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Beginner's Guide to Propulsion
Newton's Third Law and Aircraft Propulsion

If so instructed by your teacher, print out a worksheet page for these problems.

From the American Heritage Dictionary:

propulsion: is defined as the process of driving or propelling.
propel: is defined as to cause to move or sustain in motion.

The engine on an airplane or helicopter is called the propulsion system of the airplane or helicopter.

Propulsion systems are governed by Newton's Third Law:

"For every action, there is an equal and opposite reaction."
Rocket Propulsion: Thrust equals Mass Flow times Velocity
Rocket Propulsion:
Thrust = Mass Flow x Velocity

Jet Propulsion: Thrust equals Velocity multiplied by Mass Flow plus Exit Area multiplied by Pressure
Jet Propulsion:
Thrust = (Mass Flow x Velocity) + (Pressure x Exit Area)

Activity: Controlled Propulsion Experiment - Balloon

A balloon provides a simple example of how a rocket engine works. The air trapped inside the balloon pushes out the open end, causing the balloon to move forward. The force of the air escaping is the "action"; the movement of the balloon forward is the "reaction" predicted by Newton's Third Law of Motion.

The distance that a balloon will travel when restricted to a straight line is related to the amount of air trapped inside the balloon when it is released. Similarly, the distance a rocket will travel is related to the amount of fuel trapped inside the rocket engine and the properties of that fuel.

This experiment will allow us to investigate how filling balloons with different amounts of air affect how far they will travel along a straight path. In order to do this, we need a few equations....

Volume of a sphere: (the amount of air in the sphere)

Volume = 1/6 x PI x (Diameter)3

Circumference of a sphere: (how far around the sphere is )

Circumference = PI x Diameter

Diameter, as computed from the Circumference:

Diameter = Circumference/PI

PI = 3.14 (a constant value)


  • balloon
  • drinking straw
  • string
  • tape
  • printout of worksheet
  • Image of Balloon Rocket


  1. With pieces of string cut to fit around the balloon, measure the circumference of the balloon with different amounts of air in it (full,1/4 full,1/2 full, & 3/4 full ), and record the circumference in the table shown below.
  2. Set aside the pieces of string for future use.
  3. Slide the drinking straw onto a piece of string.
  4. Choose two people from your team to hold each end of the string taut and level.
  5. Place two loops of masking tape onto the straw.
  6. Blow up the balloon to its maximum capacity (greatest volume). Hold tightly onto the neck of the balloon so that no air escapes, and attach the balloon to the two pieces of tape. The neck of the balloon should be parallel to the string.
  7. Release the neck to let the air rush out of the balloon.
  8. In the table shown below, record (a) the distance that the
    balloon traveled (as marked on the string) and (b) the
    amount of air in the balloon.
  9. Repeat steps 4-7 above, filling the balloon with different amounts
    of air. Use the pieces of string you measured in Step one.
  10. Calculate the volume of the balloon with the different circumferences using the equations presented earlier.
Amount of Air
Distance Traveled
Volume of Balloon
1/4 full
1/2 full
3/4 full
  1. Make a plot showing the distance that the balloon traveled with different air volumes. (This can be done on graph paper or with a computer software package.)
  2. What conclusion can be made from this graph and how does is relate to Newton's Third Law?

Related Pages:
Propulsion Activity Index
Propulsion Index


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Editor: Tom Benson
NASA Official: Tom Benson
Last Updated: Thu, Jun 12 04:39:28 PM EDT 2014

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