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

[bewger]

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?

 

[Doc Al]

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?

 

[bewger]

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?

 

[Olly4]

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.

 

[Doc Al]

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.

 

[fizzynoob]

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

 

[bewger]

Thank you everyone for your help.
Much appreciated.

 

[Liquidxlax]

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?

 

[fizzynoob]

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