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help physics question - Page 2

Blogs > kdog3683
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SpiritoftheTunA
Profile Blog Joined August 2006
United States20903 Posts
Last Edited: 2008-02-22 05:48:32
February 22 2008 04:43 GMT
#21
On February 22 2008 12:30 Chill wrote:
Show nested quote +
On February 22 2008 09:44 SpiritoftheTuna wrote:
most of the people saying use kinetic energy = potential energy are forgetting the horizontal element of the velocity that contributes to the energy EDIT: and aren't relating it back to the force anyway

the person saying energy = force * time is just plain wrong

all the people mentioning time are thinking of impulse, not force OR energy

the time of contact between your foot and the ball is NOT instantaneous acceleration

you guys are fucked up

edit: oh, and the person mentioning elasticity (probably something to do with coefficient of restitution) is severely overestimating your physics class

the person saying force = change in momentum / distance is a fucking moron

the person who said force = change in momentum (impulse) / time is right though (assuming they meant duration of impact)


Thanks for your help and suggestions to help the OP!


yeah no problem, same to you by the way

well i mean, OP didn't mention what he's able to measure. time of impact, instant accel (easily), get a graph of position vs time up, or what. it's pretty impossible to make a suggestion from what he gave me. so hey, kdog... whatcha got?

edit: to improve on chill's pendulum idea though, ok... (just to make everything easier to measure)

tie a ball to a string, hang it vertically, make sure it's stationary
kick it lightly
measure the angle it deviates from the vertical (with a protractor facing down, oriented around the pivot point)
also measure the time it took to get to that angle

Uinitial (potential energy - set your zero point here so it's zero) + Kinitial = Ufinal + Kfinal

1/2mvsquared = mgh where h = L (length of pendulum) - Lcostheta where theta is angle from vertical

find out v (which will be initial velocity)

find out time of impact

divide for accel

multiply by mass for force

or something
posting on liquid sites in current year
skyglow1
Profile Blog Joined April 2005
New Zealand3962 Posts
Last Edited: 2008-02-22 07:12:15
February 22 2008 07:05 GMT
#22
I thought of finding out the average force because I felt measuring velocity of the ball just after it leaves the foot is much easier than finding the instantaneous acceleration at that point.

The idea of using a video cam would work really well with this I think if you could have a high enough frame rate. You could put some sort of a scale behind the ball which has really clear divisions at every 1 cm or somethinng to measure the velocity of the ball just after it leaves the foot, and you could measure how long in contact the foot is with the ball, adn measure the mass of the ball.
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