Re: [Phys-l] projectile motion lab

[fizix29@aol.com]

I used a student's sample of about 50 time measurements, that is, the time it took the marble to roll between the two photogates.  I used the mean time and the distance between the two photogates to calculate the speed of the marble as it rolled off.  I used the standard deviation of the time measurements as the error in the time and a reading error of +/- .5 mm for the distance measurement.  When I carried the calculations through (I also measured the height of the table to +/- .5 mm) I found that the range of the projectile was only certain to about 30%.  It could be as low as 20 or as high as 40 cm.  That hardly seems a triumph for newtonian mechanics!  There must be a flaw in my assumptions.  I confess I am only now beginning to include rigorous treatments of error and uncertainty in my labs.  It was a fault of my youth I am trying to correct!
 
Justin Parke
Oakland Mills High School
Columbia, MD
 
 
-----Original Message-----
From: owen.vonk@gmail.com
To: phys-l@carnot.physics.buffalo.edu
Sent: Mon, 9 Oct 2006 11:40 AM
Subject: Re: [Phys-l] projectile motion lab


Advise:
1.  As mentioned rcently - make sure the marble rolls off a level  
surface
    I place each apparatus on a board and use a 2 foot level to level it  
by slipping sheets of paper under the low end of the board (actually  
pieces of shelving from my cabinets).
2.  The hardest part for the students is aiming left and right - the  
range is almost always accurate if the release is good.
2.  A consistent release is CRITICAL.  I have the students release  
the ball several times until the time between the photogates is  
consistent.
3.  The marbles should roll smoothly on the table - bouncing marbles  
create timing problems (you want the photogate beam to cut across the  
same part of the ball at each gate)
4.  I use a target with horizontal bands labeled 10, 9, 8 and 7  
points.  The lab is scored out of 20 points: 10 possible for hitting  
the target perfectly, 10 points for the lab write-up.
       I put a strip of putty or a plastic ruler along the 10 pt band  
- so it is easy to tell if the students hit that band (you can see a  
mark in the putty or hear the click as the ball hits the ruler)
    ----------------
    7
    ---------------
    8
    --------------
    9
    --------------
    10
    --------------
    9
    --------------
    8
    ---------------
    7
    ---------------
5.  My 9th graders get great results - most score a 10.
6.  Most importantly: try it yourself before having the students do it.

I hope this helps,
-Owen


On Oct 9, 2006, at 3:42 AM, FIZIX29@aol.com wrote:

> My high school students will be performing a projectile motion lab  
> where they
> roll a marble down an inclined plane and off of the lab table.   
> Prior to the
> marble rolling off of the table in will pass between two photogates  
> which will
> measure the time elapsed between them.  The students will measure the
> distance between the photogates, the time elapsed, and the height  
> of the lab table.
> > From this they will predict where the marble will land.  Pretty  
> > standard stuff.
>
> They will measure the time between photogates multiple (>10) times and
> compute the mean and standard deviation.  My question is how to  
> grade the lab.  If
> the marble hits the line representing the predicted distance they  
> will get some
> extra points.  If the line is not hit then I was thinking of grades  
> based on
> fractions of the standard deviation but I don't have a good sense of
> reasonable values.
>
> Any advice?
>
> Thanks
>
> Justin
> _______________________________________________
> Forum for Physics Educators
> Phys-l@carnot.physics.buffalo.edu
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