AIR TRACK PHYSICS
NOTES:
An air track is a device used to minimize friction and ensure that motion is in a straight line. It consists of a hollow square tube with one corner up. The two sides adjacent to the upper corner have small holes drilled at regular intervals. An air blower blows air into the tube which escapes through the holes. A "glider" or "cart" consists of two matching metal plates that ride on the cushion of air coming out of the small holes. With care, one can perform experiments that are barely influenced by friction. Using the motion detector fro the Lab Pro to measure the position of the glider results in minimal perturbation of the motion. However, the flag that allows the glider to be 'seen' by the sonic ranger does add some air resistance - it is a trade off.
You will experimentally determine g in this experiment, and in the process get more practice with understanding position, velocity and acceleration graphs.
First level your track. A cart placed stationary on the track should not drift. Adjustments are made by raising or lowering the bolt at the end of the track.
This section lets you review what you already know about distance, velocity and acceleration graphs. Set up the motion detector so it will "see" the cart for the entire length of the track. Start the cart at the end of the track closest to the detector. When you see the graph start, give the cart a slight push so that it travels to the other end and rebounds. Keep trying until you get a graph with no anomalous spikes. Print a page containing all three graphs for each student in your group. On each of three graphs label significant points, such as when you began pushing, when you stopped pushing, when the cart hit the bumper, etc.
Raise the end of the track by placing something under the single foot. Later you will need to know the angle to which you raised the track, so go ahead and make the measurements you will need to calculate the angle. These are the spacing between the legs (center-to-center or left edge to left edge, etc.)and the height of the object lifting the end. A right triangle is formed by the track, the distance between the legs and height of the object. Since you have measured the adjacent and opposite sides, you can find the angle.
Place the cart at the high end of the track and release it. There is no need to push it this time. Be sure you get at least one rebound on the graph. (You will probably need to change the angle of the detector.) Keep trying until you get a graph with no anomalous spikes.
Print all three graphs. Label significant points as you did before.
Put the velocity graph on the screen. Select one of the sections of the graph in which the cart is moving downhill by pressing the mouse button at the point you want to start and dragging it to the point you want to stop. Choose Linear Fit from the Analyze menu which will fit a straight line to this section of the graph. The slope of the line is the acceleration. Record the acceleration. Now you can change to an uphill section of the graph and repeat the process for that segment. There should be only a small variation between the two accelerations. If you have large variations, see your instructor. Average the accelerations for the two segments. (This eliminates the small effect that friction still has.)
Change the angle of the track and find the average acceleration again. You do not need to print graphs again. Remember to repeat your measurements for finding the track angle. Repeat for two more angles. (You should have total of four angles.)
You can use your data for acceleration and track angle to find the acceleration of gravity. Plot a graph of acceleration versus sine of the angle. Use your graph to calculate g.
Last Modified Jan 2014