PHYSICS AND ENGINEERING-MAY THE FORCE BE WITH YOU
MAKING A MARVELLOUS MARSHMALLOW LAUNCHER
Experiment Objective:
Students will design and construct a catapult and use it as a
marshmallow launcher. They will learn the simple mechanics
behind the catapult and forces. The purpose of this activity is
to introduce students to use simple machine to build a device
used for launching purposes.
Learning Goals:
Students will learn the mechanics of a simple machine catapult and use its properties to
construct a marshmallow launcher. They will use their knowledge in physics and
engineering to launch marshmallows at a certain distance and angle. Students will be able
to explore how they can improve their launcher upon trials.
LESSON IMPLEMENTATION OUTLINE
Introduction:
A catapult is a mechanism used to throw missiles in
ancient and medieval warfare. At first, catapults were
specifically designed to shoot spears or other missiles
at a low trajectory. They were originally
distinguished from ballistae and trebuchets, both of
which were large military engines used to hurl stones
and other missiles, but these distinctions later blurred.
Soon after, larger catapults mounted on a single arm
also hurled stones, pots of boiling oil, and incendiaries at a high trajectory. They were
used to attack or defend fortifications. Catapults were widely employed in siege warfare,
but with the introduction of artillery they passed from use. In the 20th century catapults
using hydraulic pressure were reintroduced to launch aircraft from warships.
Lesson Background and Concepts:
What Is a Catapult?
A catapult is a machine that is similar to a giant slingshot. Objects, such as boulders, can
be positioned on the catapult and hurled over distances. Catapults were used in ancient
times to attack castles and fortresses by throwing large objects over moats and walls.
Modern soldiers have also used catapults: During World War I they were used to propel
grenades and gas bombs over long distances. In naval warfare a catapult can be used to
launch aircraft from the deck of a ship.
How Does It Work?
The catapult has a basket on the end of a movable arm strong enough to hold the weight
of object it is intended to hurl. Tension is applied to arm, which is forced down and
secured in place; springs and twisted ropes are two ways to provide the necessary tension.
After the object is added to the basket, the bindings are cut or removed. The arm then
succumbs to the tension and flips to the other side, like pulling a rubber band and then
releasing. The object is propelled forward.
General definition of a catapult: to hurl an object through the air. There are are three
primary technologies that fall into the "catapult" category.
Catapult
-winched-down bucket.
-originally, "catapult" is referred to a dartthrower. In modern times, catapult can be
used to describe any machine that hurls a
projectile. This can include a slingshot used
to hurl pebbles and a machine that launches
airplanes off aircraft carriers.
Ballista
-a very large crossbow.
-a weapon developed from earlier Greek
weapons. It relied upon different
mechanics, using two levers with torsion
springs, the springs consisting of several
loops of twisted skeins.
Trebuchet
-a weighted beam that swings a sling
carrying the projectile.
-a trebuchet tends to be easier to build
because it consists simply of a pivoting
beam and a counterweight that rotates the
beam through an arc.
Both catapults and ballistas work by storing tension either in twisted ropes or in a flexed
piece of wood (in the same way an archery bow does, but on a larger scale).
Work can be done on the catapult quite slowly, when the elastic is slowly stretched. This
provides a store of elastic potential energy, which can be expended quite quickly in
accelerating the stone of ball to a reasonably high speed.
Three properties can be exemplified in the use of a catapult: elasticity, energy, and force.
Elasticity is seen in the measure of the stretch of the spring loaded in the catapult. This
will demonstrate the accuracy of the delivery of the object being flung. Second, energy is
present in the conversion of it from potential into kinetic energy. This could demonstrate
and determine the range of the object. Finally, force can be studied and measured in the
projectile motion.
Projectile Motion
A projectile is an object upon which the only force
acting is gravity. There are a variety of examples of
projectiles. An object dropped from rest is a projectile
(provided that the influence of air resistance is
negligible). An object which is thrown vertically
upward is also a projectile (provided that the influence
of air resistance is negligible). And an object is which
thrown upward at an angle to the horizontal is also a
projectile (provided that the influence of air resistance is negligible). A projectile is any
object which once projected or dropped continues in motion by its own inertia and is
influenced only by the downward force of gravity.
By definition, a projectile has only one force acting upon it - the force of gravity. If there
was any other force acting upon an object, then that object would not be a projectile.
Thus, the free-body diagram of a projectile would show a single force acting downwards
and labeled force of gravity. Regardless of whether a projectile is moving downwards,
upwards, upwards and rightwards, or downwards and leftwards, the free-body diagram of
the projectile is still as depicted in the diagram at the right. By definition, a projectile is
any object upon which the only force is gravity.
Projectile Motion and Air Resistance
If projectiles were only launched from the surface of the moon where there is no
atmosphere, then the effects of gravity, as described in the previous section, would be
sufficient to determine the flight path. On Earth, however, the atmosphere will influence
the motion of projectiles. As opposed to the situation due to purely gravitational effects,
projectile motion with air resistance will be dependent on the weight and shape of the
object. As one would suspect, lighter objects are more strongly affected by air resistance.
In many cases, air resistance will produce a drag force which is proportional to the
velocity squared. The effects of increased air drag on an object such as a cannon ball will
cause it to fall short of its normal range without air resistance. This effect may be
significant. It was realized that cannon balls would travel farther distances if aimed at
higher elevations, due to decreased air density and decreased drag.
More subtle effects of air resistance on projectile motion are related to the shape and
rotation of the object. Clearly, the shape of an object can have an effect on its projectile
motion, as anyone has experienced by wadding up a piece of paper before tossing it into
the waste can. The rotation of an object is important also. For example, a good
quarterback always puts a spin on a football when making a pass.
Lab Activity Instructions:
Students will perform the construction of a marshmallow launcher in their groups.
Instructors will explain the how a catapult works and the basic physics concepts behind
the construction. Students are given different materials and they can choose their supply
for making the launcher. Upon completion, they will compete for the greatest launching
distance. Allow one instructor for demonstration and other instructors as assistants
around the classroom.
The Problem and Challenge: Design a catapult that will throw a marshmallow the
greatest distance.
Materials: 1 popsicle stick (the double kind, or 2 singles), 2 erasers (3-4cm long), 1
mousetrap (optional), 1 elastic band, 1 spoon, duct tape, heavy book, marshmallows
(different sizes)
Procedure:
1. You can use the mousetrap to build a catapult or construct the catapult from
popsicle sticks. Carefully pull back the mousetrap lever and watch your fingers.
Wrap the elastic band around the base of the mousetrap to hold the lever down.
2. Place one eraser against the hinge of the trap and tape it in place.
3. Place the second eraser on top of the first, slightly over the hinge. Tape it in place.
4. Carefully remove the elastic band from around the lever (watch those fingers!).
5. Break one Popsicle stick in half and tape it horizontally across the lever.
6. Tape the other popsicle stick to the lever so that it is perpendicular to the first
stick. Tape the spoon to this second stick. This is your catapult, mechanism to
used to throw missiles in ancient and medieval warfare. At first, catapults were
specifically designed to shoot spears or other missiles.
7. Tape the entire trap to a heavy book to act as your base. Ready, aim and fire. Pull
back the spoon and load it with a marshmallow and let it fly. Have a target and
shoot in that direction, this is more challenging.
Note: A catapult is a lever that not only makes it easier to lift a load, but also moves it
faster. In this case, the hinge of the mousetrap is your fulcrum, the marshmallow is
the load and the spoon is the load arm.
Expected Outcome: The catapult can throw marshmallows out and the distance of
throws will be measured. The teams will compete for the greatest distance thrown.
Checking for student understanding:
Ask students during the catapult construction process how will they design and build the
catapult. Encourage them to be creative and use the materials available to construct their
unique catapult. Promote their critical thinking and use the concepts from the lesson.
Extensions and connections:
Students can extend this activity to the making of other simple machines and devices.
They can think about where they can use catapult in real life and how they are useful in
terms of its mechanism. They can connect to the ancient catapult and explain they
CURRICULUM CONCEPTS
Physics and Engineering-Simple Machines and Catapult
References:
http://apphysicsb.homestead.com/catapult.html
http://tuhsphysics.ttsd.k12.or.us/Research/IB03/CoolJens/Catapult.htm
http://www.howstuffworks.com/question127.htm
http://www.physics.ubc.ca/~outreach/phys420/p420_03/aaron/Phys%20420%20Web%20
Home%20page-%20Aaron%20Dhillon.htm
http://teachingphysics.wordpress.com/2009/03/01/projectile-motion-activity/
http://www.physicsclassroom.com/Class/vectors/u3l2a.cfm
http://science.jrank.org/pages/728/Ballistics-Projectile-motion-with-air-resistance.html
EXAMPLES OF CATAPULT/LAUNCHER