# Homework Help: Initial upward acceleration

1. Sep 27, 2010

### bewger

1. The problem statement, all variables and given/known data

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.

2. Relevant equations

F = ma

3. The attempt at a solution

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

I get a = 210.7 m/s^2

but thats wrong.

Can anybody tell me what I'm doing wrong?

2. Sep 27, 2010

### Staff: Mentor

Not sure how you got that one.

Stick with ΣF = ma. What forces act on the rocket? What's the net force?

3. Sep 27, 2010

### 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?

4. Sep 27, 2010

### 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.

5. Sep 27, 2010

### Staff: Mentor

Yes.
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.

6. Sep 27, 2010

### fizzynoob

Sum the forces, draw a free body diagram if you need to
[PLAIN]http://img411.imageshack.us/img411/2641/blockb.png [Broken]

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: May 4, 2017
7. Sep 27, 2010

### bewger

Thank you everyone for your help.
Much appreciated.

8. Sep 27, 2010

### Liquidxlax

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: May 4, 2017
9. Sep 27, 2010

### 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