2.8.1 Dot Point Help! (1 Viewer)

[freezeice04]

A rocket has mass 400kg, 75% being fuel. It develops a thrust of 8000N. Calculate its initial acceleration.

How do I do this?

 

[khorne]

Remember, thrust is constant...you're only using f=ma, i.e 8000 = m*a i.e 8000 = 400*a, etc. this also shows, as the fuel is used up, the acceleration of the rocket increases

 

[freezeice04]

Remember, thrust is constant...you're only using f=ma, i.e 8000 = m*a i.e 8000 = 400*a, etc. this also shows, as the fuel is used up, the acceleration of the rocket increases

The answer says its 10.2m/s^2, not 8000/400m/s^2

 

[TheTutorBot]

I'm getting 20m/s^2 as well

its simple f=ma

 

[freezeice04]

Well the answer has something different. Can someone clarify if the answers is wrong or not? I have a feeling it has something to do with momentum though.

 

[TheTutorBot]

there is nothing relating to momentum in the question (except that we use the fact that force = momentum X velocity)

 

[freezeice04]

I don't think doing f=ma is right since there is another similar question and it has a completely different answer.
Dotpoint 2.8: Analyse the changing acceleration of a rocket during launch in terms of Law of Conservation of Momentum and the forces experienced by astronauts.

 

[freezeice04]

Wait I think I know now, its F=ma then you minus 9.8 (from gravity).

 

[ohexploitable]

 

[s2 SEductive]

a= T-mg/m

T = 8000N
m = 400kg
g = 9.8m/s/s

a = 8000 - ( 400 x 9.8) / 400
a = 10.2 m/s/s

 

[TheTutorBot]

Wait I think I know now, its F=ma then you minus 9.8 (from gravity).

Never subtract g from an answer...I don't recall a case where you have to anywhere in physics.

and ohexploitable, who actually knows that formula? (and why didn't I think of it?)

 

[ohexploitable]

and ohexploitable, who actually knows that formula? (and why didn't I think of it?)

 

[ohexploitable]

Never subtract g from an answer...I don't recall a case where you have to anywhere in physics.

Kay, say there is a 1000kg car and I apply a force of 1N upwards, since I don't subtract mg then that means there is a net force upwards. This means I can lift a one tonne car with a finger.

brb, I'm gonna try and lift up my parent's car lols.