How Do You Calculate a Rocket's Initial Upward Acceleration?

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Homework Help Overview

The discussion revolves around calculating the initial upward acceleration of a rocket, given its mass and thrust while considering gravitational forces. The problem is situated within the context of classical mechanics, specifically focusing on forces and acceleration.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning, Assumption checking

Approaches and Questions Raised

  • Participants discuss the application of Newton's second law (F = ma) and question the correct incorporation of gravitational force into their calculations. There are attempts to clarify the net force acting on the rocket and whether the initial acceleration can be derived directly from the thrust and weight.

Discussion Status

Several participants have provided insights into the correct approach to summing forces and calculating acceleration. There is an ongoing exploration of the implications of gravitational force and the initial conditions of the rocket's motion. While some guidance has been offered, there is no explicit consensus on the final method to be used.

Contextual Notes

Participants note the importance of not neglecting gravitational force in their calculations. There is also mention of the potential complexity introduced by variable mass due to fuel consumption, although the focus remains on the initial acceleration.

bewger
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Homework Statement



Compute the initial upward acceleration of a rocket of mass 1.2 multiplied by 10^4 kg if the initial upward force produced by its engine (the thrust) is 2.7 multiplied by 10^5 N. Do not neglect the gravitational force on the rocket.


Homework Equations



F = ma

The Attempt at a Solution



I tried doing \frac{9.8(Thrust - mass)}{mass} = a

I get a = 210.7 m/s^2

but that's wrong.

Can anybody tell me what I'm doing wrong?
 
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bewger said:
I tried doing \frac{9.8(Thrust - mass)}{mass} = a
Not sure how you got that one.

Stick with ΣF = ma. What forces act on the rocket? What's the net force?
 
So, if a = F/m, then is that also the initial acceleration also?

It stated not to neglect gravity, do I multiply gravity by the mass into the equation?
 
Your formula is correct, except for the gravity factor which you are missing.
Think about forces, gravity is downward, the thrust is upward. You'll want the net force upward for the acceleration. So that should be everything you need.
 
bewger said:
So, if a = F/m, then is that also the initial acceleration also?
Yes.
It stated not to neglect gravity, do I multiply gravity by the mass into the equation?
In order to get the force due to gravity--which is the rocket's weight--you would multiply the mass by g = 9.8 m/s^2.
 
Sum the forces, draw a free body diagram if you need to
[PLAIN]http://img411.imageshack.us/img411/2641/blockb.png

Fg = force of gravity = mass * gravity
Ft = thrust of rocket

sum of the forces in Y = Ft - Fg = ma;

solve for a
 
Last edited by a moderator:
Thank you everyone for your help.
Much appreciated.
 
fizzynoob said:
Sum the forces, draw a free body diagram if you need to
[PLAIN]http://img411.imageshack.us/img411/2641/blockb.png

Fg = force of gravity = mass * gravity
Ft = thrust of rocket

sum of the forces in Y = Ft - Fg = ma;

solve for a

wait, is this a variable weight problem, like the fuel has a burn rate so mass decreases as time goes on?
 
Last edited by a moderator:
They just want the initial acceleration of the rocket. Therefore you don't need to know the rate at which the fuel is being burned. Just sum the forces, and solve for A
 

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