Rocket question (1 Viewer)

[smallcattle]

early astronauts in space only orbited the Earth at an altitude of between 80km and 200km. A typical rocket engine would provide a thrust of 380000N to a rocket with a total mass of 29830kg prior to lift off and would use 25315kg of fuel during the flight

Q: assuming the thrust is kept constant, calculate the final acceleration of this rocket immediately before the fuel would had been used


how to calculate the final acceleration???

 

[Xayma]

Im assuming it just wants F=ma.

However that doesn't account for a_g which will change during flight.

 

[m_isk]

pretty easy a=f/m and since they want the FINAL acceleration and thrust (force) is constant we use the final mass therefore a = 380 000/ (29830- 25315)

=84.16m/s^2

 

[Li0n]

Im assuming it just wants F=ma.

However that doesn't account for a_g which will change during flight.

so gay how they dont enable html in physics forums but they do in maths.

 

[Xayma]

I think it is enabled in the main physics forum though. Oh well it really isnt worth the time at the moment to go asking for it to be changed.

 

[zeropoint]

pretty easy a=f/m and since they want the FINAL acceleration and thrust (force) is constant we use the final mass therefore a = 380 000/ (29830- 25315)

=84.16m/s^2

You're forgetting that the F in F = ma refers to the _net_ force acting on the object so you must include upward thrust as well as downward gravitational force. a = (380 000 N - (29830 kg - 25315 kg)*9.8 ms^-2)/(29830 kg - 25315 kg) = 74 ms^-2. This ignores the fact that the gravitational field vector g decreases slightly with altitude.

 

[Jase]

okay this is something you need to be extremely careful about.

the final acceleration is given by a = NET Force / Mass

now the mass is (29830- 25315)kg = 4515kg

NetForce = T(thrust) - mg (not just thrust, theres also acceleration due to gravity as Xayma mentioned)

= 380000 - (4515)*(9.8)

a = (38000 - 4515*9.8) / 4515
= 74.36 m/s^2

EDIT: woops, zeropoint posted already.. but yeah.
just use this "formula"for simplicity's sake:

a = (T - mg) / m

 

[smallcattle]

ok...i got it, thats why i lost marks there, thx!