The purpose of this lab is to prove that the acceleration of gravity is 9.8m/s/s. We did this by taking a time lapse photo of a ball dropped with a meter stick next to it. While the ball was dropped, a strobe light was flashing 3000 times per second, which created measurable points of the balls descent. Then we did a few different graphs of the data points by hand and computer.
1. Set up a meter stick vertically against a black background
2. Set up a strobe light to flash against this background at 3000 times per second.
3. Focus a camera to take in the entire length of the meter stick to ensure that the balls entire flight is captured.
4. Pull back the slide to prepare the film.
5. Press and hold the shutter open and drop the ball at the same time.
6. Release the shutter when the ball hits the ground.
7. Wait 30 sec for the picture to develop and record the measurements for each time the ball is shown.
8. Set up a graph with time as the x-axis and your distance measurements on the y-axis.
9. Plot your distance points every .02s, then sketch a best fit curve.
10. Determine the slope at every other data point by drawing tangent lines.
11. The slope found on the tangent lines is the velocity at each instance.
12. Plot a new graph using the velocity data as the new y-axis.
13. Determine the slope of that graph to determine the experimental acceleration
14. Repeat process on computer.
Distance vs. Time
Slopes of Distance vs. Time
(change in distance / change in time)
Time (s) First Point (s , cm) Second Point(s , cm) Slope (cm / s)
.02 (0 , 0) (.04 , 7.5)
.06 (.04 , 11.5) (.08 , 16.5)
.1 (.08 , 16.5) (.12 , 27.5)
.14 (.12 , 27.5) (.16 , 40.5)
.18 (.16 , 40.5) (.20 , 54.5)
.2 (.18 , 46.5) (.22 , 62.5)
Velocity vs. Time
The slope (acceleration) of the velocity vs. time graph is found:
Change in velocity / change in time
((Accepted - determined) / accepted) * 100
((980 cm / s / s - cm / s / s) / 980 cm /s /s) * 100
Percent Error =
Error could be found in two main places in this lab. They were in taking the picture, doing the graph by hand. Since the entire lab revolves around the picture you take in the beginning, it is important to take the picture correctly
The first thing that could cause error when you take the picture involves setting the strobe light. If it is not at 3000 flashes per second, then you will get little or no data points in your picture, making it impossible to draw an accurate graph. Another source of error would be in leaving the shutter open too long. This will cause your film to become overexposed and nothing will be seen on it. To correct this, be sure to watch for the ball hitting your partner's hand or the floor.
When you are doing your graph by hand, a major error can occur. When you draw the first curve, it is a best-fit curve. Since a human hand draws it, it might not actually be a best-fit curve for those data points. This would cause the slopes of you tangent lines to be off resulting in incorrect data points for your velocity vs. time graph. The only solution is to use a computer to make the graph. Also the tangent lines themselves are best guesses. The tangent lines you draw might actually touch the graph in more than one place, resulting again in incorrect slopes and incorrect data points for the velocity vs. time graph. To correct this, one could use a computer again.
This lab shows how lab techniques have progressed over the years. It also teaches how to collect distance measurements from a high-speed object. This lab also showed how gravity accelerates objects towards the earth.
One of the things learned in this lab was that mass does not affect the acceleration of gravity in the slightest. Nowhere in our calculations did mass come into play. The acceleration of gravity is constant for every object, so every object falls at the same rate. We do know from experience however that if you change an objects' surface area (make it flatter) it will fall at a slower rate. However this is not from a decrease in the force of acceleration to do gravity, but rather an increase in air resistance.
The technique that we learned to take distance measurements for a fast-moving object is very simple. To perform this lab technique all that is needed is a dark room, a strobe light, a camera, and a suspended meter stick. When you drop the ball parallel to the meter stick with the strobe light on and the cameras shutter open, you get a picture of the ball every flash of the strobe light. This results in about 16 data points along a meter stick.
Another lab technique learned in this lab is using computer-graphing software, which results in a much lower percent error due to computers being more accurate than humans are.