Find maximum thrust of a rocket blasting off

[RUphysics3]

Homework Statement

A 2.49×104-kg rocket blasts off vertically from the Earth's surface with a constant acceleration. During the motion considered in the problem, assume thatg remains constant. Inside the rocket, a 14.5-N instrument hangs from a wire that can support a maximum tension of 37.3 N .

I found that :

Acceleration = 15.42 m/s^2

Find the minimum time for this rocket to reach the sound barrier (330m/s) without breaking the inside wire.

The answer to this was 21.4 .

How far is the rocket above the Earth's surface when it breaks the sound barrier?
3530 m.

Now I have to find the maximum vertical thrust of the rocket engines under these conditions.

Homework Equations

I tried using F=ma and some basic kinematics.

The Attempt at a Solution

Using F=ma...
i tried F=(2.49x10^4) * 15.42 . This wasn't correct.

 

[Titan97]

Tension acts on the instrument and on the rocket. (Downwards on rocket and upwards on the instrument) Let acceleration of rocket be $a$. Since the string is inextensible, the instrument will also have same acceleration. Write Newtons law equation for both rocket and instrument.

 

[Mister T]

F=(2.49x10^4) * 15.42 . This wasn't correct.

Is this the net force on the rocket or the thrust force?

 

[RUphysics3]

Is this the net force on the rocket or the thrust force?

Well it's a vertical launch..So wouldn't it be the same thing? the only component is in the y direction.. I don't really understand thrust that well. I think it's something like thrust comes out the bottom of the rocket and propels it upwards so the thrust acts upwards? I really don't know

 

[RUphysics3]

Tension acts on the instrument and on the rocket. (Downwards on rocket and upwards on the instrument) Let acceleration of rocket be $a$. Since the string is inextensible, the instrument will also have same acceleration. Write Newtons law equation for both rocket and instrument.

What do you mean?
So for the rocket it would be F=(2.49x10^4) * 15.42
For the instrument.. F=(14.5N/9.81) * 15.42 ? Now what?

 

[Mister T]

Well it's a vertical launch..So wouldn't it be the same thing? the only component is in the y direction.. I don't really understand thrust that well. I think it's something like thrust comes out the bottom of the rocket and propels it upwards so the thrust acts upwards? I really don't know

Thrust is the exhaust gas pushing upward on the rocket. Gravitational force (weight) is Earth pulling downward on the rocket.

 

[RUphysics3]

Thrust is the exhaust gas pushing upward on the rocket. Gravitational force (weight) is Earth pulling downward on the rocket.

So weight would be the mass times 9.8. But i did mass * acceleration..Okay.. so should I find weight of the rocket and subtract that from the upward thrust?

 

[haruspex]

So weight would be the mass times 9.8. But i did mass * acceleration..Okay.. so should I find weight of the rocket and subtract that from the upward thrust?

To get the net force, yes.