Phy 211 General Physics I Lab PCC Cascade page 1 of 5 Experiment Real Forces acting on a Falling Body Objectives o o o Observe and record the motion of a falling body Use video analysis to analyze the motion of a falling body Apply Newton s 2nd Law to estimate the forces acting on the falling body Equipment o o o USB digital camera tripod Windows based computer LoggerPro software o o Meterstick Medium size ball Introduction During this session you will use video motion analysis technology to examine the motion of a ball in flight and apply Newton s 2nd Law to estimate the effects of air drag on the ball while it is in the air Procedure A Preliminary measurements Video Recording and Analysis 1 Obtain a medium size ball and measure the mass using a digital scale mball don t forget the units 2 Attach a webcam to a tripod It may be necessary to the remove the base and attachment screw from the webcam Important Remind the instructor to modify the Strings txt file for LoggerPro to increase the video capture rate from 15 to 30 frames per second 3 Plug the webcam into the USB slot on the computer 4 Open LoggerPro 5 Using the mouse select Insert Video Capture Options Camera Settings 6 Use the Exposure controls to i de select Full Auto Mode ii de select Auto in the Exposure control then decrease the shutter speed to 1 300 sec iii increase the gain to roughly 60 to 80 i e adjust the slide control to the right to improve contract iv set the White Balance to Fluorescent 7 Close the Camera Settings window 8 Level the webcam tripod prior to capturing video to minimize potential parallax a k a perspective errors in the video 9 Toss the ball vertically upward in the air and capture the flight of the ball using the webcam Be sure to have a meterstick for scaling the movie in the same plane as the ball 10 Describe the flight of the ball do this before performing video analysis Phy 211 General Physics I Lab PCC Cascade page 2 of 5 11 Using LoggerPro analyze the flight of the ball by generating the following graphs a vertical position vs time b vertical velocity vs time 12 Describe shape of the position vs time graph in 11 13 Describe shape of the velocity vs time graph in 11 14 Question How can the velocity of the ball can be zero at the top of its trajectory whereas its acceleration remains constant B Graphical Analysis i e measuring the acceleration of the ball while it was in the air 15 Select the region of the velocity graph that represents the upward motion of the ball Using the Linear Fit button in LoggerPro fit the graph section to a straight line 16 Observe the fit line generated by the software If the line does not adequately fit the region of the graph you selected then adjust the highlighted area and repeat this operation 17 The slope of this type of graph is the ball s acceleration during this phase of the ball s flight Record the acceleration slope for this motion in the data table below 18 Repeat the previous step for the region of the velocity time graph that represents the motion of the ball near the top of its trajectory This is the region of the graph where the velocity for the ball is low and is the least affected by air drag 19 Repeat the previous step for the region of the velocity time graph that represents the downward motion of the ball Record the acceleration in the data table 20 Copy Paste the position and velocity graphs with fit information into a Word document fit onto a single page and print out a copy the graphs one per each person in the group Phy 211 General Physics I Lab PCC Cascade page 3 of 5 Data Table Mass of Ball Phase of Motion Acceleration Net Force Net Air Drag Upward rising Near the top Downward falling C Applying Newton s 2nd Law to the motion of the ball 21 Question How do the magnitudes of accelerations measured above compare with the accepted value of g 9 8 m s2 Explain any discrepancies 22 Calculate the net force of the ball during each phase of motion above by multiplying the mass of the ball by the measured acceleration Fnet m a Record the net force values in the data table 23 Question How do the net forces Fnet calculated in the previous step compare with the weight of the ball Reminder the weight W is the mass of the ball multiplied by g i e W m g 24 Based on the measurements and calculations above use Newton s 2nd Law to estimate the net drag force Fdrag acting on the ball for each of the phases of motion up down etc you have analyzed record in the table above Note Does it make sense to use the accepted value for g or would a different value be more appropriate Explain 25 Based on your answers in 20 for each phase of motion you analyzed construct the corresponding force vector diagrams for the ball Label the force vectors Phy 211 General Physics I Lab PCC Cascade page 4 of 5 26 Calculate the Error between the air drag values during the upward vs downward motion of the ball Use the following equation F Dragrising FDragfalling 100 Error Smallest FDrag Value 27 How does the air drag during upward motion compare to air drag during downward motion Phy 211 General Physics I Lab PCC Cascade page 5 of 5 Appendix Initial Set Up and Video Capture 1 For instructor a Modify the Strings txt file for LoggerPro in each computer to increase the video capture rate from 15 to 30 frames per second 2 Hints for cameraman a The camera must remain completely still during the digital recording process a tripod is recommended b Be sure the whole ball and its complete motion is clearly in the camera field of view c Place a meter stick in the field of view to be used as a reference length in LoggerPro later on at the same distance from camera as the ball motion d Keep the ball the same horizontal distance from the camera in the field of view and focus the camera straight on to minimize parallax or visual artifacts e The video file should capture the entire trajectory from just before the initial toss through the impact with the ground f When the cameraman and the experimenter s are ready start capturing the movie then toss the ball upward g View your movie in the capture window to verify that it recorded what you want If so you are ready to upload the file to a computer and begin video analysis 3 Video Motion Analysis a Select Insert Video Analysis b Resize and move the movie field if necessary for viewability and convenience c Use the QuickTime buttons to review the movie