Lesson Ideas.
Straw rockets can be used to explore several areas qualitatively.
Variations in force from the balloon powering the system preclude extreme
precision; however, repeatable relative trials can be done.
For this experiment, we will be using multiple concepts in projectile motion,
Newtonian physics and Force/Work equations.
Forms of Energy
The straw rocket launcher converts the potential energy of the inflated
balloon to the kinetic energy of the rocket’s motion. The mechanical potential
energy of the stretched balloon is used to compress and move the air out the
balloon – converting it to kinetic energy in the motion of the air molecules (a very
small amount of the energy is lost to heat as the air is compressed).
Energy moves to the rockets as physical movement of the air mass since
the system is not sealed.
Energy Transfer and Transformations
The straw rocket launcher converts the potential energy of the air in the
balloon to the kinetic energy of the rocket’s motion. There are several stages
that can be discussed specifically.
The mechanical potential energy of the stretched balloon – converting it to kinetic
energy in the motion of the air molecules will be addressed in this lesson.
When launched, the rocket converts its kinetic energy into gravitational
potential energy as it rises. It slows since the constant (neglecting air drag) total
energy of the rocket changes from the kinetic energy of motion to the potential
energy of its additional distance from the center of the earth. Once the rocket
reaches maximum height and begins to fall it converts that gravitational potential
back into kinetic energy as it accelerates to the ground.
Forces and Changes in Motion
It takes energy to change the motion of objects. Energy change is
understood in terms of forces--pushes or pulls.
You can start by analyzing the launcher and rockets.
- Volume of the balloon for each inch of movement:
Vsphere = 4/3 π r3 (assume balloon is perfect sphere) Have students
use tape to measure circumference of balloon and then have them
rewrite the formula for volume of sphere incorporating
circumference instead of radius (Vsphere = C3 / 6π2)
You can estimate the initial velocity obtained from the balloon by weighing
the balloon and straw and using the law of conservation of energy
KEinitial + PEinitial = KEfinal + PEfinal (mvinitial2 + mghinitial = mvfinal2 + mghfinal)
Or by the simple V = d/t formula which we will use for this lesson.
A force is simply a push or pull (usually on an object). In this activity the
force to push the rocket is provided by the inflation of the balloon and the blowing
of air out of the balloon afterwards.
Forces are needed to change the motion of objects. A force can make an
object move, move more quickly, change direction, slow down, or stop.
Forces cannot be seen but their effects can be felt. The more you fill the
balloon, the more you stretch the rubber and the harder it will blow the air out of
the open end when released. The harder the air blows, the more the rocket’s
motion will change (or the more it will accelerate).
More force yields more acceleration (F = m x a) resulting in a higher
velocity (v = a x t), giving more kinetic energy (m x v2) and carrying the rocket to
a higher height (d = v x t).
Newton’s second law tells us that the force is proportional to the acceleration for
a fixed mass.
F=mxa
Each team of students will use the same rocket for the entire experiment
so the mass is constant. Doubling the force doubles the rocket’s acceleration.
Twice the acceleration produces twice the velocity:
V=axt
More force will yield more speed. The basic equation, Force = Mass x
Acceleration, indicates doubling the force will double the acceleration.
Launch Velocity = Acceleration multiplied by Time.
So, if: V = d/t = a x t
Then: a = d/t2
If: a = d/t2 = F/m
Then: F = md/t2
Using this process we can determine initial force supplied to propel the rocket to
its peak height.
Initial velocity and Force Lesson
1. Record the Mass of your balloon and straw.
2. Each team will then make a rocket (balloon taped to straws which travel up a
piece of fishing line tacked to the ceiling and taped to the floor).
3. Launch each rocket five times using 5 units (small breaths) of air.
4. Repeat for 10 and 15 units.
6. Average the distance travelled and time for each force setting, and calculate the
average velocity for each set of trials.
Rocket Mass ____________________
Trial
Number
1
2
3
4
5
Average
Launch
Force
5
5
5
5
5
Height
Time
Average speed = distance / time __________________________
Trial
Number
1
2
3
4
5
Average
Launch
Force
10
10
10
10
10
Height
Time
Average speed = distance / time __________________________
Trial
Number
1
2
3
4
5
Average
Launch
Force
15
15
15
15
15
Height
Time
Average speed = distance / time __________________________
Is there a relationship between additional air in the balloon and distance travelled?
Is there a relationship between additional initial air in the balloon and velocity?
If you did not notice any difference between velocities depending on initial air in the
balloon, what factors may have attributed to this?
For this exercise we only considered velocity and acceleration in one direction. If the
line had been tacked from the top right corner of the room to the bottom left corner of the
room how would you determine initial velocity? (hint: draw a diagram and think about
SOH CAH TOA)
Pre-MacMod
1.) Gravity acts on all objects on our planet. When a field goal kicker kicks a
football it has both velocity in the X and Y directions. If we neglect air resistance,
how much slower will the ball be going in the X direction half way through its
flight toward the goal post?
2.) Newton’s Second Law of Motion states that: F = ma.
-What force is needed to accelerate a 10 kg object to 50m/s2?
-How much more force would be needed if the object was twice as heavy?
-What force (name and value in Newtons) determines how much you weigh?
Post-MacMod
1.) Although a rocket engine relies on impulse (a continued force over time),
what force must the rocket engine overcome to send a 50,000 kg spaceship
384,403 km to the Moon?
2.) What would the average velocity be if the trip took 3 days?
3.) Do you think the amount of force needed to get the spaceship back from the
Moon to the Earth would be more, less or the same? Explain your reasoning.