Net Force On Rocket. (1 Viewer)

[dolbinau]

Ok, now that I've got your attention .

The net force acting on a body in a rocket is f=m(g+a) right? (i.e the sum of the force of gravity and the acceleration of the rocket). Why is it then, do CSSA ignore this in the first written question of the 2004 exam?

 

[dolbinau]

hey, I'm talking about the force of a body inside the rocket (e.g) human

So, a 40kg person in a rocket accelerating at 40ms^(-2) on earth would be:

F=40(40+9.8)

At least, that's what I've learnt.

 

[dolbinau]

Thanks.

 

[Jase]

Hi, I'm, a little confused by that equation you posted.

Newton's law is net Force = ma

the 'a' means acceleration. This acceleration already takes into account the effect of gravtiy (hence NET force). Thus the net force of any rocket is simply F = ma.

However.. the formula the HSC uses takes into account the individual forces by the rocket and gravity. Assume the rocket places a constant upwards force called "thrust" or T. Then.. gravity puills DOWN with a force of mg.

The true net force is then T - mg. Then subbing into newtons second law:

T - mg = ma.

a = (T - mg) / m

Thus the acceleration of the rocket will depend on the thrust of the rocket, the strength of gravity and the mass of the rocket. The acceleration that you referred to would be the acceleration due to thrust ALONE... without any gravity, which wouldn't be possible to find on earth.

I hope i havent confused you. But im not sure about the CSSA2004.. what did that question say?

 

[lolokay]

yeah that's right. F = ma is the net force; F = m(a+g) is the thrust force.

 

[darkchild69]

Be careful!

In questions such as this, it is assumed that the thrust of the engines remains constant!

Meaning that as a rocket burns fuel and decreases in mass, the Weight vector decreases and as such, the net force actually INCREASES (as the engine thrust vector remains constant).

So indeed, net force does vary, it increases, which results in an increased acceleration

I am unsure of the actual question itself, but i'd imagine they may ask you the net force on a rocket at a specific instant in time, if they give you the mass of the rocket and the thrust supplied by rocket or it's acceleration.