What would affect gravity during this experiment?

[exparrot]

I did a Newton's 2nd Law experiment for my physics lab. One of the write-up questions asks to identify at least 2 causes why your calculated value of g is not equal to 9.8 m/s². It also asks to explain how would each cause affect your data and what would make the experimental value bigger or smaller.

So in the experiment, we had set up an air track with a pulley attached to the end of the track. A cart rested on the airtrack, a string was tied to one end of the cart, over the pulley to a 5 gm weighted hanger. To keep the mass constant, 110 gram weights were placed on the cart, and after each run, the weights were transferred to the hanger. Five trials were done.

Calculations from my data gave an acceleration of 9.81540 m/s² and against the actual value of g, I get a percent error of 0.0550 %. However since I'm suppose to consider why I would not get the same value for acceleration due to gravity, would that be because of some other force, such as tension (which I don't imagine would interfere) or the normal force? I'd appreciate if anyone has some idea(s) and could help me out with this. Thanks!

 

[kuruman]

You don't say exactly what you determined with your setup, but I assume it was the acceleration of the cart and from this you got g. I will list some assumptions you probably made, but you have to figure out if they would make any difference to your value of g and in what direction.

1. The pulley is massless.
2. There is no friction on the track or the pulley.
3. There is no air resistance.
4. The track is exactly horizontal.
5. The string is massless.

 

[mgb_phys]

Another point, 9.81540 m/s2 is suspicously accurate - at that level you can't even know the local value of 'g' without doing a geological survey.

Presumably you did the experiment more than once?
Look at the range of the answers.
If you actually got results like, 9.81, 9.80, 9.82, 9.81 for example then your answer would be something like 9.81 +/- 0.1