Newton’s Second Law of Motion for Idiots

Read time: 2 minutes

Years ago I worked with a personal trainer who derailed every casual conversation about strength training with scientific mumbojumbo. His entire practice operated within this self-imposed scientific parameter.

Of course, the scientific method deserves respect. But the average person/client also deserves a better perspective on strength training than be inundated with meaningless scientific data.

A study on strength training observes very specific variables in very confined parameters, primarily that of the load used, the repetitions executed, the frequency of treatment, and all within a predetermined period.

These parameters often can’t be extrapolated to the real world, however. Resistance training in the real world is far more complex, especially as we deconstruct its relationship to Newton’s Second Law of Motion.

Force = mass x acceleration

Or, force is directly proportional to the rate of change in momentum, where momentum is the product of mass and velocity. Blah blah blah.

Strength training presents a series of forces applied to, and generated by, the tissues of the body. “Strength training” is thus “force training.”

What kind and how much force the body is accepting or generating is dictated by multiple factors:

  • Load
  • Velocity
  • Changes in velocity (acceleration and deceleration)
  • Changes in direction (angular momentum about various joints)
  • Time it takes to complete a motion
  • Distance of motion
  • Attachment of musculo-tendonous complex
  • Muscle or tendon length
  • Attachment points on bone (forming different leverages)
  • Fiber type and distribution
  • Fiber angle
  • Limb length and ratio
  • Elasticity of muscle and tendon (storing and releasing of energy)
  • Individual differences in neural patterns
  • Compensatory muscular activation patterns
  • Compensatory contribution from other muscle groups
  • Etc.

In the real world, these factors add up to very different outcomes from one individual to another. Unfortunately, in the laboratory — in a controlled scientific study — these factors are ignored.

These are the significant factors that the pedantic, self-fluffing personal trainer forgets to mention to his client, mostly from a lack of their familiarity.

The point is, none of the scientific mumbojumbo (likely borrowed from a mere abstract) means much to the average person who wants to get fit, get healthy, and look good.

In fact, the scientific conclusion of the strength-training study means little to anyone if most of Newton’s Second Law of Motion is neglected.

These factors must be considered if the science lab wants to make strength training reproducible, accurate, and scientific, and for studies on strength training to be taken seriously.

Otherwise, for most of us who want to get healthy, fit, and look good naked, why not just  keep exercise simple? Move around a lot. Lift heavy stuff. Move fast now and then. Get your heart rate up really high once in a while.

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18 Responses to Newton’s Second Law of Motion for Idiots

  1. Greg Linster says:

    Hey Johnny! I know you promote staying lean and fit through use of diet. I couldn’t agree more!

    Anyway, I was interested in hearing your thoughts on using body weight exercises and running to improve conditioning to an elite level (i.e. Elite Special Forces style conditioning). Should one train almost everyday if this is the goal? Would you consider that type of training to be detrimental to overall health and longevity?


    • Johnny says:

      Hi Greg,

      Some people can tolerate continuous intense training, even with higher density (everyday or nearly everyday) and appear physically well. Others fail to recover from it and eventually breakdown.

      Having said that, it’s been shown through blood tests that chemical markers indicative of overtraining syndrome increase in athletes who don’t exhibit any symptom of overtraining. From a performance aspect, sporting coaches would be encouraged to continue pushing the athletes. From a health aspect, well, the blood markers can tell the story.

      I guess the person doing the training almost everyday has to understand the potential for performance improvement, and the potential for health degradation, and then make his or her own decision.

      I hope this helps.


      • Greg Linster says:

        That definitely helps! Would 2 days a week of very intense body weight exercises (pushups, pullups, burpees, etc) in conjunction with walking/hiking/easy running or riding on other days be enough to maintain a relatively elite level of fitness in your opinion?

        Thanks again for sharing your thoughts.

      • Johnny says:

        I wouldn’t say elite. Probably decent for general fitness, but I’d like to see some maximum or circa-maximum strength efforts in there, too.


  2. Casey Adams says:

    I remember years ago reading in bodybuilding magazines and being amazed at how different the training styles were between really high level pros. It’s funny that they all rationalize why they trained the way trained but in the end they all stand side by side on the elite stage just nearly as good as the next. This says a lot about the limitation of scientific studies on muscle building or weight training.

    Good post!

    • Moritz says:

      Good point if we’re talking about normal people, but in my opinion the fact that all bodybuilders have to be on varying degrees and forms of enhancing substances makes them a bad example.
      basicly, they are so roided out, they walk by a gym and they will grow muscle 🙂

      • Casey Adams says:

        Hey, you’re right that pro bodybuilders are bad examples, since they’re steroided to the gills. But, even with steroids, the stimulus for hypertrophy is still exercise stress. Can’t take away that they still must train very hard… just very differently from each other. But forget about steroids for a moment, many years ago I competed in several drug-tested bodybuilding shows, and many of those shows were with a good friend of mine. Our workout routines were very different from one another, his being the Super Slow single set method while mine was the traditional regular speed multiple set methods. We both always placed in the top 5.

        But you are right though, my use of the pro bodybuilders was not a great example. The point I was making however is still valid.

  3. Lydia says:

    I am looking to increase leanness and agree that diet (or lack of) plays the major role in achieving low body fat percentage. My “problem” is that I am very active by choice…in that 5-6 days a week I am engaging in activities like rock climbing, hiking, mountain biking, yoga, backcountry skiing. All things I do because I simply love to do them and choose to make them part of my life. However, I have always used this as an excuse to eat frequently (and probably too liberally) to “fuel my activity” thus my bf% has never dropped below 20%.
    I am experimenting with IF but unsure how to encorporate it with my chosen lifestyle. For me, “working out” does not mean a set list for the gym, or tracking my heaviest deadlift. Instead I look at cleaning a route and getting up that hill on my single speed as performance markers. But I’m unsure how to IF while doing this? I want to get leaner, lower overall BF, and become less obsessed with fear of bonking while on the mountain. Everyone thinks I’m crazy if I don’t bring a bag dried fruit to snack on every 15 minutes while hiking or mountain biking. Am I doing myself (recomposition-wise) harm by restricting food until my fast is over? Thanks for any input.

    • Johnny says:

      Hi Lydia,

      I also enjoy single-speed mountain biking. So much more fun when you don’t have to worry about gears and your only concern is whether your legs can produce and sustain more power for that hill, or not and you look down to see them explode all over your downtube and chain halfway up the hill. 😦

      As far as your chosen activities, especially mountain biking, where your body continuously crosses the anaerobic threshold, glycogen is (intermittently) the primary fuel source.

      It’s difficult to IF while trying to optimize your performance. If the activity allows you to remain — at least for the majority of the time — in true aerobic metabolism, then theoretically you can go nearly forever while IF’ing.

      My wife and I occasionally hike in Half Moon Bay, and that’s a long hike with good hills, but the demand of these hills only pushes me into anaerobic metabolism for very short periods and only infrequently. I can IF the entire hike and for hours after.

      It appears that my body has become adapted at switching quickly between burning glycogen and fat, and at using ketones as fuel.

      In the end, I don’t think regular IF will work well for your lifestyle as it may be too complicated and difficult. However, I wouldn’t hesitate to experiment with which of those activities are more receptive to IF, and then make a mental note.

      You may be surprised how far you can go, even with frequent bursts of anaerobic demands. It’s uncharted territory in the literature. But real-life practices can be surprising.

      Or perhaps you can plan to IF only on a day with lighter activities, or on an off-day. There are many ways of enjoying and benefiting from IF.

      Good luck, and if you do decide to experiment (or have experimented) with IF and any of those activity, please come back and share your experience. The IF community can learn from you.


      • Lydia says:

        Johnny…thank you so much for taking the time to reply. Your response was intuitively what I was already thinking…I have been experimenting for a few months with all this and have some great results energy wise….I can’t tell you the last time I actually “bonked” whereas previously, if I didn’t eat every 3 hours I was in dire straits. I was on a backcountry ski trip in March where I was skiing for 5 hours into a hut and experience great energy and performance.

        So I think my body is making a shift to burning fat. I just haven’t been sure how long it actually takes to really get going?

        I do believe I truely get into the anearobic state only intermittently- actual climbing, that extra tough hill, etc. The rest of the time I stay in an aerobic state.

        I am just intersted in optimizing BF loss and how IF can help. Thanks again and I really enjoy your blog.

      • Johnny says:

        Lydia, I’m not sure if there’s any literature that shows how long it takes the body to learn how to switch from metabolizing glycogen to metabolizing fat, and especially how this “switch over” process can be done in progressively faster times with experience.

        I do know that it takes the average inexperienced person eating a standard American diet 2 to 4 weeks of eating a low- or no-carb diet to reach a state of ketosis; I’m just not sure how long it takes the body to get better at switching between the two metabolism effortlessly.

        But with all signs and evidence, the body does get better and faster at switching.

        I can tell you that, before my regular use of IF, missing a meal would send me into hell — emotionally and physically. Now, in looking back, I feel liberated that I can miss a meal or an entire day of eating and feel perfectly fine, even through intense and sometimes prolong physical activity.

        What a terrible constriction it was to rely on food in such short intervals.


  4. Jordan D. says:

    Johnny, that last paragraph reminded me of ghrelin. I’m normally not one to focus on things like insulin, carbs, protein, leptin, etc. I’m a “calories in-out” guy, but I do wonder if there’s something to ghrelin. For a long time, I had a bad habit of snacking in the afternoon (maybe 3-4 pm.) I would wander into the kitchen and graze, and it was hard to stop.

    Until a month ago, I was eating three meals per day: typically I would eat around 11 am, 3-4 pm, and 7-7:30 pm. Recently I switched to eating two meals a day, the first one around 9:30- 10 am, and dinner at 7-7:30 pm. (I don’t really know why I’m eating breakfast instead of lunch. It just sorta happened that way! lol. Lunch is actually more logical because it reduces the time between the first meal and the second one.)

    I’ve been eating this way for a month, and I finally stopped that snacking in the afternoon. I hardly ever think about it anymore. It just happened without me thinking about it. I wonder if my body finally made the adjustment to not eating during that time.

    • Johnny says:

      Hi Jordan,

      There’s absolutely something to ghrelin. Aside from psychological pattern, ghrelin is one of the strong physical contributors to a person’s behavior of ceaseless eating. Break the psychological pattern, break the physical pattern.


  5. Wood says:

    Since I practise IF, (1-or 2 day in a week, sometimes I dont, but thats why I love it) I don’t care if I can’t eat (as You said too). When I go fasted to gym ( I use to drink a whey protein shake before -old habits and fear lol) I don’t experice any negative effect. I log my workouts and since I dropped about 22 lbs all lift goes up. I think this is some emotional thing that “oh god my food was crap today, that’ why my workout will be too”..
    By the way very good blog. Thanks

  6. Marc says:


    From what I’ve experienced years ago and in others over the last few years, it takes about 3 months for the body to “switch over” (glycogen /fat burner)
    But…for it to happen you have to be strict with yourself for those 3 months. another interesting thing, once your system has made the switch…it’s almost like it learned a skill that it likes better then the previous method. Years ago when I would fall off the wagon, and then “eat right” again, my body would remember and make the switch quickly. I can’t really quote pubmed studies or anything, just my personal experience and it has been experienced by others thst I’ve talked to and helped make the switch over the years.

    Have a good one and hope your enjoying some summer weather.


  7. Al says:

    Off topic but two quick questions:

    1) Do you pre-plan you meals, especially your meat (cook Monday and Tuesday’s meals on Sunday, etc.)?Is this needed to make IF easier?

    2) What is you opinion on sleep: how much does one need, more the better (or less)?

    Thanks a lot.

    • Johnny says:

      Hi Al,

      Good questions.

      Occasionally I think about what we have in the fridge and pre-plan what to make for dinner. But that’s a bout it. I currently have a lifestyle where I can devote a little of my time into the act of cooking, which I enjoy very much. For others, pre-cooking on, say, a Sunday night for part of the week is necessary.

      When I was eating 5 or 6 smaller meals, that was what I had to do: cook on Sunday for the following week. I hated doing that. It was sooo obsessive.

      I find that *adequate* sleep is very important. Adequacy, though, is probably different for different people. For years I did fine with just 5.5 to 6 hours each night. Now I need 7.


  8. Pingback: How does tennis relate to the second law of motion? | Tennis Training Aids

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