Past Trial Paper (1 Viewer)

[jeremychung]

Question:

A rocket of initial mass 2000 tonnes, produces a constant thrust of 80000000N during liftoff by expelling 1500kg of exhaust gases per second.
Calculate: the net force acting on an 85kg astronaut 30s after liftoff

So how do you do this? I used the formula a = T/m (ie without the -mg) but i didn't get the answer.

The answer is 3478.26N.

Do we have to take the change of g into consideration?

 

[Lucid Scintilla]

Question:

A rocket of initial mass 2000 tonnes, produces a constant thrust of 80000000N during liftoff by expelling 1500kg of exhaust gases per second.
Calculate: the net force acting on an 85kg astronaut 30s after liftoff

So how do you do this? I used the formula a = T/m (ie without the -mg) but i didn't get the answer.

The answer is 3478.26N.

Do we have to take the change of g into consideration?

Hi, what did you do?

If you used a=T/m, then you should have got the substitution line:
a=80,000,000/(2,000,000-45,000)
a=40.92[blah]

Which you then substitute into Newton's 2nd law, F=ma, to get:
F=85 x (40.92)
F=the answer.

 

[dolbinau]

But don't you have to add gravity as well?

F=85(9.8+40.92)

Which isn't the answer according to above, but I can't see how it is wrong. Unless after 30 seconds you aren't in the atmosphere anymore? Are you? I don't know; Seems unlikely.

 

[sirfeathers]

30 sec into the flight, (1500*30)=45 000kg of fuel will already have been depleted.

This means that the total mass of the rocket 30 sec into the flight is 2 000 000 - 45 000 = 1 955 000 kg

Now since thrust on the rocket is constant at 80 000 000N, the acceleration on the rocket (and anyone inside it) 30 sec into the flight will be a = F/m = 80 000 000/ 1 955 000 = 40.92 ms-2

So an 85kg astronaut in the rocket will experience a force of F = ma = 85*40.92 = 3478.26N as required

 

[dolbinau]

But, again, don't we have to consider gravity too?

 

[dolbinau]

*bump*

 

[sirfeathers]

But, again, don't we have to consider gravity too?

Hmmm. Look, I'm just a fellow HSC student like you, so I don't have any more authority than you when it comes to Physics, but I've done many questions like this and none of them take gravity into consideration.

Don't get me wrong - this is a very valid point to make. It's confusing for me too sometimes because most of the mechanics(=physics) topics in 4U maths do take gravity into consideration.

I think that in these kinds of HSC questions where the value of g is changing due to altitude, gravity is disregarded in the same way that air resistance is - the maths that would be required to consider it is beyond what is expected of HSC physics students. So don't let it stew you over.

 

[dolbinau]

Thanks guys. I remember using the formula F=m(g+a) too, to calculate G forces where we use a as acceleration of the rocket and g as gravity has this come up?

I don't really care what the *real* answer is I just want the correct answer that will give me the marks .

If we are told a rocket is accelerating at 20ms upon liftoff on a 50KG person; calculate the net force would we also include gravity or not? (In regards to the HSC syllabus).

Hopefully we just don't get this in the HSC.

 

[sirfeathers]

I don't really care what the *real* answer is I just want the correct answer that will give me the marks .

Yep at this point in time, me too. (;

If we are told a rocket is accelerating at 20ms upon liftoff on a 50KG person; calculate the net force would we also include gravity or not? (In regards to the HSC syllabus).

In this situation I'm pretty sure you would. Because you're told it's at liftoff, you know that the value of g will remain at 9.8 so the problem of changing g is not there. So you'd have F = 50*(20+9.8)

 

[dolbinau]

In this situation I'm pretty sure you would. Because you're told it's at liftoff, you know that the value of g will remain at 9.8 so the problem of changing g is not there. So you'd have F = 50*(20+9.8)

it is likely that if you ever get a question like this in the hsc it will clearly state to assume constant gravity.

Thankyou