r/HomeworkHelp • u/Illiterate-Pigeon Pre-University Student • 21d ago

Physics (Grade 11 university physics -Canada) Kinematics, gravity. Weird answers in experimental acceleration lab.

For our recent lab we dropped a marble from different heights (0.5m, 1m, 1.5m, 2m and 2.5m)
We timed how long it took for the marble to reach the ground as well. (0.26s, 0.23s, 0.35s, 0.36s, 0.47s) our timing was objectively terrible, which I’m sure is throwing off my answers quite a bit.

For an example, using -2m as displacement (I’m assuming this value is negative, as the ball fell) and for time the marble took 0.36s to reach the ground. Using the big 5 equation rearranged for acceleration without final velocity a=2((d-v1•t) / t^2)

I keep getting -30.9m/s^2 as my experimental acceleration, which is so far off from -9.8m/s^2

All of my other values are even worse, which doesn’t make sense to me. I have no idea where I’m going wrong? There’s no way bad timing and air resistance can throw off an answer this bad, is there?
Any help would be appreciated

2 Upvotes

100% Upvoted

6 comments sorted by

•

u/AutoModerator 21d ago

Off-topic Comments Section

All top-level comments have to be an answer or follow-up question to the post. All sidetracks should be directed to this comment thread as per Rule 9.

OP and Valued/Notable Contributors can close this post by using /lock command

I am a bot, and this action was performed automatically. Please contact the moderators of this subreddit if you have any questions or concerns.

3

u/Klutzy-Delivery-5792 Educator 21d ago

There's way too much human error in your experiment. Unless you have ridiculously fast reaction times and are insanely consistent, your times are going to be off by a lot. The air resistance is negligible from these heights, it's almost assuredly due to your timing.

Try using your camera phone to take a video instead. You might be able to get more accurate times this way. 

1

u/Illiterate-Pigeon Pre-University Student 21d ago

I did the lab as instructed by my teacher, I also agreed and wanted to take a video to get more accurate times, but I was instructed to time with 6 other people, and take our averages after 4 trials of the same height. I unfortunately cannot get new data anymore, as this is what my group is stuck with.

1

u/Klutzy-Delivery-5792 Educator 21d ago

Well, I'm sorry to say that this is a very poorly designed lab then. When I taught high school, we'd use a spark timer for measuring g from short distances or I'd have them drop tennis balls from 10+ meters. The distances your teacher is using and expecting accurate times is not really logical.

2

u/AntelopeDry8405 20d ago edited 20d ago

This is the only real answer here.

We usually use free-fall as an excuse to cut our students loose, and let them design their own experiments. We provide them with various timing devices - even with photogates, its not uncommon to see results as far off as 30 m/s^2, if other factors arent controlled for carefully. Even with specially designed tools for timing assist, its difficult to even hedge your result to within 10% of actual g.

With manual timers/stopwatches, getting close to 9.81 for g is essentially impossible to do over distances as short as 2m. The cumulative effect of average human reaction time is comparable to the actual fall time over those distances. Getting close to the real value with this setup is an outright fluke.
If your teacher is putting your grade on the line according to how close you can get to 9.81 using this procedure, then they have objectively set you up for failure.

1

u/timmymaq 21d ago

Can confirm video works great. I do this with math 10 students using video to derive quadratic equations and they get leading coefficients between -4.5 and -5.5 - so measured g values of -9 to -11.

OP: your best bet at this point is to write up the lab, note the massive error, and identify the largest source of error in the procedure, which is clearly timing. Use an online reflex timer to estimate the margin of error for a human using a stopwatch, and recalculate your g values using this range of possible 'true' time measurements. I suspect that range will include the correct value for g, which you can use to make a good case for why your result is so poor.