# Linear Air Track: Measuring Acceleration (for IG and AS)

The trolley floats on a cushion of air on a V-shaped track, so there’s no friction to slow it down. When the light beam is interrupted by the passage of the trolley it starts a clock. The clock stops when the beam is restored. The logger can be connected to a computer for easier display and configured to calculate average speed.

IG: The card on top of the trolley measured 199mm = the distance travelled. The time for the trolley’s journey through the light gates can be read off the display.

Example:

Average speed = distance / time = 199mm/0.403s = 493.8mm/s (1d.p)

AS students. You MUST read this part: If one end of the trolley was inclined (tipped up), the trolley will accelerate down the slope since the component of its weight acting parallel to the slope will provide the unbalanced force. Can we design and carry out an experiment to compare the acceleration of the trolley with the angle of inclination which will determine the size of the unbalanced force?

I want you to develop your own procedure for this and try it out in the lab, thinking through each stage carefully.

The new light gates are more sensitive and better than the old ones.

If we use two light gates the logger can calculate the average speed of the trolley as it passes through the first and then the second light gate. The onboard timer will then be used to calculate the acceleration of the trolley being the difference in average speed between gate 1 and gate 2.

Perhaps a better way might be to use just one light gate with a split card, like the one shown below.

If we input the dimensions of the card into the logger, it will calculate the average speed during the first and then the second pass of the card and calculate the acceleration for us automatically. If you can’t remember how to do this, look here.

Important design points:

1. How are you going to ensure that the air track is horizontal to start with? Should each run start from the same place? How will you make sure that this happens?

2. You can use plywood spacers or even new exercise books to prop up one end of the track. How are you going to measure the angle to the horizontal? (not with a protractor)

3. What about accuracy? How many times should each measurement be repeated and what are you going to do with the values?  Can you identify any errors in your procedure or results? What are you going to do about them?

4. How are you going to show what your results look like – a table, a graph, or both?

5. Can you predict what the shape of the graph ought to be? What information is obtainable from it?

Solution. Check carefully..All of you should have found time to use the apparatus. AS asks about datalogging principles.

An exaggerated view. The free body force diagram plus resolved components is shown. Weight W acts vertically downwards and normal reaction acts R at 90 degrees to the surface. The resolved component of W, W sin ϑ causes the trolley to accelerate down the slope. F(R) is a zero frictional resistance so can be ignored.