SPH4U – CATAPULT PROJECT
THE DEVICE
A catapult is a device used to launch heavy projectiles long distances. It has often been used in
warfare to throw rocks at enemy fortifications. Your assignment is to construct a catapult that will
launch a marshmallow (large size) at a target on the floor at the other end of the classroom (a distance
of about 6 m). You will compete against other catapults for accuracy. Each catapult will be allowed 3
shots and the distance from the initial contact point with the ground to the centre of the target will be
measured. The total of the three distances will constitute the score. Lowest score wins.
Rules:
i.
The energy to launch the marshmallow cannot come directly from the motion of
a human being. There must be some source of stored potential energy in the
catapult (elastic potential, compressed air, chemical potential, electric potential,
gravitational potential, magnetic potential, etc.).
ii.
The catapult must be designed to launch from ground level.
iii.
The device must be designed and constructed by the student. No store bought
catapults are allowed.
iv.
The marshmallow must actually be launched from the catapult with an initial,
upwards velocity component.
Assignment
i.
With a partner or on your own, design and construct a catapult. It may be as
simple as an elastic band, a cup and a couple support arms. Compete in the
competition which will be held in class on ______________________. Some
research on the internet may help with ideas, but don’t let that limit your
imagination!
ii.
Write a report on the device you designed. The report is described below.
The catapult device mark is an Applications Mark and will be based on (Total 20)
-effort in construction and design
/6
-ease of use
/4
-accuracy
/6
-adjustability (two methods)
/4
THE REPORT
1.
The first step is to gather relevant data from your catapult. You will measure things like the mass of
your throwing arm, the length of the arm, the location of the centre of mass, the height above the ground
the marshmallow is released, the angle to the horizontal the marshmallow’s initial velocity makes, etc.
This data should be clearly defined and listed. INCLUDE A DIAGRAM OF YOUR CATAPULT AND
LABEL IT WITH YOUR DATA.
2.
If applicable, do an experiment to gather force-length data for your “spring” and plot a F vs. x graph to
find the spring constant.
3.
Use your knowledge of conservation of energy to predict the speed the marshmallow will have when it is
released from your catapult.
4.
Use the VideoPoint software to analyse the video of your catapult in operation. Follow these steps:
A.
Loading your Movie.
Open the VideoPoint Program by clicking on the
Close the “About VideoPoint” window that appears.
The following screen will appear:
icon on the right side of the desktop.
Click on “Open Movie...” and select the movie of your catapult.
When you see the following:
Just select “OK” since you are only analysing one object (the marshmallow). It will take about
30 s for the program to load the entire video.
B.
Set Your Scale
You must tell the program how to scale your movie. We know that the front edge of the back lab
counter measures 3.5 m in length. Click on the
button on the left side.
In the box that appears, enter 3.5 for “Known Length:”. Click “Continue.”
Place your cursor over one end of the front edge of the lab bench in the photo. Click once. Move
your cursor to the other end. Click once more. You have now set the scale.
C.
Gather Data
Click on the
button on the left of your screen. This is your data point button. Place your
cursor directly over the marshmallow. Click once. The program will record the data point and
move forward one frame. Continue clicking on the marshmallow until it hits the floor. If you
cannot see the marshmallow in a certain frame, use the ► arrow at the bottom right corner to
advance one frame at a time until you find your marshmallow again.
If you make a mistake, click on the
button, and use the ◄►arrows on the right end of the
slider below your movie to locate the frame(s) with the bad data points. Click once on the data
point. Press the “Delete” key. Now click on the
button and redo that data point if necessary.
D.
Set Your Coordinate Origin
Use the slider below your movie to fast-forward to the point where the marshmallow is just
leaving your catapult. Use the ◄► arrows on the right end of the slider to move frame by frame
until you have found the exact frame where the marshmallow is leaving. A frame counter
appears in the top right of your video.
To the left of your movie, there is a vertical column of buttons. Click on the
button. A
yellow coordinate axis appears on your movie. Click once on the origin of the coordinate system.
Drag the origin of your coordinate system until it is located at the base of your catapult directly
below where the marshmallow is leaving your catapult.
E.
Create Your Graph and Equation
Click on the
button. The following screen will appear:
For the “Horizontal Axis” click on the drop down menu where it says “Time” and select “Point
S1”. A new menu appears to the right. It should say “x-component”. Below this, select
“Position.
For the “Vertical Axis”, “Point S1” should be selected and the second field should be changed to
“y-component”. Below, select “Position” again.
On the graph that appears, click on the “F” button on the top right corner. You will see a dialog
like:
Change the “Type of Fit:” to “Polynomial” and the “Order of Fit:” should be “2” for a quadratic.
Click “OK”. Check your vertical axis on the graph. If the y-values do not start at 0, position
your cursor on the axis and click on it. You should see a window where you can change your
minimum y-value to 0. Leave the other values where they are.
F.
5.
Saving Your Work
Call your teacher over. If your teacher approves of your graph, you now need to save it. Ensure
your graph is the selected window and under the edit menu, select “Copy Window”. Minimize
the Videopoint program and open Microsoft Word. Paste the graph into the word document.
Save it to the desktop and e-mail it to yourself. Once that is done, exit Videopoint without saving
anything and delete the Word document you created off the desktop. You are done.
You can now make several calculations.
First, calculate the initial velocity of your marshmallow from the VideoPoint Analysis. The equation
you have above your graph comes from the following analysis or projectile motion:
vertical
horizontal
y = 12 at 2 + v1 y t
x = v1x t
∴t =
x
v1x
sub t in vertical
2
 x 
 x 
y = g   + v1 y   where g is the acceleration due to gravity
 v1x 
 v1x 
 g  2  v1 y 
∴y =
x + x
2 
 2v1x 
 v1x 
1
2
By matching the above equation with the corresponding terms of your equation on your graph, you can
determine the initial velocity v1 (including angle). Compare this result with the one you obtained in step
3 above.
Now determine the horizontal distance travelled in three ways:
a.
Use the velocity you found in step 3 and calculate how far your marshmallow goes.
b.
Use the velocity you found from your graph and calculate the how far the marshmallow
goes.
c.
Read how far your marshmallow goes from your graph.
Using the result in c above as the accepted value, determine the percent error in the other two values.
6.
Write a paragraph or two explaining the reasons for the differences in your initial velocity results and in
your distance results.
Marks
Your report will be marked as follows:
THINKING: Diagram and Measurements – completeness and accuracy
Calculations and Comparisons – correct and well explained
Analysis of Differences – completeness and correctness
COMMUNICATION:
Grammar, spelling, organization clarity, neatness
10 marks
10 marks
5 marks
10 marks