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# Determining g by free-fall - shorter distances? watch

1. Hey

In the experiment (ball dropped through light gates, pretty common stuff), the height dropped was never lower than 0.250m because it would have been "difficult to obtain accurate timings" - why is it so difficult? I understand the larger % uncertainty but wouldn't it be easier as there is less chance of the ball making an irregular (not perpendicular) path?
2. (Original post by h8skoooooool)
Hey

In the experiment (ball dropped through light gates, pretty common stuff), the height dropped was never lower than 0.250m because it would have been "difficult to obtain accurate timings" - why is it so difficult? I understand the larger % uncertainty but wouldn't it be easier as there is less chance of the ball making an irregular (not perpendicular) path?
IMO air resistance would cause the ball to reach terminal velocity. since the acceleration at terminal velocity is zero, you would get a lower value for g.

3. (Original post by Kyx)
IMO air resistance would cause the ball to reach terminal velocity. since the acceleration at terminal velocity is zero, you would get a lower value for g.

Yeah I get the science behind the experiment, I don't see why lower values for the height would be inaccurate or difficult to measure when the light gate is very precise (can't remember how many decimal places but approx 5 or 6?)
4. (Original post by h8skoooooool)
Yeah I get the science behind the experiment, I don't see why lower values for the height would be inaccurate or difficult to measure when the light gate is very precise (can't remember how many decimal places but approx 5 or 6?)

thought it said 'higher than', not 'lower than'...
5. (Original post by Kyx)

thought it said 'higher than', not 'lower than'...
Ah no problem, thank you anyway!
6. (Original post by h8skoooooool)
Hey

In the experiment (ball dropped through light gates, pretty common stuff), the height dropped was never lower than 0.250m because it would have been "difficult to obtain accurate timings" - why is it so difficult? I understand the larger % uncertainty but wouldn't it be easier as there is less chance of the ball making an irregular (not perpendicular) path?
Well it depends on what you're using to time it....?
7. (Original post by h8skoooooool)
Yeah I get the science behind the experiment, I don't see why lower values for the height would be inaccurate or difficult to measure when the light gate is very precise (can't remember how many decimal places but approx 5 or 6?)
Well afaik the light gate is going to trigger when the ball is some way through the light beam... but how far through the beam?

width of the beam is fixed and starts to become a significant %age of the drop distance as the drop distance is reduced.

the proper solution would be to try measure the light gate's trigger points for that ball as accurately as possible and do a lot of repeat runs... but probably there isn't time in a lesson.

---
if you were doing it with a pendulum method otoh you can time over as many swings as you like to reduce uncertainty.
8. (Original post by Joinedup)
Well afaik the light gate is going to trigger when the ball is some way through the light beam... but how far through the beam?

width of the beam is fixed and starts to become a significant %age of the drop distance as the drop distance is reduced.

the proper solution would be to try measure the light gate's trigger points for that ball as accurately as possible and do a lot of repeat runs... but probably there isn't time in a lesson.

---
if you were doing it with a pendulum method otoh you can time over as many swings as you like to reduce uncertainty.
There was a line going across the centre of the light gates, and we were assuming that was the point at which the ball crossed the beam.

Sounds like this is an issue for really really small distances rather than something like 10-20m though. I just don't get why AQA used 25m as their cut-off point (I'm probably thinking too much into it :s) Thanks for your help though!
9. (Original post by cbreef)
Well it depends on what you're using to time it....?
Light gates connected to data loggers
10. (Original post by h8skoooooool)
Light gates connected to data loggers
Oooh right, then I've no idea. Shouldn't matter surely...?
11. (Original post by cbreef)
Oooh right, then I've no idea. Shouldn't matter surely...?
That's what I thought, it's just something on this AQA practical handout - I thought it needed explaining in my lab book because I don't really get it :s

But ah well it's probably just some exam board BS, thanks anyway!
12. (Original post by h8skoooooool)
There was a line going across the centre of the light gates, and we were assuming that was the point at which the ball crossed the beam.

Sounds like this is an issue for really really small distances rather than something like 10-20m though. I just don't get why AQA used 25m as their cut-off point (I'm probably thinking too much into it :s) Thanks for your help though!
You mean cm, surely?

Does the AQA handout specifically state to use light gates? Not all schools are going to have those, it might be assuming that you're going to use a stopwatch (in which case using larger distances is prudent).
13. (Original post by lerjj)
You mean cm, surely?

Does the AQA handout specifically state to use light gates? Not all schools are going to have those, it might be assuming that you're going to use a stopwatch (in which case using larger distances is prudent).
Ah yes sorry, I meant 0.25m

And yes it does, it even suggests an electromagnet thing to release the ball (but we didn't have those).

I also used a stopwatch in the experiment, just to see what it's like, and it was physically impossible to see at heights of 0.5-0.25m. I can't imagine any school doing that and taking it seriously.

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