The Acceleration Required Practical Without Light Gates PART DEUX

I have written about completing the acceleration practical without light gates before but I thought I’d share a slight variation on the original method that I have found to work well with my teaching groups. Links to some digital resources (spreadsheet, powerpoint and worksheet) will be included.

The method does not require light gates or a data logger. In fact, the only measuring instruments needed are a metre rule and a stop clock. The other items are standard laboratory equipment (dynamic trolley, bench pulley, string. 4 x 10 g masses on a hanger, 4 x 100 g masses on a hanger, and wooden runway). If your class can access IT then a rather clever spreadsheet is included, but this is not essential.

We use small 10 g masses to accelerate the trolley so the time it takes to travel a certain distance (between 0.50 to 0.90 m) can be timed manually with a stop clock (typical time for the 10 g mass is between 3 and 5 seconds).

This works well as a class practical, especially if you follow Adam Boxer’s excellent ‘Slow Practical’ method.

The Powerpoint that I use to run this practical can be downloaded here.

Set up a friction-compensated slope

The F in Newton’s Second Law stands for the resultant force (or total force) so ideally we should eliminate any frictional force tending to slow down the trolley. This can be done by tilting the runway slightly as shown.

Using one or two 100 g slotted masses propped under one end of the runway provides enough of a slope so that the trolley continues moving at a steady speed when given a short, gentle push. Use trial and error to find the precise angle of the slope needed.

Students should mark START and STOP lines on the runway and measure the distance s between them and record it on the worksheet (or in the spreadsheet).

Make sure the weight stack does not hit the ground before the trolley crosses the stop line, otherwise the results will be unreliable as the trolley will not be accelerating over the full distance.

Use a system of constant mass

Increase the mass of the trolley, but keep F fixed

Calculate the acceleration

The force of the weight stack on the trolley can be calculated using W=mg where m is the mass in kilograms and g is the gravitational field strength of 9.81 N/kg, although the approximation 10 g = 0.10 N can be useful if students are performing the calculations and plotting the graph manually.

Students can use the formula a = 2s/t2 to calculate the acceleration manually. Note that the units of this expression are m/s2 as we would expect for a valid equation for acceleration.

A derivation of this expression suitable for GCSE students is outlined on Slide 5 of the Powerpoint.

If students have access to tablets or computers, they can use this spreadsheet to automatically calculate the results and plot the graph. Students can print the graph if they click on the relevant tab. (The line of best fit is not included as all students generally benefit from practicing this skill!)

Evaluate the results

Students can evaluate the results using Slide 7 of the Powerpoint.

Note that in the graph shown, although there is a convincing straight line of best fit, there is also a noticeable systematic error: the acceleration is slightly too small for the indicated force. This would suggest that the runway was not tilted steeply enough to eliminate all frictional forces.

If you find this blog and resources useful, please leave a comment and/or share it on Twitter 🙂

5 thoughts on “The Acceleration Required Practical Without Light Gates PART DEUX

  1. paul martin November 8, 2020 / 5:35 pm

    I attended an excellent IoP teach trainee Physics day-long session before March in a 70’s era school. I was taken by the ancient equipment for the above experiment which I have written about elsewhere (I just checked Part Uno of yours & it was not there).

    There were four groups and from my elderly perspective the people (sans your Powerpoint) were struggling to get the equipment to work. Only when light gates were introduced was it easy to do. Admittedly I am adept at SUVAT (from 40yrs ago). The trauma on the group was exemplified by their approach to the air track practical – no one wanted to take the lead with that piece of equipment.

    I can see if the school was poorer your approach would be useful but progress is progress. (Hope I have not been too -ve)

    • e=mc2andallthat November 8, 2020 / 9:16 pm

      Not at all negative — light gates undoubtedly do the job more reliably (most of the time — we sometimes have problems as ours are wireless). The issue I have is that often LGs are simply a ‘black box’ for students — if they return just the value of the acceleration, why do the prac? Student may simply see it as the relationship of KAKSJJSKALALSLSL (random characters) to force. At least here they actually get to calculate the value of the acceleration.

  2. LB (@linbak1234) November 25, 2020 / 9:46 pm

    Thank you for this! We’ve just changed to AQA (first cohort in yr11 now) so I thought I’d give this prac a whirl and it was good. One or two groups had odd results, but the prac highlighted some rather dodgy practical/measurement skills that they got to fix so there was plenty of skill development which wouldn’t have happened with just a ramp and light gates. Thanks!!

    • e=mc2andallthat November 25, 2020 / 10:28 pm

      Excellent! So pleased it went well. What were the dodgy practical/measurement skills — I’m dying to know 🙂

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