rocket question (2 Viewers)

[Farmerism]

a rocket of inital mass 2000 tonnes, produces a constant thrust of 8 x 10^7N during lift off by expelling 1500kg of exhaust gases per second.

calculate the net force acting on an 85kg astronaut 30s after liftoff

i dont get how to go about calculating it

 

[passion89]

This equation is not specified in the syllabus, it is a special case of Newton's second law so it's probable that this type of question wont be asked...but it can happen.

For the first part find the acceleration:
a = (T - mg)/m (Where a = acceleration, T = thrust, m = mass, g = 9.8m/s^2

I'm kinda stuck as to what you do afterwards though and I have no idea if you actually use that equation it just looked like it'd fit. I'm gonna try solve it and if I do i'll post the answer up. I really hope this helps you!

 

[spurnicious]

don't know if you still need this, but

if you use a=(t-w)/m with the values you gave, a=30.2
then with net force= ma, net force=2567N

But, does anyone know how to calculate thrust without knowing the acceleration?

 

[zeropoint]

Hi HSC stresser outers

I get this question all the time because people have a habit of plugging into ``formulas'' without the faintest idea about what they're doing.

It turns out you can answer every part of a question like this using only a few simple principles you've probably known since year 10.

Firstly, remember Newton's second law. It makes the following (simplified) statement

(net force) = (mass) x (acceleration).

What this means is that if you add up all of the external forces impinging on an object (say a rocket) at any one time, the answer you get (in newtons say) is equal to the product of the mass of the object multiplied by it's rate of change of velocity at that point in time.

Now you're done. All that is left to do is count up the forces (taking into account directions) and you get

(net force) = (mass) x (acceleration).

(thrust) - (weight) = (mass) x (acceleration)

Now remember what the question asked you for: What is the net force?

Well, we know that

(net force) = (mass) x (acceleration).

But hang on, we also know that net force is the sum of the forces impinging on the body.

In this question, the easiest way to evaluate the net force is to get it directly by performing the sum:

(net force) = (thrust) - (weight)

By definition.

Hope this helps.

James