explainlikeimfive
ELI5: Isn't the more powerful lifter still technically using more force and power via their muscles when lifting a weight to the same height more quickly?
The classic introductory physics example for differentiating power from energy is two powerlifters lifting the same barbell to the same height, except one does it more quickly. The amount of gravitational potential energy given is the same, but done in different amounts of times and thus at different speeds. But i get confused because lifting the same barbell quicker requires a higher net force to be applied, which means the more powerful display of lifting required the muscles to output more force. And heat is also expended by the working muscles as well.
Discussion
You’re confusing work and energy with force. Both lifters move the same weight the same distance, creating the same amount of potential energy (mgh). But to accelerate the weight faster requires more force (F=ma).
Since they both end up in the same place you could plot force over time. The faster one will have a higher peak, but they will have the same area under the curve because they did the same amount of work.
https://m.youtube.com/watch?v=jm-ewXYs2yQ&pp=0gcJCf0Ao7VqN5tD
You're not applying the higher force for the entire distance as you need to stop at some point to let the momentum settle.
Work is mass x displacement and doesn't have the time element. So is energy which is why it doesn't matter fast or slow.
I'm the box example, you are storing energy in the box. That energy potential is mass x gravity.
If the weight stops at the same height, then an equal amount of work was done, as that is the amount of work that is required to raise weight X to height Y.
The slow version will be applying a constant amount of force the whole way (more or less), whereas the fast lifter will accelerate the weight at the bottom in order to build speed, but then has to stop applying force in order to let the momentum carry it to the same height. Or he could keep applying force, and then apply a counter force in order to stop the weight at the top. In this latter example the work is more, as he had to do work to stop the weights as well.
If we get in to the *really* fine print, there is probably some miniscule difference due to air displacement in fast vs slow.. A faster weight would cause more of an airdisturbance, and that work has to be provided by someone as well... But I don't think you are looking for that granular of an answer.
Yes, the total energy is the same for both, but the peak power is higher for the faster lifter.
Think of it this way. Energy is power multiplied by time. Power * Time = Energy
Let's say it takes 10 units of energy to lift a weight to some height. If lifter A can lift with 10 units of power, then it'll take him 1 second for the lift. 10 powers * 1 second = 10 energies
If lifter B is twice as strong and can lift with 20 units of power, then it'll take him 0.5 seconds for the lift. 20 powers * 0.5 seconds = 10 energies
For both lifters, the energy required to lift the weight to the same height is the same, but the faster lifter has to use more power since he did it more quickly.
Total work done to the object is the same for fast or slow moving
But the body is not a well oiled perfect machine. It has loads of inefficiencies, and one of them is when doing high power activities you are particularly less efficient. So therefore the calories burned, and fatigue gained is higher for high power work, even if its the same total amount of work on the objects
Yes, faster energy is more force and power. If energy is distance then power is speed.
Lifting competitions simply aren't judged on force and power that I know of.
Energy is like the total fuel in a car (in the tank, in the fuel lines, in the engine) and power is like how fast you're burning the fuel. A car revving it's engine in neutral would be high power but zero work. It's still still burning fuel but not turning the wheels.
So energy ≠ power ≠ force ≠ work
Energy = potential/capacity (the fundamental "stuff" of physics)
Force = specific push/pull (vector, what causes change)
Work = energy transfer process (what happens)
Power = rate of energy transfer (how fast it happens)
lifting 1 pound up one foot takes the exact same amount of "work" if done in a minute or in an hour.
Doing it faster requires more "power", you are correct.
a mouse on wheel could lift a pound in an hour.
to lift a pound 1 ft in one second you would need a bigger animal proving significantly more power.. like a dog running on the wheel..
a horsepower (1) is defined as the ability to lift 550pounds 1 ft in one second..
Yes, the same amount of work can be done via more force over a shorter time. What is the question here?
Clarification of technical terminology when trying to understand and compare feats of power and articulate it correctly. I tend to conflate force with energy colloqually and then it confuses me.
Well it sounds like you answered your own question...
Yes and no. I want to be able to confidently explain it to another person without confusing them, not quite there yet. Like how an ice cube melts at room temperature or on a blazing hot day, but one will melt it quicker due to more heat energy.
Connective heat transfer is a totally different topic. The real answer to your question is that colloquial and physics definitions of "power" are just not the same.
The weight gains and requires the same energy/work no matter how fast you lift it.
The difference is physiology, not physics. Muscles become less efficient as the force applied increase, since they are oxygen limited (actually circulation limited). With repetitions at higher force and with less time, they can't get enough oxygen from the blood stream, so they shift to a less efficient method of using fuel. That incidentally causes lactic acid to build up, which causes pain and suffering. That's why lifting heavier weights or lifting faster seems different, even though the work might be the same overall.
I mean, sure? But who is contesting otherwise? If you are trying to argue that should be a tie-breaker in a weightlifting meet (for example), that’s just not how the rules of the game are written. If Steph Curry shoots a high-arcing shot, the ball travels a longer distance but it’s still worth the same 3 points.
My question is about building the conceptual understanding of the difference.
It's a difference in what you're measuring. In much of physics, you compare the energy of the starting and end states.
To lift a box a certain height, you need to overcome a specific amount of potential energy that depends on the box's mass, the acceleration from gravity, and the distance you want to move the box. It doesn't matter how quickly it's done. Power is a measure of how quickly the energy is changing/being exerted, but doesn't affect how much energy is needed to be supplied in this simple instance.
If you add some form of time dependence to the energy equation (such as an elevator accelerating upwards at an ever increasing pace while the box is being lifted), lifting the box more quickly would reduce the amount of energy the lifter had to supply because the acceleration term in U = mah would be getting more positive with time. On the other hand, if the elevator was accelerating downward at an increasing rate, the slower lifter would have to supply less energy since the acceleration term would be getting less positive with time.
Since the acceleration from gravity is effectively constant with respect to time, the time does not enter into the equation of how much energy is required.
I find looking at the units of a given equation to be helpful in determining what's happening physically. U = mgh = kg (meters/second2 ) (meters). The units end up being kgm2 /s2 which is energy. If you want another unit to be involved but still measure energy, you need something in there to cancel out the units you add to the right side of the equation. In the increasing acceleration example, the units would be kg (meters/second3 ) (meters), and thus the new potential energy equation would be dependent on time, being U = [(mah)/t] * t(lifting). I separated the a and the (1/t) just to show the units, but they'd be part of the same term, just adding extra time dependence. To get the actual value at any time (and not just be able to input given values), you'd need to determine the Lagrangian for the system to see how the energy changes with time, but that's far beyond U = mgh.
For your question about heat from the muscles, that's far more complex and would need someone well versed in biophysics and specific information about the individual doing the lifting. The focus in your link is on the mass being lifted, not what's happening with whatever is doing the lifting. That's more of an efficiency question.
I think, maybe it is the speed vs slow factor he is hearing?
But who is contesting otherwise?
So, this is a common arguement within aspects of fitness, and many people believe for instance, slow = better.
Now, that is also an issue with muscle fiber types and issues like endurance etc.
It is often that slower lifts are more valued. So, I believe part of the question is centered around the concept of this net force being that the faster lifter of the same weight is technically "stronger" for a single lift PR type thing.
I would, now thinking about it, say that the reason slow is valued is that generally, someone who can slow lift, can still fast lift more, due to our design.
Meaning let's say, Lifter A lifts 100lbs in 1 second and lifter B does it in 2 seconds.
A technically lifted with more force. But if B can lift 100 in 2 seconds, he assuredly can lift 120 in 1 second.
A who lifted 100 in 1 second, might not make it to 120 in 1 second.
Exactly how/why that is how we work, idk. But that is probably the things leading to the question.
50 pushups in 50 seconds, or you do 2 second pushups and only hit 30 or so type thing....
Other Ape vs Human? They typically can do more Explosive lifts, but lack endurance.
Goes back to the basic Force = Mass * Acceleration equation. Mass is the same in both cases, but acceleration is larger/faster in the fast lifter, faster acceleration means more force.
Slow lifter: 1 mass * 1 speed = 1 force
Fast lifter: 1 mass * 2 speed = 2 force
Technically speed isn’t acceleration, but I’m trying to ELI5
Yes. But the part that confuses me is the energy/work aspect.
The lifter that lifts it with more acceleration over the same distance and therefore more force does more work according to:
W=f*d
So the muscles of the faster lifter have expended more energy, but the change in gravitation energy of the barbell is the same for both lifters. As you surmised the extra energy expended by the faster lifter is in heat.
If the faster lifter used that greater force over the entire distance, he would indeed have done more work -- but the weight would still be traveling upward at the end of his lift. In other words, the extra work results in leftover kinetic energy in the weight, so that it flies upward past the target height.
For the faster lifter to end up with a motionless weight at the target height, he would have to apply the greater force over a shorter distance, so that the total work would come out the same as the slower lifter.
Yeah the same amount of energy went into lifting the bar itself. The difference in energy use is a matter of what speeds human muscles work efficiently at.
Similar to the above
Work = force * displacement
Displacement is the same in both cases, but force is double in the above example, so work is doubled
Yes. But there must be a way to reconcile it in technical terms. The work done in both cases is the same because the change in GPE is the same. But the power is different because the velocity is different. And the velocity is different because the applied forces in lifting it were different.
the lifter that does the lift in a shorter time period has a greater power output for a shorter time period.
its like saying: lifter 1 can do ten horse power for 1 second. and lifter 2 can do twenty horsepower for half a second. obviously the power numbers I have picked do not line up to the current question.
You’re using more force to perform the same work over a shorter time. To get the same work over a shorter time means more power.
Two different rockets raising the same weight the same distance.
One rocket has a more power motor. It raises the weight to the same height faster than the other.
But both rockets take the same amount of energy to lift the weight. Or they do the same amount of work.
I feel like a lot of the responses you're getting are making it more confusing, so i'll try to change perspectives.
When you lift a weight, no matter how fast you lift it, you're not accelerating it upward the whole time. It's accelerating upward at the beginning and decelerating when you get close to the top. From the perspective of the weight, it only experiences a net upward force (lift > gravity) while it is accelerating up. It experiences a net downward force (lift < gravity) while its decelerating.
You could try to measure this force over the course of the lift but that would be sort of infeasible. Instead, you can figure out that if the weight starts at velocity zero (bottom) and ends at velocity zero (top) that means that the acceleration and deceleration have to perfect balance out. That means if you average over the course of the lift, the upward force has to exactly balance gravity, otherwise the weight would keep moving after you got to the top.
Lifting faster means you're applying more force (greater acceleration) at the beginning but less force from the middle to end (more deceleration). The points about Power (which is a rate measurement) are true but don't necessarily help with understanding the net forces at play.
Whether or not this feels harder has to do with a lot of factors beyond the basic physics including biological processes and bone-angle-leverage and stuff.
The faster lift is less efficient, more effort to achieve the same 'result'.
The higher force will lead to a higher kinetic energy, but both lifters have the weight stationary at the end, which means that the faster lifter puts much less force at the end, as the weights kinetic energy is put into potential energy. In the end, its the same amount of work in both cases.
Thank you for providing the actual answer. Some of the other answers say correct information but it's not actually the question OP is asking.
Replace "speed" in your two equations with "acceleration", otherwise you're implying that speed and acceleration are the same thing, which they are unequivocally not.
Yeah, I get it and probably simultaneously made my note at the same time as your comment but trying to parse accelerations in 5 year old terms can be challenging
This is not a subreddit for literal five year olds. Please refer to rule 4. Your comment should target adult laymen. In that vein, I recommend not writing your comments to be objectively incorrect just because that lets you avoid a slightly bigger word.
Yes.
Power is energy over time. They both use the same amount of energy, but one uses less time than the other. If they did it twice as quickly, they'd use twice the power.
Power is also force x distance. And the distance is the same, so the force will be greater. At least in this simplistic case.
But the energy to do both is the same.
Power is not force x distance. It's force x distance over time, or in other words, force x velocity.
The faster lifter applies higher force but for for a shorter time. Energy is the power "summed up" for the duration of the event.
So if he uses 10W for 1sec while a slower lifter uses 1W for 10sec they both used the same "energy"
Also don't get too much into the muscle part of this example. Human muscles are complicated and the soreness/tiredness you feel when using them is (also) due to other biological reasons, not only because of the energy output. In fact you can get tired muscles by (technically) not producing any mechanical work (holding a heavy weight while standing for example)
On paper, most weighlifting excersizes have a net zero work since you start and end at the same spot.
Not if you account for gravity. Lowering a weight slower than freefall still requires upward force.
Edit: wasn't thinking of "work" in the physics sense, my bad. Raising and lowering a weight requires energy but it doesn't do work.
It’s force x displacement. As long as you end where you started, no work is done. And lowering slower vs faster affects power in the same way as lifting slower vs faster, but the time factor goes away when you integrate power to get work.
Yeah, fair enough. I wasn't thinking of the physics definition of "work."
That seems like a pretty important factor to account for.
Athletes and bodybuilders have been using negative or eccentric reps for a long time. Recently it's started to gain popularity with the amateur crowd too.
For example, maybe you can't do a pull-up so to start you climb up to the bar and just lower yourself as slowly as possible, doing a negative rep.
It is. Lifting slowly takes more energy for the same reason.
That's actually a bit more complicated than it seems. A lot of the additional energy is being spent on maintaining balance and spending more time in the middle part between extensions where your muscles are least efficient at supporting the weight. There's a lot more to it even than that but yeah, as a combustion driven lifting machine, a biological creature is an unfathomably complex system of work and thermodynamics.
A lot of the additional energy is used to counteract gravity.
See "Gravity losses". It's a rocketry term but it explains quite nicely.
https://en.wikipedia.org/wiki/Gravity_loss
No, they said work. That's force times displacement, so if net displacement is zero, so is the net work done no matter how much force / effort was applied.
But you can absolutely design a mechanical system that lifts and lowers that weight with zero energy consumption other than friction losses.
Wow thanks. I try so hard.