The Projectile Lab

Purpose

 

The purpose of this lab is to confirm the key aspects of projectile motion – that an object under the sole influence of gravity moves horizontally with constant velocity and vertically with constant acceleration equal to g.

 

Procedure

 

A video clip of a projectile will be analyzed using Logger Pro 3 software in order to create data and graphs showing the x and y components of the objectÕs position and velocity.  The video clip can be one that you make yourself or one of the videos available on the physics USB drive or the physics Canvas page (the same three videos are found in each of these locations).

 

Opening the video

1.     Start up the computer and run Logger Pro 3.

2.     The video file needs to be located either on the hard drive of the computer or on a USB drive attached to the computer.  Take care of this before proceeding.

3.     Under the Insert menu, choose MovieÉ and then find and open the video.

4.     You should then see a window appear in Logger Pro in which you can play the video.  Experiment with the controls at the bottom of the video that allow you to step forward or backward through the individual frames or rewind to the beginning.

 

Getting data from the video

1.     Before you do anything else select the video and drag the corners of its window to make it as large as possible – this will allow you to be most precise.

2.     Click on the button at the bottom right of your videoÕs window to Enable Video Analysis.

3.     Click on the Set Scale button (yellow ruler) and then click and drag across an object or feature with a known length that can be clearly seen in the video.  A green line appears in the video and a box in which you should enter the correct actual distance that the line spans.  You are establishing the scale of the video.  In the provided videos you will see a meter stick that serves as a convenient object for this purpose.  But any object of known length will suffice.  It is important that this reference object be the same distance from the camera as the projectile.  It is best if a relatively large object is used to set the scale because you will be able to more precisely match the length of the green line to the size of the object.

4.     Advance through the frames of the movie to the first point at which the object is clearly under the sole influence of gravity.

5.     Click on the Add Point button (red dot and crosshairs) to activate data collection.  At this point you are ready to produce the actual data.  Every time you click with the mouse, the computer will record the crosshairÕs position and then advance one frame through the movie.  Center the crosshairs on the object and click on it.  You will see a dot appear where you clicked and you should see the movie advance one frame.  Click on the object again and again to produce a set of dots that follow its trajectory – stopping at the last frame before it ceases to be under the sole influence of gravity. 


 

6.     Once you have clicked on each usable frame then click on the Add Point button again to deactivate data collection.  Or you can click on a different tool button.

7.     Note:  if you make a mistake you can use the Select Point tool (button looks like an arrow) to select any data point that you have created in the video and then delete it or drag it to a new position.

8.     Optional:  there is a Set Origin button that allows you to click anywhere in the video and change the location of the origin of the x-y coordinate system.  It doesnÕt really matter where the origin is located so this is a matter of personal preference.  The choice of origin can have an effect on the appearance of the position graph – you can experiment with it if you like.  This can be done at any point before or after the collection of data and creation of the graphs. 

9.     Once you are satisfied with the points of the objectÕs trajectory you should move and/or resize the video to the right side of the page so that the data table and the video are both visible. 

 

 

Analyzing the Data

 

Edit the Data Table

q  Find the data table and notice that the computer has tabulated x and y position and velocity components based on the points that you clicked on.  You may change the size of this window by dragging its handles so that you can see all five of the columns.  Double click on the data tableÕs title and change it to a one word description of the object you tossed. 

q  Double click on the column header labeled X to open the Column Options menu.  Change the long name to x-Position.  Change the short name to x.  Under the Options tab choose an ÒemptyÓ symbol instead of a ÒfilledÓ symbol.  You can pick a different color if you want. 

q  Repeat the process to edit the other columns using long and short names:  y-Position, y; x-Velocity, Vx; y-Velocity, Vy.  Also give each column a unique ÒemptyÓ symbol and color.

 

Create and edit Position vs. Time and Velocity vs. Time graphs

q  Under the Page menu use the Add Page command to insert a blank page.  Then use the Insert menu to insert a Graph.  Drag the corners of the graph to make it as large as possible. 

q  Click on the label of the y-axis, click on More and select both the x and y components of the objectÕs position. 

q  The range of the scales of the graph may be adjusted – the Autoscale button is handy!

q  Double click on the graphs grid to open the Graph Options menu.  Give it an appropriate title.  Check the Legend box, Check Point Protectors, Uncheck Connect Points.  Make the Minor Tick Style Solid and gray (same as the Major Tick Style). 

q  Use the Analyze menu and Linear Fit to do an automatic fit for the x-Position.  Use the Analyze menu and Curve Fit to do an automatic fit for the y-Position.  Choose a quadratic form and click on Try Fit and then OK.  You can drag the equations and legend wherever you want on the graph – just make it look the way you want it.  Note:  if you have any obvious outliers then you can choose to select only a portion of the data by dragging the brackets that show the range of data being analyzed (or by highlighting a portion of the graph before performing the analysis).

q  Repeat the process to produce another page and graph showing both components of the objectÕs velocity, including appropriate automatic best fit and equation for each component.

 

 

Final Review and Printing

 

Check that you have the following in your file (which you may save to the USB or other location):

1.     Page 1 – Data Table with 5 columns visible and video analysis window.  (Note: there may be a graph here too but you may delete it if you want.)

2.     Page 2 – Position Graph showing x and y components with best fit analysis for each.

3.     Page 3 – Velocity Graph showing x and y components with best fit analysis for each.

 

Read and follow these directions to print out one copy of your results for each person in your group:
 

1.     Under the File menu choose Printing Options and choose to create a footer that shows the last name of each person in your group. 

2.     Under the File menu choose Page Setup and select a Landscape orientation.

3.     Under the File menu use Print Preview to make sure everything looks good!

4.     Print the results.

 

 

A complete report (50 pts.) will consist of the following (in this order):

 

q  Completed data table and still shot of video. (12)

q  Position vs. Time graph with best fit and equation for both x and y components  (12)

q  Velocity vs. Time graph with best fit and equation for both x and y components  (12)

q  Answers to the following questions on separate paper  (12)

 

Questions

1.     Describe the horizontal motion of the object. (Is it constant acceleration, constant velocity, or what?)  Explain your answer by referring specifically to one or more of the results shown on the graphs.  (2)

2.     Determine the horizontal component of the object's velocity by using the results of one or more of the curve fits.  Show your work and/or explain your method.  (2)

3.     Describe the vertical motion of the object.  (Is it constant acceleration, constant velocity, or what?)  Explain your answer by referring specifically to one or more of the results shown on the graphs.  (2)

4.     Determine the vertical component of the object's acceleration by using the results of one or more of the curve fits.  Show your work and/or explain your method.  (2)

5.     Determine the relative (percent) error in the vertical acceleration found above.  Show all work.  (2)

6.     Write an intelligent, grammatically correct, and concise discussion of error in this lab.  Any complete discussion of error will include:  indication or evidence of error(s) and speculation or explanation of the most likely sources of the same.  Remember to consider both types of error – random and systematic.  Your goal here is to satisfactorily explain how and why your results are not perfect.  (2)