Science of Everyday Life
Family Activities
Full of Hot Air
Overview
Have you ever said to anyone, “It’s not rocket science.” What did you mean by that?
People often imply that “rocket science” is a mystifying subject that only a select few can
understand. In this activity, you will see that the basic principles behind rocketry are the
same as blowing up a balloon and letting it go! Have fun building and racing balloon
rockets.
Materials
 two chairs
 round balloons
 long balloons (the type used for making balloon animals)
 a pump to blow up long balloons
 Scotch® Duct Tape
 cotton string
 fishing line, kite string and/or soap water
 plastic straws
 scissors
 binder clips (optional)
Thought Starters
Ask these questions before you begin:
 What are Newton’s Laws of Motion?
o First Law: An object in motion remains in motion at constant velocity
unless acted upon by a net force.
o Second Law: The acceleration of an object is proportional to the net force
acting on it and inversely proportional to the mass of the object. F = ma
o Third Law: If object A exerts a force on object B, object B exerts an equal
and opposite force on A.
 What is a net force?
o A net force is an unbalanced force.
 How does a rocket propel itself upward?
o A rocket expels hot gases downward and this gives the rocket momentum
upward.
 What is friction?
o A: Friction is the force that opposes motion between two surfaces in
contact.
 How do you think a rocket reduces friction with the air?
o A: A cone or rounded nose on the rocket helps it slice through the air
with less resistance.
A good scientist is a safe scientist. Do not conduct any experiment without adult supervision. The content is provided for informational purposes only. Discovery
Education and 3M assume no liability for your use of this information. 3M is a trademark of 3M. Published by Discovery Education. © 2014. All rights reserved.
Activity
 Cut two three-inch pieces of a plastic straw.
 Take two chairs and put one at each end of the room. Tie a string from one chair
to the other. Before securing the string, slip one of the straw segments onto the
string. Make sure the string is taught. Repeat with another string so that you
have two parallel strings running the length of the room, each with a straw on it.
 Blow up two round balloons. Do not tie off the balloons. Instead twist the end of
each balloon and secure each with a binder clip. If you have enough helpers, you
can just hold the end of the balloon.
 Use a small piece of Scotch® Duct Tape to attach one balloon to each straw.
 You have just built two balloon rockets that are now going to race. Which one do
you think will win the race? Why? Remove each binder clip, and use fingers to
keep the air in the balloons. On the count of three, release both rockets at the
same time. Repeat several times. (You can re-inflate the balloons while they are
hanging from the string.) Are you getting the results you expected? If not, why?
 Remove one of the round balloons and replace it with a long balloon. Which
rocket will be faster? Make a prediction and test it several times. Try to explain
your results.
 Now, race two long balloons. Make one balloon longer than the other. Which
rocket will win and why? Do several races. Do your results make sense?
 Can you decrease the friction between the string and straw? Try changing from
cotton string to glossy kite string or fishing line. Or, rub oil or soap water on your
string.
 Do you think fins will improve the speed of your rocket? Make fins out of a
Styrofoam plate, and tape them to your balloon rocket.
 Have each family member take what they have learned, and make what they
think will be the best rocket. Then, have a race to see who is the best engineer!
 Ready for more rocketry? Go vertical! Run your string from the floor to the
ceiling and see how explosive your rocket really is!
Discussion Points
 Blowing up a balloon is work. Really! Air you are blowing into the balloon is
applying force on the walls of the balloon. This force is acting through a distance.
In physics, that’s work – applying a force on an object through a distance. The
work that you (and your air) are doing becomes elastic potential energy stored in
the stretched balloon. That energy is stored until you release the balloon. What
type of energy do you think it becomes?
 When you blow up a balloon, you are filling it with a gas. The molecules of the
gas are colliding with the inside of the balloon. This creates air pressure in the
balloon and the balloon stays inflated. When the balloon is released, the air is no
longer trapped and rushes out of the balloon and creates a flow of air. This flow
of air is what propels the rocket forward. To find out why, we need to remember
Newton’s Third Law of Motion. Newton’s Third Law of Motion states that when
object A exerts a force on object B, object B exerts an equal and opposite force
on object A. The forces are equal in magnitude and opposite in direction. In this
case, object A is the balloon rocket and object B is a collection of air molecules.
The balloon rocket applies a force on the air, and the air applies an opposite
force on the rocket, propelling it forward. Newton’s Third Law can also be stated
with the idea of momentum. Actually, Issac Newton originally explained his law
A good scientist is a safe scientist. Do not conduct any experiment without adult supervision. The content is provided for informational purposes only. Discovery
Education and 3M assume no liability for your use of this information. 3M is a trademark of 3M. Published by Discovery Education. © 2014. All rights reserved.


in terms of momentum, not force. The air gains momentum out the back of the
rocket and the rocket gains an equal amount of momentum forward.
Conservation of momentum predicts that the balloon rocket that expels the most
air would gain the most momentum and win the race. This probably held true for
you for the long balloons but perhaps not the round balloons. The instability of
the round balloon causes inconsistent results. Some rockets have fins to provide
stability and keep them moving in a straight line. Did you try fins on your balloon
rockets?
What forces are acting on the balloon rocket?
normal force
friction and drag force
thrust
weight
o
o

Thrust is the force of air molecules pushing the rocket forward.
Friction is the contact force between the string and straw that resists
sliding motion.
o Weight is the downward pull of gravity on the rocket.
o Normal force is the upward force of the string on the rocket (specifically
where the string is in contact with the straw).
o Drag force is the resisting force of a fluid, in our case air, on an object
moving through that fluid. Often referred to as air resistance. The long
thin balloon has less drag on it than a round balloon.
Newton’s First Law tells us that an object will not accelerate unless there is an
unbalanced force on it. For us, that means the thrust force must be greater than
the force of friction for the rocket to accelerate along the string. The normal force
on the rocket upward is equal to the weight of the rocket downward. That is why
the rocket is not accelerating in the vertical direction
To Learn More
 Build a virtual rocket at
http://www.nasa.gov/externalflash/RocketScience101/RocketScience101.html
A good scientist is a safe scientist. Do not conduct any experiment without adult supervision. The content is provided for informational purposes only. Discovery
Education and 3M assume no liability for your use of this information. 3M is a trademark of 3M. Published by Discovery Education. © 2014. All rights reserved.