ATAR Notes: Forum

HSC Stuff => HSC Science Stuff => HSC Subjects + Help => HSC Physics => Topic started by: shreya_ajoshi on December 26, 2016, 02:04:35 pm

Title: Physics Space Topic
Post by: shreya_ajoshi on December 26, 2016, 02:04:35 pm: Hi!
How do you do this question?
1a) A rocket has a mass of 400kg. 75% being fuel. It develops a thrust of 8000N. Calculate its initial acceleration

Thank you!!! :)
Title: Re: Physics Space Topic
Post by: RuiAce on December 26, 2016, 02:18:44 pm: Quote from: shreya_ajoshi on December 26, 2016, 02:04:35 pm
Hi!
How do you do this question?
1a) A rocket has a mass of 400kg. 75% being fuel. It develops a thrust of 8000N. Calculate its initial acceleration

Thank you!!! :)
$\text{Initially, its mass is still }400\text{kg}$
$\text{The initial acceleration provided by the thrust is found by}\\ \begin{align*}F&=ma\\ 8000&=400a\\ a&=20\text{ m s}^{-2}\end{align*}$
$\text{But this force is pulling the rocket up. Gravity is pulling down}\\ \text{Note that the gravitational acceleration on the rocket is }-9.8\text{ m s}^{-2}$
$\text{So our net acceleration would be the sum of the two, which is }a=10.<br />2\text{ m s }^{-2}$
Title: Re: Physics Space Topic
Post by: kiwiberry on December 26, 2016, 02:39:12 pm: wait, does the net force on the rocket need to include the weight force as well?

so Fnet = ma
T - mg = ma
a = (T-mg)/m
= ( 8000-400x9.8 )/400
= 10.2 ms^-2
Title: Re: Physics Space Topic
Post by: RuiAce on December 26, 2016, 02:41:05 pm: Oh, no wonder I got a weird result. I can't do simple arithmetic.
Title: Re: Physics Space Topic
Post by: shreya_ajoshi on January 02, 2017, 07:17:46 pm: Thank you!!
So the '75% being fuel" has nothing to do with the question?

(Answer: Yes)
Title: Re: Physics Space Topic
Post by: shreya_ajoshi on January 02, 2017, 07:25:55 pm: Hi
Part b) of the same question asks
"Calculate its acceleration when half its fuel has been consumed"

Actually, don't worry. I got the answer :)
Thanks anyways

Actually how do you do this
c) Calculate the g-force on a 60kg astronaut in the rocket when half the fuel has been consumed. Assume the rocket is still within Earth's gravitational field. The acceleration of the rocket when half the fuel is consumed is 22.2ms^-2

Mod edit: Merged duplicate posts. At times like this, please resort to the modify button instead of posting multiple times.
Title: Re: Physics Space Topic
Post by: jamonwindeyer on January 03, 2017, 10:14:14 pm: Quote from: shreya_ajoshi on January 02, 2017, 07:25:55 pm
Hi
Part b) of the same question asks
"Calculate its acceleration when half its fuel has been consumed"

Actually, don't worry. I got the answer :)
Thanks anyways

Actually how do you do this
c) Calculate the g-force on a 60kg astronaut in the rocket when half the fuel has been consumed. Assume the rocket is still within Earth's gravitational field. The acceleration of the rocket when half the fuel is consumed is 22.2ms^-2

Mod edit: Merged duplicate posts. At times like this, please resort to the modify button instead of posting multiple times.

Hey! So for this question, we just need to figure out how much extra acceleration the astronaut is experiencing! The astronaut is travelling upwards with an acceleration of 22.2 metres per second per second, and the astronaut experiences an extra 1G of force for every 9.8 metres per second per second of acceleration upwards (due to the reaction forces).

$\frac{22.2}{9.8}=2.265$

So, the astronaut experiences 2.265 G's of extra forces due to the rockets motion. Add this to the normal 1G of weight force, and you get 3.265G :)

Hope this helps!