PHY 211 Lab: Free Fall - Portland Community College



Experiment #2: Free Fall

Objects tossed up or down accelerate at a constant rate, in the absence of air resistance. This rate is called the free-fall acceleration, or acceleration due to gravity, and its magnitude is represented by g. The value for g does not depend on the object’s mass or shape, it is constant for all objects.

For objects traveling at relatively low speed, air resistance is not significant and therefore will not be considered in this experiment. During this experiment you will record and analyze the motion of an object in free fall. This experiment is intended to help you better understand the concept of acceleration and the kinematic equations.

OBJECTIVES

o Record and analyze free fall motion using video technology

o Model the free-fall motion

o Compare the free fall model to the kinematic equations of the accelerated motion

o Determine the acceleration due to gravity.

MATERIALS

A Windows-based computer workstation Rubber ball

Logger Pro Software Nikon CoolPix digital camera

PRELIMINARY QUESTIONS

1. For an object tossed vertically upward with an initial velocity of 2m/s, sketch the velocity vs. time graph. What is the significance of the slope of this graph, in the absence of air resistance?

2. For the same object, what is the value of the acceleration at the beginning of the motion? At the top of the path?

3. For the same object, predict the shape of the distance vs. time graph.

4. At the top of the path, what is the value of instantaneous velocity? What about instantaneous acceleration?

A. Making a Movie

1. Place a vertical meter stick in the background. The meter stick will be used to scale the movie.

2. Obtain a ball (dark color against the white background preferred) and position yourself next to the meter stick.

3. Have your lab partner place the camera on a firm surface and prepare to take the movie. Practice throwing the ball vertically and keeping the ball inside the screen. Slower velocities are preferable (better image resolution).

4. Record the movie

5. Review the movie to make sure that the entire path of the ball is recorded and that the meter stick is visible. If it is not, delete the movie and record another one.

6. When you have an acceptable movie, turn off the camera

7. Connect the camera to the computer using the USB cord provided. Turn the camera on. The computer will display the message" Found new hardware". Wait until the computer displays the message" New hardware installed and ready to use"

8. From" My Computer", open" Removable Disk (F)"/DCIM/lOONikon. Move your movie (file of type DSCNOO**) to the desktop, wait for the file to be copied and then close the window.

B. Analyzing the movie

1. Open All Programs/Vernier Software/LoggerPro.3.0. From the top toolbar, select Insert/Movie. From the dialog box, open "Desktop" and select your file (DSCNOO**).The first frame of your movie will be displayed.

2. Play the movie by clicking on the arrow at the bottom left side of the movie►. Go back frame by frame using the double arrows ►►to the first frame where the ball is in motion.

3. Click on the right-bottom arrow to enable the video analysis. From the vertical toolbar, click on to scale the movie. On the screen, click on the bottom end of the meterstick and drag to the top end. In the dialog box, enter the length of the meterstick.

4. You are now ready to analyze the movie. From the vertical toolbar, click on icon select your pointer. Click on the ball in the first frame where the ball is moving. The movie will advance by one frame. Click on the ball on each frame, until the last frame of the movie.

5. Click on the icon again to de-activate your pointer. Minimize the movie, so you could see the graphs.

6. Click between the two arrows on the y axis and select "y". The graph of the vertical position will be displayed. What is the shape of the graph? What type of motion is the motion on the vertical direction?

7. Click and drag on the graph and select the appropriate fit. What kind of curve fit best matched the graph? Print the graph.

8. What is the physical significance of the coefficients a, b and c?

9. Click between the two arrows on the y axis label and select "y velocity". The graph of the vertical velocity will be displayed. What is the shape of the graph? What type of motion is the motion on the vertical direction?

10. Click and drag on the graph and then try a linear fit. What is physical significance of the slope of this graph? Print the graph

Slope = _______.

11. Explain why there is a portion of positive velocity and then negative velocity on the graph.

12. Is there a point where the vertical velocity is zero? Where is this point? Is the acceleration zero at this point? Do these results match your prediction in the Preliminary Question?

13. Is there a relationship between the slope of the y-velocity graph and the parameter a from the y graph? Why?

14. Using Vy vs time graph, find the acceleration in the y direction. Explain the significance of the portion with negative Vy. Where did the ball slow down, and where did it speed up?

15. Is the acceleration zero at the top of the path?

16. Did you prove that the acceleration is constant? How does your data support this affirmation?

17. Do your experimental results match your predictions in the Preliminary Questions?

18. The accepted value for the acceleration due to gravity (magnitude) is 9.8 m/s2. How does this value compare to the acceleration calculated in step 14 above? Calculate the % error between these values and list several possible reasons.

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