Kinematics and Dynamics Kinematics and Dynamics Physics Help Forum 
Sep 16th 2014, 04:38 PM

#21  Physics Team
Join Date: Jun 2010 Location: Morristown, NJ USA
Posts: 2,347

Originally Posted by waynexk8 By that, are you saying that with both lifts the impulse {force over time} is the same ??? 
The average force is the same, because while for the fast lift the peak upward force is much greater, when being lowered the fast cycle requires very little force  basically gravity does most of the work. Meanwhile for the slow lift the peak force is less, but more constant  during lowering you must still apply a significant upward force to keep the weight from falling too fast.
I should also point out that in physics force is a vector, which means it has direction. So when I say the average forces are the same that;s because part of the force used in the fast lift is negative, which cancels some of the positive force. But in human physiology your muscles don't know about that  they only care about the magnitude of force, not whether it's positive or negative. If you push on something then pull on it, physics say no work is done and average force is 0, but it's clear that your muscles still get tired.
Originally Posted by waynexk8 So if I am right following you, not saying I am, but the slow cycle makes up all the impulse as the fast cycle decelerates for the transition from positive to negative, I am no Physicist, but that does not seem possible to me. 
Originally Posted by waynexk8 Also is that right, that if you move a weight faster in the same time frame, and move it twice the distance, that the same impulse is required ??? 
No. First, please stop using the term "impulse," because I have no idea what you mean. Let's talk in terms of force applied. Given that the force for the fast lift varies so widely its peak is much greater than for the slow lift  this is needed to accelerate the weight faster upward. But as already mentioned  the minimum force for the fast lift is much less than for the slow, in order to let gravity do its thing. Essentially the weight lifter gets a rest during the down part of the cycle. This is a consequence of the parameters you specified, or 1/2 meter amplitude with time periods of 1 and 2 seconds. If you specified lifts of, say, 0.75 m with times of 2 seconds and 4 seconds I think the results may be less confusing to you, as the forces would never go negative.
Originally Posted by waynexk8 If I used a weight that was 80% of what I could lift once, would not the magnitude of force required for the fast lift be just 20% more ???  Sorry, but I do not understand what you are trying to say.
Originally Posted by waynexk8 If the fast moves 4m in 2 seconds and slow 2m in 2 seconds, is not the range two times ???  By "range" I meant the difference between maximum and minimum values. The fast lift ranges from +300N to 100N, for a range of 400N.. The slow lift ranged from +150N to +50N, for a range of only 100N.
Originally Posted by waynexk8 I always thought that as you are more tried from fast lifting with the same weight in the same time frame, that it was because you used more impulse ???  You use more force (not impulse), and that makes you tired.
Originally Posted by waynexk8 So in the slow cycle on the eccentric, the fast uses more impulse you are saying I think ???  It uses more peak force.
Originally Posted by waynexk8 is not higher reading for the fast a higher usage of impulse ???  Higher force, yes.
Last edited by ChipB; Sep 17th 2014 at 08:23 AM.

 
Sep 17th 2014, 04:40 AM

#22  Junior Member
Join Date: Sep 2014
Posts: 16

Originally Posted by ChipB The average force is the same, because while for the fast lift the peak upward force is much greater, when being lowered the fast cycle requires very little force  basically gravity does most of the work. Meanwhile for the slow lift the peak force is less, but more constant  during lowering you must still apply a significant upward force to keep the weight from falling too fast.
I should also point out that in physics force is a vector, whihc means it has direction. So when I say the average forces are the same that;s because part of the force used in the fast lift is negative, which cancels some of the positive force. But in human physiology your muscles don't know about that  they only care about the magnitude of force, not whether it's positive or negative. If you pushg on something then pull on it, physics say no work is done and average force is 0, but it's clear that your muscles still get tired.
[/FONT][/COLOR][COLOR=black][FONT=Verdana]
No. First, please stop using the term "impulse," because I have no idea what you mean. Let's talk in terms of force applied. Given that the force for the fast lift varies so widely its peak is much greater than for the slow lift  this is needed to accelerate the weight faster upward. But as already mentioned  the minimum force for the fast lift is much less than for the slow, in order to let gravity do its thing. Essentially the weight lifter gets a rest during the down part of the cycle. This is a consequence of the parameters you specified, or 1/2 meter amplitude with time periods of 1 and 2 seconds. If you specified lifts of, say, 0.75 m with times of 2 seconds and 4 seconds I think the results may be less confusing to you, as the forces would never go negative.
[COLOR=black][FONT=Verdana]
Sorry, but I do not understand what you are trying to say.
[COLOR=black][FONT=Verdana]
By "range" I meant the difference between maximum and minimum values. The fast lift ranges from +300N to 100N, for a range of 400N.. The slow lift ranged from +150N to +50N, for a range of only 100N.
[COLOR=black][FONT=Verdana]
You use more force (not impulse), and that makes you tired.
[COLOR=black][FONT=Verdana]
It uses more peak force.
[COLOR=#545454][FONT=Arial][SIZE=3] Higher force, yes. 
Hi there,
Going have to get back to the rest tomorrow, but thank you for explaining in full. I thought that impulse was the force added up over time ??? Aha, now I think I get you. So if I am reading this right, you said the impulse for the fast and slow cycle looks the same on paper, however, as on the eccentric of the fast cycle, we are using negative forces, that’s forces below that of the weight being moved which in this case is 80, Physics does not add these on, and this is the same for when on the concentric cycle of the fast when we hit the last stage to decelerate for the transition, we again like the whole eccentric are using force, but as its below 80, its not added on, please have I got that right ??? AND IF SO A VERY VERY VERY BIG THANK YOU, WILL EXPLAIN MORE ON THAT AFTER. So if the impulse is the same for the fast and slow cycle, it actually must be higher for the fast if we ignore Physics and add on the negative forces, could we please do that ??? And then that will be as we agree, why you fail far faster in the fast cycles, as we are in fact not only using higher forces, but more impulse. Wayne 
 
Sep 18th 2014, 06:00 AM

#23  Physics Team
Join Date: Jun 2010 Location: Morristown, NJ USA
Posts: 2,347

Rather than talking about "impulse" let's consider the work done by the lifter's muscles. Work is calculated by multiplying the force applied times the distance traveled. In this case where force varies with position we can integrate Fdx. Further, lets take the absolute value of this work for each increment of distance covered so as to avoid having negative work. I think this accomplishes what you're looking for.
The result is that doing two fast lifts in the same time as one slow lift requires twice as much work. It's equivalent to doing the slow lift with twice the weight. So  the reason the fast lift stresses the muscles more is because it requires twice as much work to make it happen.

 
Sep 22nd 2014, 04:11 PM

#24  Junior Member
Join Date: Sep 2014
Posts: 16

Originally Posted by ChipB Rather than talking about "impulse" 
Sorry, was called away from Town for a few Days. I think I agree with you, as if as Physics says, as another Physics Friend said; If you pushing on something then pull on it, physics say no work is done and average force is 0, but it's clear that your muscles still get tired. That is precisely why the physical quantities of impulse and work done are not good measurements for the biological phenomenon of “tiredness”.
So that mean Force and Impulse in the way I was looking at it, or/and the way the Muscles feel it, can definitely be Worked out by Physics definition of Force and Impulse, as to Me and the Muscles, they feel and use Force and Impulse when they are producing more Force than the Weight as in Acceleration, and the same Force and Impulse as in pushing up at a Constant Force, and when Decelerating, as in just before the Transition from Positive to Negative, and the Muscles also feel and use Force and Impulse when Lowering the Weight.
But could We First is possible clear up something ??? So You are saying that when I lift 100 for 1m Up and Down for One Cycle 1 Second each way, 2m in all, and 2 Seconds over all, and lift 100 for 1m Up, and 1m Down .5 of a Second each way Twice each way, 4m in all and 2 Seconds over all, that the Average Impulse {force over Time} is the same, but am I Right in saying that when the Fast is Decelerating after the Acceleration just before the Transition from Positive to Negative, and the whole Negative, as in both the Force is Less than the Weight, that this Force in Physics is not Added in ???
Let Me explain more, let’s say the Impulse was 1000N for the Fast and Slow, but on the fast there is the Deceleration after the Acceleration from just before the Transition to Negative, thus I am using less Force than the Weight being Lifted, which in this Case could be 100, and the Whole Negative where the Muscles are still using Force and Impulse, but again because its less Force and Impulse than the Weight which in this case is a 100, AM I RIGHT IN SAYING THAT ANY FORCE USED UNDER A 100 IS NOT ADDED TO THE EQUATION ???
Originally Posted by ChipB let's consider the work done by the lifter's muscles. Work is calculated by multiplying the force applied times the distance traveled. In this case where force varies with position we can integrate Fdx. Further, lets take the absolute value of this work for each increment of distance covered so as to avoid having negative work. I think this accomplishes what you're looking for.
The result is that doing two fast lifts in the same time as one slow lift requires twice as much work. It's equivalent to doing the slow lift with twice the weight. So  the reason the fast lift stresses the muscles more is because it requires twice as much work to make it happen. 
Thank you very much, as you say, I think this accomplishes what I am looking for far more than force and Impulse, multiplying the force applied times the distance travelled. Further, let’s take the absolute value of this work for each increment of distance covered so as to avoid having negative work. as they do not Add in the Negative Forces, and these must be in the case of when Working out the Total Impulse used by Muscles when being used. That’s why My EMG Machine Stated a Higher Reading for the Fast.
Wayne
Last edited by waynexk8; Sep 23rd 2014 at 04:13 PM.

 
Sep 26th 2014, 03:25 PM

#25  Junior Member
Join Date: Sep 2014
Posts: 16

Hi ChipB, and Thank You Very Much for Your Time and Help.
The Only Conclusion that I can come too, is that My Definition of Force, that being Force over a Distance and Time is Work in Physics; Work is the Amount of Force Applied over a Distance, thus that takes a “Time” so Force in Moving Weight Back and Forth = Work = Force + Distance/Time. Then We have Power, Force x Velocity. Or We Could have, Strength is Generally the Ability to Resist Deformation, so Crushing Strength for Example is Measured in Megapascals.
Work would be in My Thinking on Lifting Weights Up and Down, the Application of Force by human beings, or the Machines they have designed, to an activity that creates a product or service. Work cannot happen unless Human beings apply themselves to an Activity and use a Force. Work makes use of the Forces of Nature, and Causes them, the Forces to Move things Fast and a Great Distance, or Slow and Little Distance.
So I think {if I am Right} the Words I was Looking for when Moving Weights was Work/Force ??? That would then be why My EMG stated More Muscle Activity for the Fast Lifting of Weighs in the same Time Frame as the Slow Lifting of Weights ???
Wayne

 
Sep 30th 2014, 04:24 PM

#26  Junior Member
Join Date: Sep 2014
Posts: 16
 Mathematically, work can be expressed by; where F is the force, D is the Displacement. In order to Accomplish Work on an Object there must be a Force Exerted on the Object and it must move in the Direction of the Force. Work is done whenever a Force Results in a Displacement. All other things being Equal, applying a Greater Force should Result in more Work being done. Likewise, Exerting a given Force over a Greater Distance should Result in more Work being done. Two Questions, 1, I still do not Undersand or Think, You Can use the same Impulse, {Force over Time} if You Move 100 for 2m and then for 1m in the same Time Frame ??? As the Definition of Work is defined (in calculus terms) as the Integral of the Force over a Distance of Displacement. And if I Move 100 for 2m and then for 1m in the same Time Frame, You have done more Work, Right ??? So as Work = Force x Distance, and in this Case I have used more Work, I “must” have used more Force, thus more Impulse ??? 2,
Applying/Exerting a Greater Force should result in more Work being done, would not that also mean more Impulse being used ??? http://www.lightandmatter.com/html_books/lm/ch04/ch04.html https://www.google.co.uk/?gws_rd=ssl#q=work+is+said+to+put+force+on+an+obje ct Wayne 
 
Sep 30th 2014, 05:19 PM

#27  Physics Team
Join Date: Jun 2010 Location: Morristown, NJ USA
Posts: 2,347

I think I've said about everything there is to say on this subject, which is why I've not responded to your last few posts. However, I will gamely try again.
Originally Posted by waynexk8 Mathematically, work can be expressed by; where F is the force, D is the Displacement. In order to Accomplish Work on an Object there must be a Force Exerted on the Object and it must move in the Direction of the Force. Work is done whenever a Force Results in a Displacement. All other things being Equal, applying a Greater Force should Result in more Work being done. Likewise, Exerting a given Force over a Greater Distance should Result in more Work being done. 
Bravo! Well said.
Originally Posted by waynexk8 Two Questions
1, I still do not Undersand or Think, You Can use the same Impulse, {Force over Time} if You Move 100 for 2m and then for 1m in the same Time Frame ??? 
Please, please, please  stop using this term impulse = force over time! It's hooey, and means nothing. In physics we use the term "impulse" to mean force times time, so if you want to invent a new term for force divided by time you will need to call it something else.
Originally Posted by waynexk8 As the Definition of Work is defined (in calculus terms) as the Integral of the Force over a Distance of Displacement. And if I Move 100 for 2m and then for 1m in the same Time Frame, You have done more Work, Right ??? 
Yes, applying 100N for 2 m results in more work being performed than 100N for 1 meter.
Originally Posted by waynexk8 So as Work = Force x Distance, and in this Case I have used more Work, I “must” have used more Force, thus more Impulse ??? 
No, no, no! If distance increases for a given force, then force times distance increases. That does not mean that force increases. And since I have no idea what you mean by "impulse" I can't respond to whether this mysterious thing you call "impulse" increases or not.
Originally Posted by waynexk8 2,
Applying/Exerting a Greater Force should result in more Work being done, would not that also mean more Impulse being used ?? 
More work is done if force increases for a set distance. I do not know whether your definition of "impulse" increases or not, since I have no idea what you mean by the term "impulse."
Last edited by ChipB; Sep 30th 2014 at 05:25 PM.

 
Oct 1st 2014, 06:06 AM

#28  Junior Member
Join Date: Sep 2014
Posts: 16
 Hi ChrisB, And Thank You for saying with Me. I Wrote the Below before Reading what You just Wrote, so Best Explain Impulse First and then will get back to You Post. My meaning of Impulse, is all the Forces Added Up over Time, let’s say I use 10N for .33 of a Second to Accelerate the Weight, 5N for .33 of a Second to Move the Weight at a Constant Speed, then 3N To Decelerate the Wight, for .33 of a Second, so the Total Force {strength used by My muscles} used over One Second = 18N, have I got that Right ??? We all agree We use more Energy over the same Time Frame and move it more Distance, but only I say that is because We use more Impulse, no One else says why We move it more Distance or use more Energy, as there must be a Physics Answer to why more Distance was covered and more Energy used in the same Time Frame and moving the same Weight. Going to have to Read and Think this One though more, However, I thought Work was Basically Very Slimier to Impulse, as Impulse is Force over Time added up, and is not Work, force x Distance, so if We Add Up Force over Distance, and as this does take Time, will not Work = Force x Distance Actually mean Impulse x Distance ??? So I lift 325N for 2m = 650J, So I lift 325N for 1m = 325N, so how would the Question go if We Added Time in ??? So I lift 325N in 1 Second for 2m = 650J, So I lift 325N in 1 Second for 1m = 325N. Or Dose Work just mean the Energy Used ??? but then it does say Force x Impulse, and Power is more like that, it’s the Amount of Energy You use Per Unit of Time. This get Confusing, Force is said too do Work, so Surely the More Work You do over a Distance that has to have Time, You do More Work, think We agree there, as I have Moved the Weight Further, so if I have done more Work, which Equals doing and using more Force over Distance, if I use more Force {over Distance and Time} I must have used more Impulse ??? Otherwise how can I use more Force/Work over Distance that takes Time, without using more Impulse ??? I do not get do anything like 100 Pounds can be moved 1m in 1 Second, and then be moved 2m in 1 Second and both use the same Impulse {Force over Time} ??? Is there any Practical Experiments to See this please ??? We all agree We use more Energy over the same Time Frame and move it more Distance, but only I say that is because We use more Impulse, no One else says why We move it more Distance or use more Energy. Wayne 
 
Oct 1st 2014, 10:37 AM

#29  Physics Team
Join Date: Jun 2010 Location: Morristown, NJ USA
Posts: 2,347

I get it now  you are defining impulse as force time time (not force divided by time). It maikes a abig difference.
Originally Posted by waynexk8 My meaning of Impulse, is all the Forces Added Up over Time, let’s say I use 10N for .33 of a Second to Accelerate the Weight, 5N for .33 of a Second to Move the Weight at a Constant Speed, then 3N To Decelerate the Wight, for .33 of a Second, so the Total Force {strength used by My muscles} used over One Second = 18N, have I got that Right ???  No, not right. Forces don't add over time. However, using your impulse definition: 10N x 0.33s + 5N x 0.33s + 3N x 0.33 s = 6Ns. Note the units are newtonseconds.
Originally Posted by waynexk8 We all agree We use more Energy over the same Time Frame and move it more Distance, but only I say that is because We use more Impulse, no One else says why We move it more Distance or use more Energy, as there must be a Physics Answer to why more Distance was covered and more Energy used in the same Time Frame and moving the same Weight. 
As alreasdy discussed several times  moving a weight faster requires more force to accelerate it. If you do two reps instead of one rep in 1 second then F must be greater (to accelerate the weight faster) and the distance covered is twice as great (2 reps versus one). Hence the work done is 4 times greater. Using your impukse definition, force is greater but time is the same, so the amount of impulse is two times greater. I think the analysis using work is a better approach than using impulse.
Originally Posted by waynexk8 Going to have to Read and Think this One though more, However, I thought Work was Basically Very Slimier to Impulse, as Impulse is Force over Time added up, and is not Work, force x Distance, so if We Add Up Force over Distance, and as this does take Time, will not Work = Force x Distance Actually mean Impulse x Distance ??? 
No. Check the units: Nm is not the same as Ns. But they may be correlated  in general pushing a weight for a longer period of time will cover more distance. But they aren't the same thing. For example  pushing on a brick wall that doesn't move results in 0 work being done on the wall, although the force applied times time may be quite substantial.
Originally Posted by waynexk8 So I lift 325N for 2m = 650J, So I lift 325N for 1m = 325N, so how would the Question go if We Added Time in ??? 
Originally Posted by waynexk8 So I lift 325N in 1 Second for 2m = 650J, So I lift 325N in 1 Second for 1m = 325N.  You're leaving out the fact that tp lift a 325N weight in 1/2 second requires more force than to lift it in one second.
Originally Posted by waynexk8 Or Dose Work just mean the Energy Used ??? 
Yes.
Originally Posted by waynexk8 but then it does say Force x Impulse, and Power is more like that, it’s the Amount of Energy You use Per Unit of Time. 
Yes, power = energy/time, or force x velocity.
Originally Posted by waynexk8 This get Confusing, Force is said too do Work, so Surely the More Work You do over a Distance that has to have Time, You do More Work, think We agree there, as I have Moved the Weight Further, so if I have done more Work, which Equals doing and using more Force over Distance, if I use more Force {over Distance and Time} I must have used more Impulse ??? Otherwise how can I use more Force/Work over Distance that takes Time, without using more Impulse ??? 
As noted abovem, in general this is true.
Originally Posted by waynexk8 I do not get do anything like 100 Pounds can be moved 1m in 1 Second, and then be moved 2m in 1 Second and both use the same Impulse {Force over Time} ??? Is there any Practical Experiments to See this please ??? 
I don't follow what you're asking for. As already noted, moving the weight 2 meters in one second versus 1 meter in one second requires more force to accelerate it from v=0. So both impulse is increased (by a factor of 2) and work done is increased (by a factor of 4). We all agree We use more Energy over the same Time Frame and move it more Distance, but only I say that is because We use more Impulse, no One else says why We move it more Distance or use more Energy. Wayne
[/QUOTE]
I think I showed why above.

 
Nov 15th 2014, 09:06 AM

#30  Junior Member
Join Date: Sep 2014
Posts: 16

Hi there all, sorry, been on a long trip to Australia.
Originally Posted by ChipB I get it now  you are defining impulse as force time time (not force divided by time). It maikes a abig difference.
[/B][/SIZE][/FONT] No, not right. Forces don't add over time. However, using your impulse definition: 10N x 0.33s + 5N x 0.33s + 3N x 0.33 s = 6Ns. Note the units are newtonseconds. 
Here is where I get that Impulse is Force over Time. So take the Tennis Ball hitting the Bat, the Longer the ball stays in contact with the bat, there will be a Force on the muscles longer, thus a higher Impulse. {Ignore the Numbers} Bat hits ball with a Force of 50 for .5 of a Second, ball hits bat with a Force of 50 {and as the batter follows though} for 1 Seconds, thus the Impulse is higher. This is how I am trying to Work the Weightlifting out. I put a higher Force on the Wight, as it moves twice as far in the same Time frame, thus more Power used, more Force, and more Impulse.
Impulse is an important concept in the study of momentum. Time passes as a force is applied to an object. When this happens we say that an impulse is applied to the object. For example, when a tennis racket strikes a tennis ball, an impulse is applied to the ball. The racket puts a force on the ball for a short time period. By the way, according to Newton's third law of motion, forces come in pairs. So, the ball also puts a force on the racket, and the racket, therefore also has an impulse applied to it.
An impulse is equal to the net force on the object times the time period over which this force is applied. Below, we derive impulse from the equation F = ma, which comes from Newton's second law of motion. Study the following three lines and read the commentary under them. http://zonalandeducation.com/mstm/ph...mSummary2.html http://spiff.rit.edu/classes/phys311...e/impulse.html
Originally Posted by ChipB As alreasdy discussed several times  moving a weight faster requires more force to accelerate it. 
Right.
Originally Posted by ChipB If you do two reps instead of one rep in 1 second then F must be greater (to accelerate the weight faster) and the distance covered is twice as great (2 reps versus one). Hence the work done is 4 times greater. Using your impukse definition, force is greater but time is the same, so the amount of impulse is two times greater. I think the analysis using work is a better approach than using impulse. 
Originally Posted by ChipB 
Right total agree, this is what I have been try to say.
Originally Posted by ChipB No. Check the units: Nm is not the same as Ns. But they may be correlated  in general pushing a weight for a longer period of time will cover more distance. But they aren't the same thing. For example  pushing on a brick wall that doesn't move results in 0 work being done on the wall, although the force applied times time may be quite substantial.  But if you push a weight for the same time frame and move it further, would not you use more Force and Impulse ???
Originally Posted by ChipB You're leaving out the fact that tp lift a 325N weight in 1/2 second requires more force than to lift it in one second.

Yes.
Originally Posted by ChipB Yes, power = energy/time, or force x velocity. 
Originally Posted by ChipB
As noted abovem, in general this is true.
I don't follow what you're asking for. As already noted, moving the weight 2 meters in one second versus 1 meter in one second requires more force to accelerate it from v=0. So both impulse is increased (by a factor of 2) and work done is increased (by a factor of 4). 
Yes agree, that’s what I was trying to Work out, yes more Force and more Impulse.
We all agree We use more Energy over the same Time Frame and move it more Distance, but only I say that is because We use more Impulse, no One else says why We move it more Distance or use more Energy.
Originally Posted by ChipB I think I showed why above. 
If you mean more Force and Impulse then we agree.
Thank you again for your time and help; sorry I was away for so long.
Wayne

  Thread Tools   Display Modes  Linear Mode  