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 Concepts:
 force, direction, balanced forces, unbalanced forces, force of gravity,
air resistance, thrust, lift,
 flight, flight time, fuel,
 variable, dependent variable, independent variable, controlled
variable, hypothesis, prediction
 Skills:
 Be able to draw graphs using Excel.
 Be able to identify the variables involved in an investigation.
 Be able to identify independent, dependent and controlled variables in
an investigation.
 Be able to determine resultant force on an object in flight given the
relevant forces acting on it.

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 Automotive vehicular transportation involves a fuel and a vehicle.
 While some automobiles such as motorbikes, cars and trucks, transport people, animals and objects
from one location to another by land and use fuels such as petrol,
diesel etc. others such as ships and boats transport people and things
by sea or water, while still others such as hot air balloons, airplanes,
and rockets, transport people and
things through space.
 In this topic we will study the physics and chemistry involved in two
such vehicles, rockets and hot air balloons.
 The topics will include:
 Motion
 Forces involved in their flight
 Energy involved (i.e. fuels) and their nature

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 A squid propels itself by filling its body with water and ejecting it
backwards in order to move forwards.
 This is the principle used by Rocket Engineers.
 Space rockets use fuels which are burning in a chamber shaped rather
like a bottle with a neck pointing backwards.
 The burning of a fuel (called combustion) produces a large volume of gas
expanded by heat and this is ejected at high velocity forcing the Rocket
in the opposite direction.

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 Your rocket like the squid uses water as the driving agent and
compressed air to provide the energy instead of heat.
 The pressure in the Rocket body at launch is about 18 ´ 10^{4} N/m^{2} and the area of the jet
through which the water is forced is 1 cm^{2}.
 This produces a theoretical thrust of 18 N (3.9 lbs) at launch.
 As water is ejected the Rocket gets lighter producing increased
acceleration and ‘g’ force.
 This increase in ‘g’ force as fuel is used up is one of many discomforts
Astronauts have to endure.
 In the case of a real rocket the acceleration is maintained for several
minutes rather than about one second for your Rocket.
 Don’t worry our rocket is not likely to go into orbit.

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 Three forces:
 Gravity pulls the rocket towards the earth,
 Air resistance retards the flight of the rocket through air, and
 The energy and force generated by the rocket engine burning the fuel
propels the rocket into and through the air (thrust).
 Of the three forces, two are pull forces and the third a push force.
 The pull forces, as you might have already realized, are gravity and air
resistance (pulling the rocket towards the earth), and the push force is
the that coming from the burning fuel.
 The fuel for your bottle rocket will be water.
 The water will be forced out of the bottle by the pressure build up
inside it when air is pumped into it which will propel the rocket into
the sky as you will observe in the demonstration.

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 So when the bottle rocket is launched, it will fly but for a limited
time.
 Task 1: What are the factors that affect the flight time of the rocket?
 (List the factors that you believe will affect the duration for which
the rocket will stay in the air.)

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 Factors that can be measured (quantified) and may affect the result of
an experiment are called variables in science.
 In a scientific investigation, we distinguish between three variables:
 Independent variable, dependent variable and controlled variable.
 Independent variable in an investigation is the variable that the
experimenter changes from one experiment to the other.
 It is the variable the experimenter is studying the effect of, it’s the
variable the experimenter is interested in.
 The dependent variable is the variable which may change from one
experiment to the next in response to the changes in the independent
variable.
 Controlled variables are the variables that are kept constant from one
experiment to the next to ensure that the experiments are “fair tests”
and therefore the results are valid.

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 What you will investigate is the effect of the mass of water in the
bottle rocket on the flight time of the bottle rocket.
 The aim of this investigation therefore is to determine the effect of
mass of water on the flight time of the bottle rocket.
 The capacity of the water bottle rocket will be 2000 g (2 liters, 2000
cm^{3 }).
 But only the following mass (volume) of the water will be investigated.

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 Therefore, for this investigation,
 the independent variable is the mass of water (mass of fuel)
 The dependent variable is the flight time.
 That means the following variables, which you will keep the same from
one experiment to the next, are the controlled variables:
 The mass of the bottle rocket (the same rocket will be used every time)
 The angle at which the rocket is launched (it will be launched at 90° from the ground).
 The force with which air is pumped into the bottle.
 The speed at which air is pumped into the bottle.

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 Task 2. Create a hypothesis as to how you believe the flight time will
change with increasing mass of water in the bottle and include an explanation.

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 Plot a graph of flight time (vertical axis) vs mass of water (horizontal
axis). Draw a smooth curve along the data points.
 When the curve is extrapolated from the first data point, it intercepts
the yaxis. What does this yintercept represent?
 What does the yintercept corresponding to the peak of the curve
represent?
 What does the xintercept corresponding to the peak of the curve
represent?
 When the curve is extrapolated from the last data point, it intercepts
the xaxis. What does this xintercept represent?
 How does the wind speed affect the flight time of the bottle rocket and
therefore the result of the experiment?
 Was the result, the graph, consistent with your hypothesis? Was the
graph you drew based on your hypothesis anything like the graph you
obtained for the experiment? Why or why not?

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 8. Draw a diagram of the bottle rocket in flight. Draw arrows to
indicating the three different forces acting on it.
 9. a) When the bottle rocket takes off it takes off at a really high
speed, but as it gains height, it appears to slow down. Which force is
the strongest when the rocket takes off from it’s launching site?
 b) What happens to that force as the rocket gains in height?
 c) Do the other two forces change in magnitude (size) during its
flight? If they do how?
 d) What causes the rocket to slow down during it’s flight?
