Physics Help Forum Theoretical vs. Expermental values

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 Oct 7th 2018, 08:54 AM #1 Junior Member   Join Date: Oct 2018 Posts: 3 Theoretical vs. Expermental values I did a Half-Atwood machine experiment using a car where I obtained 3.6% of difference between the theoretical and experimental force, while the percent difference between the theoretical and experimental acceleration was 18%!! The experimental force was the mean of a Force vs. Time graph, while the experimental acceleration was the slope of a Velocity vs. Time graph. What are the reasons for the differences between theoretical and experimental values? Does the experimental acceleration have a higher percent of difference because of friction? Attached Thumbnails
 Oct 7th 2018, 03:59 PM #2 Senior Member   Join Date: Oct 2017 Location: Glasgow Posts: 364 Well, according to propagation of errors in quadrature, you would expect a percentage error in the acceleration to be the same as the percentage error in the resultant force. However, all sorts of things might be responsible for the greater error. For example: 1. Your apparatus may be set up imprecisely (e.g. horizontal beam isn't horizontal, mass is different to the model value, etc.) 2. Inaccurate representation of the resultant force because of "intrusive" forces (like air resistance, friction, movement of apparatus, etc.) 3. The precision on your speedometer or clock may be poor There's more too, but you'd have to consider the situation you have and try and gauge what may be the biggest factors affecting your uncertainty.
 Oct 8th 2018, 05:29 AM #3 Senior Member     Join Date: Jun 2016 Location: England Posts: 882 You mention that the force was the mean of a force vs time graph How was the force determined? was it not just mass times gravity? in which case how did it vary with time? It can be informative to do the experiment many times, and analyse the statistical variability of the results. __________________ ~\o/~
 Oct 8th 2018, 07:10 AM #4 Junior Member   Join Date: Oct 2018 Posts: 3 I did various runs and still obtained the same acceleration. The experimental force was the mean in a force vs. time graph and the theoretical one was determined by multiplying the car + sensors mass x the theoretical acceleration. Attached Thumbnails
Oct 8th 2018, 07:11 AM   #5
Junior Member

Join Date: Oct 2018
Posts: 3
 Originally Posted by benit13 Well, according to propagation of errors in quadrature, you would expect a percentage error in the acceleration to be the same as the percentage error in the resultant force. However, all sorts of things might be responsible for the greater error. For example: 1. Your apparatus may be set up imprecisely (e.g. horizontal beam isn't horizontal, mass is different to the model value, etc.) 2. Inaccurate representation of the resultant force because of "intrusive" forces (like air resistance, friction, movement of apparatus, etc.) 3. The precision on your speedometer or clock may be poor There's more too, but you'd have to consider the situation you have and try and gauge what may be the biggest factors affecting your uncertainty.
Thank you, I will evaluate my results considering those factors.

 Oct 8th 2018, 01:38 PM #6 Physics Team     Join Date: Jun 2010 Location: Morristown, NJ USA Posts: 2,344 Don't forget that there is friction in the apparatus - in the pulleys and especially in the wheel/axle bearings and between wheels and the table. This may the major source of reduction of acceleration from theoretical. benit13 and acarolina like this.

 Tags expermental, theoretical, values