
#19
Nov2411, 10:57 PM

P: 253

No, in my example the height was 1m for both, slow and fast, I did not work it out for slow, short distance.
The same is true on the way up, whatever impulse was applied must have been equaled by gravity, or else it would be in motion at the top, and we would not be looking at a 3/3 1m model, we would have different numbers. So for the 1 rep, over 6 seconds, the force applied must have been equal to gravities impulse over that 6 seconds, or else we would still be in motion and not on the model. 352.8*6s = 2116.8N*s of impulse for a .5/.5 1m, 6 reps, we are looking at 352.8*.5 = 176.4N*s impulse 1 way, 352.8N8s impulse round trip, so 6 over those is equal to the previous answer of 2116.8N*s, since we still have 6 seconds of gravity acting on 36kg of mass. and as per the models, we still have a beginning and ending momentum of 0. change in momentum = force * time This is because momentum and force are really just velocity and acceleration, respectively, with mass taken into consideration momentum = mv force = ma anyone would agree that v = at, so momentum = force * t is really the same thing. The answer I am helping you with are for the FORCES, it would be correct for the forces measured on a forceplate, I think, but it has no DIRECT link to how tired you get, or how much muscle activity is going on, there are alot of other things at play here, This is a simplification to certain aspects, in this case, the "Force applied" to the weights. It is very well possible that more signal is required to make your muscle apply more force over a shorter time, I don't know how muscles work that well, I would take that specific question to a biology forum, The fact of the matter though is that whatever is required to get your body apply that force, the result is an equal amount of acceleration(force)*time 



#20
Nov2511, 02:27 AM

P: 148

God knows how many times the above has been explained to Wayne. So Wayne...you got your answer once again.I'll wait for your next thread after a few weeks asking the same thing. 



#21
Nov2511, 08:17 AM

P: 399

Hi everyone,
No time to read anything in work. Here is a study/test showing the there was more force/strength in the faster lifts as what I am saying, but D. says they are leaving something out, but he can’t or will not explain to me what, in scientific terms or layman’s. Could anyone here please say whets been left out if anything. http://docs.google.com/viewer?a=v&q=...ObsxJUbHpDqdXQ If you note, the graph, table 2, normal 170kg produced 17.9N force/strength to the slow 6.2N. That’s about 180% MORE force/strength, and in less time, the slow force/strength was calculated in 10.9 seconds, and the fast in 2.8 seconds. Imagine the slow person had 6.2N of force/strength or should I say that ammount of tension on his muscles for 10.9 seconds, the fast person had 17.9N of force/strength or should I say that ammount of tension on his muscles for 2.8 seconds. But if the fast person had done the fast for the same amount of time, he would have had 17.9 of force/strength used by his muscles for 10.9 seconds. This as I said is a huge difference, 180% more force/strength used by the muscles, or should I say tension on the muscles from the force/strength used, in the same time frame. Wayne 



#22
Nov2511, 09:11 AM

Sci Advisor
PF Gold
P: 11,354

@Wayne
Why do you keep using the two terms force and strength together? They just don't mean the same thing. 'Force' (an active word) is something that changes shape of an object or produces acceleration. 'Strength' (a passive word) refers to how an object will deform when a force is applied. If you really want a scientific answer then you could at least use scientific words appropriately in your questions. 



#23
Nov2511, 09:42 AM

P: 148

For the negative acceleration the propulsive force takes negative values which are not measured in the study. The average propulsive force is zero and the force equal with the weight regardless the speed. 



#24
Nov2511, 12:07 PM

P: 253

Equation 3 from that source points out that a linear impulse is equal to change in linear acceleration
If we take an force of 6.2N over 10.9 seconds, that is an impulse of 67.58N*s This means we must see a change in momentum of 67.58kg*m/s The weight presumably started at a velocity of 0, so this means it is left with a velocity of .4m/s, thus, it has not yet stopped moving, if you stop applying force, it will take gravity another .04s to stop it, and during that time there is a negative propulsive force, an applied force less than the force required to hold the object up. So in my opinion the data is not good, because they did not wait for it to stop to take their readings. If you take 1672.2N(the applied force required to get a propulsive force of 6.2N) * 10.9+ 0N*.04s = 1672.2 and then divide that by the total time 10.94 = 1666.0859N avg which is really close to the 1666N that were required to hold it up. For the faster rep, we have an avg propulsive force of 45.3N * 2.8s = 126.8N*s, so we know it is left with a velocity of .75m/s which would take gravity .076s to decelerate, so again, add the 45.3 to 1666 = 1711.3*2.8s = 4791.64N*s from the force applied +0n*.076s =4791.64/(2.8+.076) = 1666.078 once again really close to the force required to hold it still in the first place. So overall, sure, it is more force to get the same weight going faster, anyone will agree there, the problem is that unless you apply a force to pull the weight down toward you before it stop moving up, then comparing the force for the whole cycle should be the same in other words, if you apply more than required to hold it up, and then never less than required to hold it up, it will keep moving up, since that is not the case there is more going on than listed in that expiriment 



#25
Nov2511, 12:38 PM

P: 253

@sophiecentaur
In layman's terms, strength and force are very similar, a strong person can supply more force than a weak person, a force with a large magnitude is a strong force, a force with a small magnitude is a weak force, so strength is like the magnitude of force in a way degree of intensity or concentration I agree with the problem in wording it "used more strength/force" because it's not something you use up per se I guess it is that you think if he comes here to ask a question he should have it in the correct format for physics, and he assumes he can use terms how HE uses them and we being the ones knowledgeable in physics will know the physics terms to work out the problem he is outlining. It depends on what you feel the point of this forum is. 



#26
Nov2511, 02:36 PM

Sci Advisor
PF Gold
P: 11,354

So how would we describe the ability of a structure to resist a force if we've already used the word strength?
It's surely not too demanding to use the right (common) words if you want serious help about a scientific topic. Unless it's just a trivial query which it clearly is not. 



#27
Nov2511, 03:26 PM

P: 253

Doesn't really matter, unless you expect everyone on this forum to be able to ask questions using appropriate physics terms when they may be coming because they are not very familiar with physics. I believe anyone can infer that he means strength as a measure of your overall ability to apply force, so the more of that ability you use, the more strength you use. It's what is meant that counts, in my opinion.




#28
Nov2511, 04:24 PM

P: 148

The true mean propulsive force is 0 because the net delta V is zero. The average force is always equal with the load and...to use Wayne's terms...the "total applied force" is equal with gravity's impulse over the duration of the rep(force*time). So to summarize...either you lift 100N once with 3/3 or 6 times with .5/.5 the total applied force is 600N*s in both cases. 



#29
Nov2511, 05:17 PM

Sci Advisor
PF Gold
P: 11,354

@Wayne  I'm not just 'picking on you' and I'm sure that you have a genuine wish to get something out of this thread. Your terminology is just one example of what I'm talking about. There are far far worse things to be seen elsewhere. Only today, the moderators have actually pointed out the necessity of 'reporting' contributions that are potentially harmful to understanding. We owe it to people to keep a high standard where possible if we are to maintain our present level credibility. It's ok to be laid back about some things when you know your audience but it's really not fair to people if we don't keep our technical house in order. What is this forum for, when you get down to it? 



#30
Nov2511, 06:21 PM

P: 399

Sorry about the wording sophiecentaur, will try better, buts it’s hard when I have not been shown. And I am genuine and want to learn. One thing is no one will answer me is the distance, force problem I am having, could you ??? If impossible to lift 80% in .5 of a second to 1m, the time could be less, but I am sure I could lift that in the time, will try it on video> 1, We both have a 100 pounds of maximum strength that we can bench press. We you 80% 80 pounds, I move the weight 1000mm in .5 of a second, I will be accelerating this weight for 60% for the ROM, {range of motion} of for the concentric rep, you will be moving the same weight in .5 of a second for only 166mm. Question, If I have moved the weight more distance in the same time frame, does that not that mean that I have used more force ??? {force and strength are the same I would have thought, but let’s just go for force now} As even if you just count my acceleration its 600mm, that far moiré distance than the slow. 2, I bench press the same weight same distance of 80% {and this weight of 80% is important, as my maximum is only 20% more than my maximum} I lift at .5/.5 for 6 reps = 6 seconds covering a distance of 12m. The slow lifts at 3/3 for 1 rep = 6 seconds, covering a distance of only 2m. Question, basically the same as the above. As a force that causes an object with a mass of 1 kg to accelerate at 1 m/s is equivalent to 1 Newton, if you move it futher in the same time from you have to use more force, N’s. 



#31
Nov2511, 06:42 PM

P: 399

If I lift 80% up to 1m from a still start, and then lift 80% up from when it is being lowered under control at .5 of a second down, the weight will gather acceleration/ movement thus appear to be heavier than it is. Meaning if you put the 80% on a scales on the standing start it wound register 80 pounds. However if you could immediately put the scales under the weight at 1m when it had dropped 1m in .5 of a second, it would register far far far more. So it would take more force to lift the same weight up and down on the second moving rep, than to just move it up and down from a still start. Also, is not power in mechanics, the combination of forces and movement ??? I thought power was the product of a force on an object and the object's velocity ??? As we all will agree that the fast uses more power, {even I can work that out with some simple equations} so if power is the of a force on an object and the object's velocity, does that not mean more force more power ??? Wayne 



#32
Nov2511, 11:40 PM

P: 253





#33
Nov2611, 05:15 AM

P: 253

@wayne
I am not sure how exactly your EMG machine works, but would you be able to test the muscle activity that results from holding a variety of weights at a variety of heights, holding it in place for a specified time? If so that would give you an idea of what a change in forceapplied impulse gives in the EMG results. 



#34
Nov2611, 08:01 AM

P: 399

Still on this dongo, and it very slow and does not hold a connection. This time I will copy and paste all and read in full and get back when my broadband is up and running fully. Wrote this before going online. Douglas, you are telling me that a few people have worked this debate down on paper, and some say the forces are the same, first, I have put four electrical pads on my muscles that measure the muscle activity = muscles forces and strengths with a dual EMG, and in a real life experiment done several times in front of me, I have seen the reading that state there in more muscle activity in the faster reps every time. So do you not think that you have left something out of your equations maybe ??? Look, 40 + 40 x 0 + 1 = ??? I would say 81, as 40 + 40 = 80 X 0 = 80 + 1 = 81, but some say its 41 and some 1, just for fun could all please answer this, AS IT MIGHT HELP ME SEE IF I AM GOING WRONG HERE. Thx all. Wayne 



#35
Nov2611, 09:03 AM

P: 148




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