While I was checking the worksheets at the end of class I was noticing that a few groups had “interesting” values for their cart masses on the final table of Unit 5.
- The mass values you put in the table derive from the accelerations you measure, not the spring scale measurements
- If you did things right then the Force column (the forces from the fan) should be the same for all cases where the same number of batteries were used.
- The masses you calculate this way are systematically wrong because you ignore friction. But if they are wrong by more than about 20% then they are too wrong. (Later we’ll learn how to correct for the friction to get a better value.)
- If they are too wrong then check your method of calculation. The ratio of accelerations should be equal to the ratio of the total mass that is being accelerated. That means you have to do some algebra to solve for the mass of the fan-cart in terms of the bar’s mass.
- The question asking for g want the value implied by your data. If you measured the weight of the cart with a spring scale that weight would be in newtons. If you used to the spring scale and read the mass scale you have to use g = 9.8 N/kg to get the weight. Then you report another value of g in the question which will be somewhat wrong because friction was ignored.
- If you find you just can’t get any reasonable values from your data then try to understand what’s wrong and explain your analysis of the issues. We don’t expect you to always get perfect results or to fudge your data and we don’t necessarily grade on the nearness of your answer to the “right” answer.