Answer this for me (1 Viewer)

[jamesfirst]

A rocket of intial mass 2000tonnes, produces a constant thrust of 8x 10^7 newtons during liftoff by 1500kg of exhaust gas per second
Calculate the net force acting on an 85kg astronaut 30 seconds after liftoff

 

[rawrence]

ohmagad completely forgot physics

 

[jamesfirst]

Anyone ?

 

[comm-u-nist]

i think by net force they mean g-forces, which is calculated by (g + a)/(g)

'g' is 9.8 m.s^-2 based on the presumption that we are on earth. as for 'a', the rocket is 1955000 kilograms after thirty seconds. thrust divided by this figure gives us acceleration upwards (F=ma), so just plug the figures into your calculator.

i got 5.175.

answer your question?

 

[comm-u-nist]

or it could be the upwards force of the rocket, (thrust minus weight) minus the weight force of the astronaut. which i will calculate now.

 

[comm-u-nist]

60838354.74 N up. the wording of the question is rather confusing, was there a diagram?

 

[K4M1N3]

Ok so we need to find mass after 30 secs = 2000000-(30 x 1500) (on converting to kg)
T = 8x10^7

using the formula:
a = (T-mg)/m
a = 31.12 (to 2 d.p.)

then use F = ma
where m = 85 (mass of astronaut)
a = 31.12 (as we found above)

therefore F = 2645 N

 

[jamesfirst]

Oh that was rather simple..

I was thinking along the a = (T-mg)/m

but didn't know what I could do with the acceleration. But what about the gravitational acceleration ??

 

[K4M1N3]

Hmmm yeah. I did use 9.8, but not sure if you need to use the acceleration due to gravity at a specific height after 30s. How many marks was the question?

 

[jamesfirst]

4


I think you're right. There's no fault in the working... (I think).

 

[hscishard]

Hmmm...getting confused by the answers made by other people

Forces acting on an astronaut are the ground reaction force (more like the rocket floor reaction force) and the force on the person produced from the force on the rocket.

Letting acceleration due to gravity remain constant (9.8) throughout it's flight, I got 3478.22 N

 

[hscishard]

Hmmm yeah. I did use 9.8, but not sure if you need to use the acceleration due to gravity at a specific height after 30s. How many marks was the question?

You wouldn't be able to do that with hsc phyics

 

[Fizzy_Cyst]

A rocket of intial mass 2000tonnes, produces a constant thrust of 8x 10^7 newtons during liftoff by 1500kg of exhaust gas per second
Calculate the net force acting on an 85kg astronaut 30 seconds after liftoff

Initial Weight Force:
= 2x10^6kg x 9.8
= 1.96x10^7N down

Weight Force after 30s:
= 1.96x10^7 - (1500*30*9.8)
= 1.9159x10^7N down

Thrust force:
= 8x10^7N up

Net Force:
= 8x10^7 - (1.9159x10^7)
= 6.0841x10^7N up

acceleration of rocket:
= Fnet/m
= 60.841x10^6/(2,000,000-(1500*30))
= 31.12ms-2 up

Fnet on astro:
= ma
= 85 * 31.12
= 2645.2N up

This is assuming constant 'g'

 

[hscishard]

Wouldn't the force be 2645.2 + mg?

 

[hscishard]

Oh..it's net force, not weight experienced