- #1
marksyncm
- 100
- 5
I ran into this problem and need some guidance. Here's the question exactly as it appears in my sheet:
"A 500 kg rocket is set for vertical take-off. If the exhaust speed is 1,000 m/s, the mass of gas ejected per second to supply the thrust needed to overcome the weight of the rocket is:" (multiple choices follow)
The answer to this is supposed to be the following:
[tex]F = ma = m * \frac{dv}{dt} = 500 * 9.81[/tex]
[tex]m * \frac{1000-0}{dt} = 500 * 9.81[/tex]
[tex]\frac{m}{dt} = 4.9 Kg /s[/tex]
What I do not understand is how we are able to do this only knowing the velocity of the exhaust and without knowing the acceleration of the exhaust. For all we know, the exhaust could have been accelerated to 1,000 m/s over the course of 0.0001 second, or 0.00423 seconds. Isn't it the acceleration, not velocity, which determines how much force was imparted to the gas, and hence how much force was imparted to the rocket?
What am I missing?
Thank you.
"A 500 kg rocket is set for vertical take-off. If the exhaust speed is 1,000 m/s, the mass of gas ejected per second to supply the thrust needed to overcome the weight of the rocket is:" (multiple choices follow)
The answer to this is supposed to be the following:
[tex]F = ma = m * \frac{dv}{dt} = 500 * 9.81[/tex]
[tex]m * \frac{1000-0}{dt} = 500 * 9.81[/tex]
[tex]\frac{m}{dt} = 4.9 Kg /s[/tex]
What I do not understand is how we are able to do this only knowing the velocity of the exhaust and without knowing the acceleration of the exhaust. For all we know, the exhaust could have been accelerated to 1,000 m/s over the course of 0.0001 second, or 0.00423 seconds. Isn't it the acceleration, not velocity, which determines how much force was imparted to the gas, and hence how much force was imparted to the rocket?
What am I missing?
Thank you.