Virtual Science Ltd
Determining Acceleration Using an AirTrack
Introduction
In this experiment we are going to investigate acceleration using an AirTrack. An AirTrack is a very useful device for performing a variety of experiments about motion, forces, and momentum. What makes it so effective is its use of pumped air to create a virtually frictionless track upon which various glider configurations can be put in motion and collided against.
This picture shows you the main components.
Here you can see the main track in blue with many small holes along its length. Air is pumped through these holes which creates a virtually friction free surface for the glider to travel along. There are two sets of support legs. The left-hand supporting legs can be raised and lowered. The buffers at the ends of the track have rubber bands which reverse the direction and speed of the gliders. Notice that there is a ruler alongside the track with a moveable pointer that can be used to reset the position of the glider.
The photogate timers are used to time the gliders travelling through them. The photogate timer has a light and a detector inside the U-shaped gantry. When the glider’s white rectangular card interrupts the beam the timer runs. The time that is recorded can be used to calculate the speed as its length is known.
In this practical we are going to measure acceleration. In this configuration we will simply raise one end of the track and let gravity provide the force that will accelerate the glider along the track.
The objective
To measure the acceleration of the glider on an AirTrack when subjected to a constant force from gravity.
The apparatus
You will need:
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An AirTrack
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An air pump
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A glider
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Two photogate timers, paired and not paired (when paired, the timers act together and record the time between entering the first gate and the second gate; when not paired, a photogate records the time that the glider takes to pass through this timer only)
The track should be configured as shown as above, it does not need to be level.
The variables
The dependent variables are the time taken to travel between the paired photogate timers and the time to pass through an independent photogate timer.
The Physics
Average speed through each timer gate is simply calculated by dividing the length of the card on the glider by the time it takes to travel through each timer gate. This gives two speeds: V1 and V2. The time it takes to travel from one timer gate to another is Dt. The acceleration is given by the change in velocity over the time Dt:
A = (V2 – V1)/Dt
If we take as an assumption that the acceleration is constant, then this result will be accurate. We can attempt to verify this by making the distance between the gates smaller and smaller and examining the calculated acceleration for each.
The method
Measure and make a note of the length of the glider’s fin. Move the position indicator on the track to about one-fifth of the way along from the left. Raise the left part of the track (the opposite end to the pump), using the height adjuster, so that there is an appreciable acceleration when the pump is on.
Make sure the photogate timers are in paired mode, with the glider at the position of the indicator as shown below.
Switch on the pump (or with the pump on, hold it there and then release). Make a note of how long it took the glider to travel between the two gates; this is Dt. You may want to stop the pump at this point.
Swap to using two independent timer gates in exactly the same positions as the two paired gates. Make sure these are both switched on and are not paired.
Take the glider back to the same position and, with the pump on, release it. Take note of the times through each gate; these are T1 and T2.
Take repeated readings with different distances between the photogate timers, always starting the glider from the same position. Complete the following table. Where Dt is the time taken to travel between the gates (measured using the paired gates). T1 is the time through the first gate. T2 is the time through the second gate. V1 is the average speed through the first gate. V2 is the average speed through the second gate. A is the acceleration.
The Video
Watch a video of the Acceleration e-practical here.
This shows how to use it and how to collect the data.
The e-Practical
Perform the experiment yourself, collect your own data, make mistakes and be able to correct them. The e-practical requires that your browser canrun WebGL 2 (usually found on Windows browsers, safari on iOS, and various Mobile browsers, test with https://get.webgl.org/webgl2/). This link is for students and evaluation only, schools should purchase a site licence.
The e-practical will run on laptops and desktops for PCs and Apple computers and will run on mid to high spec tablets and 'phones.
On a portable device, make sure you click on 'Toggle onscreen controls'. The left joystick controls movement, the right joystick controls direction and where you are looking.
The length of the glider’s fin is 16.5 cm.
You can change from paired timer gates, which are used to measure the time of the glider passing between both, and non-paired timer gates that will independently measure the time taken for the glider to move through each, by clicking on the large red cube (You can see this in the picture at the start of this section at left of the track). Note that when the timer gates have been swapped, the new gate goes to the exact same position as where the old gate was. This can be convenient when taking a series of measurements that involve both sorts of timer gate.
The glider can be stopped by holding down the mouse button when the cursor is over the fin (the rectangular upper part). It can be released by releasing the mouse button.
An impulse can be applied to the glider by placing the cursor over the rectangular area and using the mouse wheel or dragging when using onscreen controls. This can be useful when you want to place the glider at a particular position on the track. Even with the pump off, it is possible to move the glider downhill by giving it an impulse. This is very useful when you want to position the glider accurately.
The green pointer can be moved left or right by putting the mouse cursor over it and then using the mouse wheel or dragging when using onscreen controls.
The two timers can be moved left or right by putting the mouse cursor over their bases and using the mouse wheel or dragging when using onscreen controls.
The exact position of a timer can be accurately estimated from its shadow on the AirTrack’s scale.
The Results
From your table you should find that the acceleration is the same in all cases.
Further Discussion
What happens if you start the glider from a position further from the first photogate timer? Can you explain the result?
By noting the amount that the track has been raised you can calculate the angle of the track. You can use this to calculate a value for g, the acceleration due to gravity.
E-practical users beware the gravity here is about 15% of the Earth’s!
If you tried to calculate g using a sloped ramp and a cylinder that rolled down the ramp, why would it not give a good result? (Hint: think about the energy of the cylinder).
This section is adapted from material developed by Dr Robert Lucas and is related to the book High School and Undergraduate Physics Practicals, published by CRC Press.