Rocket Launch

Law of Conservation of momentum.

• This is how the spaceship is able to propagate in space at all.
• The law states: "In a closed system, the sum of the momenta before a change is equal to the sum of momenta after the change."
• Before a launch, the spacecraft and its fuel are not moving, so their momenta add up to zero
(1)
\begin{equation} P_{rocket} + P_{fuel} = 0 \end{equation}
• As the rocket ejects the fuel, it will move in the opposite direction that the fuel was ejected at.
(2)
\begin{equation} P_{rocket} = - P_{fuel} \end{equation}
• Also, assuming the exhaust speed is constant, therefore the thrust of the rocket is also constant.
• Yet the rocket also has to also overcome gravity, so the net force on a rocket is given by:
(3)
\begin{equation} F_{net} = F_{thrust} - F_{gravity} \end{equation}
(4)
\begin{equation} F_{net} = F_{thrust} - mg \end{equation}
• From Newton's second law, we can find the acceleration on the rocket.
(5)
\begin{align} a = \frac{T-mg}{m} \end{align}
• However, as fuel is being ejected, mass decreases.
• And as shown by the previous equation, if mass decreases, acceleration increases exponentially

Forces on Astronauts.

page revision: 0, last edited: 03 Nov 2011 03:26