rocket question... from past trial paper (1 Viewer)

[HSC*is*near]

1) why does the acceleration of a rocket increase as it travels up through the atmosphere? (2 marks)

2) rockets used to put satellites into orbit have more than one stage. why? (3 marks)

i am not sure how to answer these questions.. i mean what exactly should i write...

 

[Riviet]

1) why does the acceleration of a rocket increase as it travels up through the atmosphere? (2 marks)

Basically, the mass gradually decreases since it is constantly burning its fuel, so as the fuel is used up, the rocket loses mass as it travels up into the atmosphere. Since the total force on the rocket remains constant and the mass decreases, the acceleration increases. We can observe this in the equation F=ma.

 

[alcalder]

Basically, the mass gradually decreases since it is constantly burning its fuel, so as the fuel is used up, the rocket loses mass as it travels up into the atmosphere. Since the total force on the rocket remains constant and the mass decreases, the acceleration increases. We can observe this in the equation F=ma.

Additional to that, as you get further away from earth, the pull of gravity (acceleration due to gravity) reduces.

Since:

g = Gm/r²

So as r (the distance between the space ship and earth) increases, g decreases.

Now the reason rockets have more than one stage - I think that also has something to do with jettisoning mass as each fuel stage is used up. Thus, you need less thrust to lift the smaller space ship after each stage is jettisoned. I saw something about this on Wikipedia not long ago. Yep, here it is:

http://en.wikipedia.org/wiki/Staged_Rocket

 

[alcalder]

Well indeed. THere are 4 forces on a rocket (there is a paper on this on the NASA website - google Rockets forces) - THRUST, GRAVITY, LIFT and DRAG.

THRUST is due to the engines
GRAVITY - would decrease as the rocket lifts due to less mass and greater distance from the Earth.
DRAG - would decrease as the atmosphere thins
LIFT - would also decrease to some extent as the atmosphere thins (this is the lift that an aeroplane feels on its wing) because there would be less pressure on the fins of the rocket.

 

[Mountain.Dew]

alcalder, be careful when u call gravity a 'force'. gravity is not a force, its a field. WEIGHT FORCE is the force involved under the influence of gravity. think of gravity as a field, a field which exerts influence on objects of mass.

just a point of clarification, all other reasoning is good.

you explained the 2nd point well, just want to add that there are the economic ramficiations involved. less fuel = less expenses = more funds in budget to improve safety, supplies or nagivation technology, etc...this reasoning applies when we talk about the slingshot effect as well.

 

[alcalder]

*palm forehead* Indeed, gravity is not a force - it is weight. I do apologise. F=mg. It is the force due to gravity - weight force.

 

[Mountain.Dew]

*palm forehead* Indeed, gravity is not a force - it is weight. I do apologise. F=mg. It is the force due to gravity - weight force.

please, no need to apologise. we are all here to help

 

[Forbidden.]

*palm forehead* Indeed, gravity is not a force - it is weight. I do apologise. F=mg. It is the force due to gravity - weight force.

Aside from electromagnetic, strong and weak nuclear forces ... I swore I remember that gravity was a force ...

 

[SoulSearcher]

Aside from electromagnetic, strong and weak nuclear forces ... I swore I remember that gravity was a force ...

Misconception.
http://en.wikipedia.org/wiki/Fundamental_interaction might explain it better than I do, in fact most people would be able to explain it better than I do

 

[Forbidden.]

Misconception.
http://en.wikipedia.org/wiki/Fundamental_interaction might explain it better than I do, in fact most people would be able to explain it better than I do

Well if forces are actually fundamental interactions, could YOU explain it to me ?

 

[SoulSearcher]

Only gravity, electromagnetic force, weak nuclear force and strong nuclear force are fundamental interactions. The rest are indeed forces.

 

[M-turkey]

I think it might get confusing for some to state that Gravity is not considered a force.

For all partical purposes, we do consider Gravity as a force, it acts just the same as any other force. When we say gravity, we usually are refering to the effective force.

For people doing Year 11 and 12, I think it's easier to say that Gravity is a force. Wait until Uni physics (and some philiosophy) to talk about whether interactions are indeed forces, fictional forces etc.