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Geeky physics related question (Read 5343 times)

Nibile

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Geeky physics related question
July 22, 2016, 03:11:27 pm
Ok, I was wondering what's the way of calculating the amount of force produced in some of my lifts. Especially in the clean.
The weight is 53 kilos. It travels a vertical - more or less - distance of around 95 cm in 0,73 seconds.
Are these data enough?
Does my question make any sense?
Cheers.

i.munro

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#1 Re: Geeky physics related question
July 22, 2016, 03:42:00 pm
Ok, I was wondering what's the way of calculating the amount of force produced in some of my lifts. Especially in the clean.
The weight is 53 kilos. It travels a vertical - more or less - distance of around 95 cm in 0,73 seconds.
Are these data enough?
Does my question make any sense?
Cheers.

Enough to calcultae the amount of energy used yes. For Force you need to know how fast the weight accelerates.

Nibile

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#2 Re: Geeky physics related question
July 22, 2016, 03:43:47 pm
Cheers.
So, how much energy?
And how can one calculate acceleration? I bet this second think is quite difficult. Which data are missing for this?

JamieG

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#3 Re: Geeky physics related question
July 22, 2016, 03:47:02 pm
Hi Nibile,

According to my rough maths/mechanics. If you assume that you are lifting the weight with a constant acceleration, then the accleration is the distance divided by the time squared. Therefore you are acclerating the weight at 1.3014m/s2. Force is equal to mass multiplied by acceleration. However you also need to overcome gravitational acceleration at 9.81m/s2. Total acceleration is 11.5927m/s2 multiplied by 53kg, gives a force of 614.413N. Bear in mind it takes 519.93N just to hold the weight up.

In reality the acceleration won't be constant, probably highest near the start. Therefore the forces would be higher at this point versus later in the lift, but without more information it is impossible to calculate.

I hope i've got my maths/mechanics right . . . .

Jamie

JamieG

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#4 Re: Geeky physics related question
July 22, 2016, 03:51:43 pm
Potential energy gained is easier to calcualte. It is just the mass multiplied by gravitational acceleration multiplied by height gained. 53*9.81*0.95 = 493.93J

i.munro

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#5 Re: Geeky physics related question
July 22, 2016, 03:55:31 pm
So let's assume that you accelerate the weight at the start of the lift and then lift at a constant speed after that. (Bit of a guess but hey!)

If you knew how far you accelerated the weight over then we could get an approximate figure.
Could you video yourself and then slow-mo  the video.

JamieG

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#6 Re: Geeky physics related question
July 22, 2016, 04:09:49 pm
Oops the acceleration you generate is double what I said. Missed the 2 in my calc.  :slap:

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#7 Re: Geeky physics related question
July 22, 2016, 04:15:24 pm
Force is nearer 660N

Fultonius

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#8 Re: Geeky physics related question
July 22, 2016, 04:19:32 pm
Ok, I was wondering what's the way of calculating the amount of force produced in some of my lifts. Especially in the clean.
The weight is 53 kilos. It travels a vertical - more or less - distance of around 95 cm in 0,73 seconds.
Are these data enough?
Does my question make any sense?
Cheers.

Ha, just saw i.munro saying the same about slow-mo...

Do you have a decent camera that can film in 60p?  You could probably roughly calculate the peak acceleration and peak force by measuring the distance per frame.

Alternatively, do it on a set of scales and see what the max is.

These guys were doing 3500N on a force plate. https://ojs.ub.uni-konstanz.de/cpa/article/download/697/617

Nibile

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#9 Re: Geeky physics related question
July 22, 2016, 04:25:01 pm
Cheers guys!
Much appreciated.
I have a couple of videos but I'm not as curious as to try and figure out the distance covered during each frame, it was just curiosity.
I like the idea of the scales. I remember trying that for high jumps and the scale almost went out of... haem... scale.

Fultonius

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#10 Re: Geeky physics related question
July 22, 2016, 04:36:17 pm
You'd need scales that went up to 400kg! (or maybe more, BEAST)

cjsheps

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#11 Re: Geeky physics related question
July 22, 2016, 06:04:55 pm
1. Put a measuring tape behind the weight, video a rep and watch it back in slo-mo.
2. Plot the position of the weight as a function of time. You should get a curved profile (acceleration) before something straight (constant speed).
3. Calculate the gradient of the plot, in metres per second, at the very start of the linear bit. This is your moving speed post-acceleration.
4. Divide this maximum speed by the time in seconds required to achieve the speed. This is your (approximately linear) acceleratioin.
5. F=ma. Multiply this acceleration by the mass (in kg) and you have the force you've applied (in Newtons).

Also, multiply the maximum speed by the mass and by the acceleration due to gravity (around 9.81 metres per second per second), and you'll have the POWER being expended on lifting the weight (an underestimate due to muscular inefficiency).

ALTERNATIVE METHOD for misogynists:

1.Google a bunch of pro weightlifters.
2. Establish a correlation between the hotness out of 10 of groupies they get.
3. Go to the beach and perform your lift in front of a large sample of babes.
4. Determine the hotness of your groupies and find your place on the trend established in (2).

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#12 Re: Geeky physics related question
July 22, 2016, 07:14:45 pm
Forget video analysis, if you have a modern iphone, they have an accelerometer built in.  You can download a variety of apps to record acceleration events.  If you were to firmly attach your phone to whatever you are moving around (e.g. tape it to the weight) you could pretty easily record the acceleration of any lift.   If you wanted force vs time, it would be pretty easy to determine from the acceleration plot and the known mass of the weight.  Come to think of it, this would be a pretty stupid-fun way to check out explosive pull up power, just attach your iPhone to your upper body.

JamieG

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#13 Re: Geeky physics related question
July 22, 2016, 07:20:25 pm
you'll have the POWER being expended on lifting the weight (an underestimate due to muscular inefficiency).

Muscle efficiency is quite bad if I remember correctly (around 20% ish). You are right that you are calculating mechanical power, but the metabolic power you use may be several times higher.

Nibile

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#14 Re: Geeky physics related question
July 22, 2016, 09:46:13 pm
The accelerometer app is the dog's bollocks. Shame I don't know how to use it, I just watch the graph move. It's beautifully hypnotic.

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#15 Re: Geeky physics related question
July 22, 2016, 09:52:29 pm
Accelerometer is a great shout!

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#16 Re: Geeky physics related question
July 23, 2016, 11:07:51 am
The accelerometer app is the dog's bollocks. Shame I don't know how to use it, I just watch the graph move. It's beautifully hypnotic.

Just be sure to attach the phone really well as long as you are using the borrowed one  ;D

Nibile

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#17 Re: Geeky physics related question
July 23, 2016, 04:13:25 pm
The accelerometer app is the dog's bollocks. Shame I don't know how to use it, I just watch the graph move. It's beautifully hypnotic.

Just be sure to attach the phone really well as long as you are using the borrowed one  ;D
Too late.

 

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