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Catapults
Sources: http://www.life.uiuc.edu/boast1/sciencelessons/levers.htm
http://csep10.phys.utk.edu/astr161/lect/history/newton3laws.html
http://science-class.net/Lessons/Physics/Simple%20Machines/Marshmallow%20Catapults.pdf
http://pbskids.org/zoom/activities/sci/cottonballcatapult.html?print
Materials Needed:
Plastic Spoons
Chalk
Plastic cups
Pencils and paper
Cotton balls and Marshmellows
Rubber bands
Masking Tape
Popsicle Sticks
Tape Measure
Buckets/containers
Activity
1) What is a catapult? A simple machine designed to throw things.
2) Have you ever seen a catapult? When were they used? In the middle ages, but they are
also used today! The Trebuchet is probably the oldest kind. Catapults are used to launch
planes off ships! Hoe big can they be?
3) Catapults are “simple machines”. Now let’s take a step back. WHY are catapults so
cool? Can you throw things as far by hand? Which do you think would be easier?
Newton’s three laws of motion:
I.
II.
III.
Every object in a state of uniform motion tends to remain in that state of
motion unless an external force is applied to it.
The relationship between an object's mass m, its acceleration a, and the
applied force F is F = ma. (Acceleration and force are vectors (as indicated by
their symbols being displayed in slant bold font); in this law the direction of the
force vector is the same as the direction of the acceleration vector.)
For every action there is an equal and opposite reaction.
Force- a push or pull. A force can make something start moving, stop moving, or make
change direction.
Work– work is done when we use a force (a push or pull) to move something over a
distance.
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Energy- the ability to do work. If I have no energy, I can’t use force to make something
move!
There are two different kinds of energy
Kinetic Energy– the energy of motion.
Potential Energy– is stored energy – it has “potential” to do something. WE are
going to convert potential energy into Kinetic energy with our catapults!
Elastic potential energy– an elastic can store energy and convert it to
motion when it is released. (Stretch the rubber band, and shoot it across the
room)
Gravitational potential energy– is the energy an object has when we raise
it off the ground. (raise the spoon and drop it on the table)
Chemical potential energy is the energy stored in chemical bonds of
molecules (remember our “volcano and rocket juice”? What happened when we
mixed the different chemicals together?)
4) Simple machines make our lives easier by allowing us to use less energy & force to do
work. They can be combined to form compound machines, allowing simple machines to
be used in greater variety of ways.
5) The lever is one of these simple machines. A lever consists of a plank that is free at both
ends, and a steady object on which the plank can rest.
(Take the chalk and tape it to the table with the masking tape –one for each child.
Balance the tongue depressor on the chalk – like a seasaw on a play ground. )
6) Place a cotton ball on one side. The pivot point is called the fulcrum. The object that
one is trying to move is called the load. The distance from the load to the fulcrum is
called the effort arm.
(Have each student measure the distance between the cotton ball and the pivot.)
7) What do you think will happen if we push down on the other side, the “lever”. How
high can we make the cotton ball go? What can we change to make it go higher? WRITE
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DOWN WHAT THEY SAY FIRST! Then do it. Will it change with what we use as the
“load”. (Force of pushing on the lever, changing where the fulcrum is. What about
making the fulcrum bigger - increase from chalk to something larger to act as the pivot
point? What happens if you use a small marshmellow instead of the cotton ball? A big
marshmellow?) Were they correct?
8) NOW – Let’s build the catapult! Let each child build their own. Ask them what they
think will help make the load fly further. Which will fly further – a small or big
marshmellow , the cotton ball? How long should they make their lever (use just a
spoon? Use a spoon taped to a popsicle stick, taped to the cup?). How should they
attach it (to the cup upside down? on its side? Does height matter?)
Something like this. Remember to hold down the plastic
container when shooting! Use the rubberband as the method for pulling the spoon back
for better launching ;)
9) Now go outside (inside ok if raining) and start shooting.
a. First – work on who can shoot their’s the farthest. Who wins? Why? Have the
students measure the distance. Could you modify your catapult to make it
launch the marshmellow farther? Try it with cotton balls too. Which is better?
b. Now work on accuracy, who can hit the different buckets at a target far away?
Did anyone modify their catapults to make them better for accuracy?
c. Who is the most consistent?
d. Explain the difference between accuracy and precision.
How did you increase the accuracy of your catapult? What kind of changes increased the
distance the catapult launched the marshmallow?
WHY did these changes increase the distance?
Depending on how rambunctious they are –we can have a cotton ball war (not the
marshmellows – they get sticky!) 
10) Conclusions:
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Your catapult is a simple machine, a lever. In moving a marshmallow your catapult is
doing work.




What are levers used in society?
What types of levers are around us?
What is the relationship between force and distance?
How can force be measured? Mass x acceleration! (see newton’s laws
above!)
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