Physics

Mr. Buxman

Catapult Lab

Goals:

  1. Use a catapult system to learn to take careful measurements.
  2. To become familiar with the relationship between launch angle and range.
  3. Calibrate the catapult to allow use in a predictive manner

 

Background

A catapult is a launching device that allows us to experimentally observe projectile motion (see Figure 1).

Figure 1 - Catapult Schematic

Once the ball is launched, the only forces acting are gravity and air resistance. Gravity accelerates the ball in the negative y direction. Air resistance accelerates the ball in the opposite direction of travel. In order to make a catapult useful, we will want to be able to predict where a projectile will land after launch. To do this, we need to calibrate the catapult. Calibration will take into account all accelerations (including air resistance).

 

Our catapults will consist of a base, a plastic spoon, and a duct-tape sling. We will use a plastic golf ball as the projectile. For the first part of the experiment, you will need to calibrate the catapult. This will involve determining experimentally the relationship between launch angle and projectile travel distance. The second part of the experiment will consist of using your Part I results to hit a target with your projectile. As you can see, it will be important to complete the first part carefully. Bonus points will be awarded to successful target hits in part two.

 

Part I: Calibration of the Catapult

Our catapults will look like Figure 2. Here we see a base and a spoon. In the top picture, the ball is launched with a launch angle q = 0°. In the bottom version, the ball is launched with an angle greater than zero.

Figure 2 - Catapult (two views)

 

Step 1: Familiarize yourself with the catapult

You will need to build a sling out of duct tape that will support the ball when the catapult is at zero degrees launch angle. After you are familiar with your catapult and can launch the ball successfully, set up the apparatus for measurements. Remember that you should note your procedures and record all your data in your lab notebook. Be sure to note any special techniques that you use to hold the spoon to the base, to insure a consistent launch velocity, or in the creation of your sling. You will also want to record the ID number of the catapult base and put your name on your spoon to make sure you get the same one in Part II.

 

Step 2: Calibrate the catapult

Take a series of measurements of distance of projectile travel as a function of launch angle. Note: The base of the catapult (the side with the number) should be closest to the lab table. You should create a table in your lab notebook as follows to record your data. Record all data in cm since that is what targets will be given as in Part 2.

q

d1

d2

d3

daverage

0°

 

 

 

 

15°

 

 

 

 

30°

 

 

 

 

 

You should take at least three distance measurements for each angle and you should take measurements for as many angles between 0 and 90 degrees as you have time. Don’t stop at 30° like the sample table above. Remember, the more data you have, the more accurate your calibration. If you have time, take additional data at intermediate angles (for example, fill in with 5° increments).

 

Step 3: Graph your results

Draw a graph of daverage vs. q (daverage on the y-axis, q on the x-axis). This graph will serve as your calibration curve. After plotting the points, draw a smooth line through the data.

 

With a completed graph, you should be able to predict what launch angle will be necessary to launch your projectile any distance. Answer the following questions in your notebook.

 

If you have time left over, you may want to practice hitting a target of arbitrary distance.

 

Part II: Hitting a Target

Make sure that you have the same equipment that you used for part one.

 

Step 1: Predicting the Angle

Upon entering class, you will be given four (4) target distances that you will need to hit. The distances will be measured from the center of the target to the pivot point of your catapult. You will be given 10 minutes from the start of class to calculate the necessary launch angles.

 

Step 2: Launch the Projectile

Two teams will be at each target at the same time. One team will setup and launch their catapult while the other team will act as judges. The decision of the judges is final, but remember, judges will also be judged. So be strict, but fair. Each team will have a maximum of 3 minutes to set up their catapult to the correct angle and launch exactly two (2) projectiles. The score from these two projectiles will be added together. Projectiles that have not been launched after the 3-minute time limit are null and void (i.e. cannot score points). After scores have been recorded, the judging team and launching team will exchange places. After one full rotation, the two teams will move to the next station, with a different distance.

 

Step 3: Score totaling

The team with the most successful target hits (i.e. most points scored) will receive 2 bonus points.

 

Step 4: Analysis

In your lab notebook, reflect on what went right and what went wrong in the targeting phase of the experiment. Include a discussion of what you would do differently to improve your overall results, if you were to repeat the entire experience. What advice would you give to next years students?

 

 

Hints: Suggestions from previous physics students