Dreskillz89**, Sunday, September 20, 2009 at 4:05pm
By the way, the A = 121.0 and B = 12.5 refers to the equation F=A+Bt^2, not Part A and Part B.

- Physics -
**MathMate**, Sunday, September 20, 2009 at 4:32pm
A, B, are correct.

a) and b) are also correct.

For part (c), the question is asking for the force, F(t) = A+Bt², using the values of A and B that you have found earlier, and t=4.1 s. F(4.1) should be over 300 m/s/s.

Once you have calculated the force, you can proceed to calculate the acceleration as in part b).

- Physics -
**Dreskillz89**, Sunday, September 20, 2009 at 8:36pm
For Part C, I calculated it to be 331.125N, but am still getting a wrong answer. I know you said it was over 300, is that what you got too?

- Physics -
**MathMate**, Sunday, September 20, 2009 at 9:40pm
I have the same number as you have, but based on your values of A and B, which I found minor discrepancies. If it is a computer answer, you may want to recheck from there on.

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