Physics
Lab 1.2 Balloon Rockets
Experiment 1.3.1
Balloon Rocket Races
Purpose: The
purpose of this lab is to observe the operation of
balloon rockets and to identify the factors that affect
their performance. This will enable us to build better
rockets and test their performance by racing them against
each other. Two or more nylon lines are used for guiding
the competing rockets. Rocket performance can be judged
according to the distance covered by the rocket or by the
rate of acceleration over a set distance.
To estimate the rate of
acceleration, use a stopwatch to measure the time taken
for the rocket to cover a known distance (starting at
rest).
If we assume that the rocket accelerates constantly over
this distance, the following equation relates the time
taken and the distance covered to the rate of
acceleration:
d = ½at2
Distance = ½ x
acceleration x time2 (for accelerating object)
Where: d = distance
(m), a = acceleration (m/s2), t
= time (s)
Balloon
rockets
Balloons can act as rockets because they move under the
influence of air escaping from an open end. Like all
rockets, the thrust is produced at one end while the mass
or inertia of the rocket itself tends to oppose movement.
The result is that unless the rocket if fitted with tail
fins or a stabilizing device, it will tend to spin or
follow a random path. A balloon rocket can be made to
move in a fixed direction (guided) by attaching it to a
drinking straw that has a taut piece of fishing line
threaded through it.
A balloon rocket converts
pressure energy and stress energy into kinetic energy.
The energy stored in the balloon is also used to overcome
friction between the rocket and the air and between the
straw and the nylon line. If the rocket gains height
during its movement, some of the rockets initial energy
is converted to gravitational potential energy.
Materials
& Equipment
30 to 50 meters of nylon fishing line or light twine.
Drinking straws
Balloons
Adhesive tape
Stopwatches – 2
Tape measure
Procedure
String 2 or 3 pieces of nylon fishing line, each roughly
15 meters long, between anchor points. The lines should
be roughly horizontal and parallel to each other. Thread
a drinking straw over each of the lines before anchoring
the lines. The drinking straws will be used to guide the
rockets along the nylon lines.
To construct a rocket,
blow up a balloon, hold the open end closed, tape the
balloon to a drinking straw on a nylon line. To launch
the rocket, release the open end of the balloon.
After determining the
maximum distance covered by a particular rocket, use a
marker to mark a point on the line at roughly 50% of this
distance from the starting point. Measure the distance
from the starting point to this mark.
Return the straw to the
starting point, inflate the balloon and re-tape it to the
straw.
Start one stopwatch as
soon as the rocket is released. Start the second
stopwatch as soon as it passes the mark on the line. Stop
both watches at exactly the same time. The difference
between the times displayed on the watches will be the
time taken for the balloon rocket to cover the distance.
The
Process
The air leaving the
balloon will create a reaction force at the open end of
the balloon that will drive the balloon in the opposite
direction to that of the escaping air. Rockets work by
reaction.
The rocket will accelerate
as long as air leaves the balloon. After running out of
air, friction between the straw and the line and friction
between the empty balloon and the surrounding air will
slow the rocket until it stops.
Measurements
- Rocket Number (or
name):……………………………………
- Maximum distance
covered by balloon rocket:
………………… meters
- Selected distance (i.e.
distance from starting point to mark on line)
………………… (d)
meters
- Time taken to cover
this distance:
…………………. (t)
seconds
Average speed over this distance =
.............................m/s
Estimated rate of acceleration over this distance:
(using the equation below) a =
…………………. (m/s2)
d = ½at2 Where: d = distance (m), a =
acceleration (m/s2)
and t = time (s)
Repeat these measurements
for each type of rocket tested.
From
your observations and the results of your experiments
submit answers to the following questions:
Questions:
- Would a long narrow
balloon tend to be a better rocket than a
spherical balloon with the same amount of air in
it? Why?
- Would a larger
balloon tend to be a better rocket than a smaller
balloon with the same amount of air in it - but
at a higher pressure? Explain.
- Does the air escaping
from a balloon rocket have to have something to
push against to enable the rocket to move? Why?
- For balloons with the
same size, shape and pressure, does the size of
the opening in the balloon affect: a) the
distance covered by the rocket?, b) the average
speed over this distance?, c) the rate of
acceleration of the rocket? Explain.
Possible
answers to questions:
(Test these answers
against the results of your experiments.)
- It might be better
for two reasons: The shape of the balloon will
allow it to create less drag. The shape of the
balloon may also result in the air taking longer
to escape. (The opening may also be a bit smaller)
In this case, the balloon may not accelerate as
quickly but it could travel further.
- The smaller balloon
may accelerate at a greater rate initially but
the larger balloon would probably travel further.
- No. Rockets move for
the same reason that fire hoses are very
difficult to hold when a large stream of water
leaves the opening at the end of the hose. The
action of expelling the gases from the rocket
creates an equal and opposite reaction in the
rocket. Something that slows down the rate at
which gasses leave the rocket may in fact slow it
down. (We’ll study reaction in more detail
later in the program.)
- A larger opening
could be expected to a) reduce the distance, b)
increase the average speed and c) increase the
rate of acceleration.
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