Stomp
Rocket
The goal of this lab is to connect the
motion of a simple object with physics and chemistry you have learned
previously. The object is a “stomp rocket,” a lightweight plastic tube which
can be launched with surprising effectiveness by compressed air, which is
generated by stepping down hard on a flexible plastic box connected to a tube
the rocket fits over.
The basic task: measure
the height to which the rocket rises, and use this number (and your background
knowledge) to estimate energies, pressures, and other characteristics of the
launch process.
The basic model: The stomp
rocket is basically a cylinder full of an ideal gas (air) with a piston at each
end. Rapid compression at one piston can create enough gas pressure to launch
the other piston high in the air.
(a) Assemble your gear:
stomp rocket / launcher / survey tape /
meter stick / protractor / anything else you think may be useful
Find a place where the top of the rocket’s
trajectory can easily be observed with an altitude angle of 45 degrees or less,
and where the rocket can easily be recovered after its flights. Use some
practice flights to set up a trigonometric method for measuring the height of
the trajectory.
(b) Make several measurement runs, i.e.
launches where you take data. (If you look ahead, you’ll see that vertical
launches will be easiest to interpret.)
(c) Measure the mass of the rocket
(top-loading balance provided) and estimate the potential energy change of the
rocket between launch level and the top of its trajectory. Calculate how much work needed to be done on
the rocket to launch it on the trajectory you observed.
(d) Now try varying the angle at which you
launch the rocket. Predict
quantitatively how this will affect the potential energy of the rocket and
hence the height the rocket will travel, then perform several runs to see if
you measurements compare with your predictions. Be careful to try to get a reproducible stomp so you can compare
these measurements with your vertical launch measurements.
(e) Inspect the launch apparatus (if you
haven’t already) to develop a clear picture of how it works. Use the dimensions
of the apparatus (and any other information you need) to estimate the number of
moles of gas expelled during the launch process.
(f) Use the dimensions of the rocket to
estimate the average pressure exerted by the gas while the work of launching
was being done.
(g) Do you think the gas process during
launch is more likely to be isothermal or adiabatic? Explain. Notice there are
two phases of the process, initial compression and subsequent expansion.
Estimate the temperature change in the gas during each phase.
(h) How many whole or fractional cashews
do you estimate is the minimum it takes to provide the energy a person needs to
do the work that launches the rocket?
(i) Think about the sensitivity of your
trigonometric method to determine the height the rocket traveled. To what extent does deviation in measuring
the angle to the highest point traveled translate to deviation in calculating
the height traveled? Can you think of a
way to express this mathematically?
(j)
How would you go about designing a better stomp rocket? To address this, start by asking what the
present model tries to be good at, then think about what direction of improvement
you’d like to go: Would you want it to
go higher, carry a greater load, require less pressure, etc. How, at least theoretically, could this be
accomplished?