1
$\begingroup$

To simplify the question we assume the same man run on a straight and flat road with constant speed. Personally I don't believe the man would spend a similar energy. But how to prove it in physics?

I have a raw answer to this question, probable it is not right. I separate the running process into two phases. First is the start point from 0 velocity to V. as we known

$$ mV_{fast}t_{short} = mV_{slow}t_{long} $$ because $$ mV = Ft $$ then $$ F_{fast}t_{short}^{2} = F_{slow}t_{long}^{2} $$ V as velocity; F as force; t as time; m as mass;

if decrease t will lead to increase F keep equal the equation. The energy spend by the man is direct proportional to the inverse of $$1/t^{2}$$

The second period is constant speed, the energy is mainly spend on resist the resistance. Because it's the same road so it is relative to the air resistance. The air resistance equation is $$ F = 1/2C\rho SV^{2} $$ The energy spend by the man is direct proportional to the $$V^{2}$$ C as const value of air resistance; $$ \rho $$ as density of air; S as frontal area; V as velocity;

I'm not expert in physics so if someone could supply a more professional answer that would be create! I will really appreciated the help and thanks in advance!

$\endgroup$
6
  • 1
    $\begingroup$ I'm not quite following how you're defining your variables there, but I think that the simple answer is this: which option is more energy-efficient is going to be entirely dependent on what mechanism energy is being lost (e.g. friction, air-resistance, biological processes, etc.). I imagine for the complex losses in humans there is going to be some maximally efficient speed that's neither too fast nor too slow. $\endgroup$ Commented Dec 22, 2015 at 6:59
  • $\begingroup$ @aquirdturtle Thanks for your feedback. I will improve the question. I want to ask if there is a way to prove the results with some physical formula equation. I don't want to find the efficient speed instead I want to prove my assumption that if someone run faster then much more energy is required. $\endgroup$
    – Erxin
    Commented Dec 22, 2015 at 7:07
  • $\begingroup$ @aquirdturtle the question is updated $\endgroup$
    – Erxin
    Commented Dec 22, 2015 at 7:16
  • $\begingroup$ I'm still not sure where you are going with your derivation. It looks like you've equated the forces in the two situations, where the forces are the forces required to accelerate an object of mass m to velocity V in time t. I think that what you want to do is equate the distances: V_1*t_1 = V_2*t_2. If you don't include any model for dissipation, than you will always win by going slower because it will take less energy to accelerate to that speed and if you don't lose the energy you will eventually reach your destination. $\endgroup$ Commented Dec 22, 2015 at 7:28
  • $\begingroup$ What I suggest is using some model for dissipation, e.g. friction F = -mmu*(v^hat), linear or quadratic air resistance (F=-aV or F=-b*V^2 for some factors a and b depending on the geometry). Then, figure out what force the person would need to exert to keep themselves going given the resistive force and figure out how long they'd need to exert the force at a given velocity. Then, figure out how much energy a person needs to use to exert a given force. Knowing how long they need to exert that energy, you have the total energy they need to exert. $\endgroup$ Commented Dec 22, 2015 at 7:32

2 Answers 2

0
$\begingroup$

This question is an example of the role played by forces internal to a system. In the case of the human body, complex chains of metabolic processes provide the energy used to contract muscles. Furthermore, different metabolic pathways with different efficiencies will be deployed depending on the human's speed, current dietary situation (does their body have access to carbohydrates? Is the body burning more fat or ketones?), and many more variables. This heavily complicates the question.

Fortunately, to answer your question, we don't have to worry about that. All we have to do is treat the human like a black box and measure its energetic expenditure in different situations. For the sake of getting a clear answer, let us assume that we are dealing with an average human who just had a nice big plate of pasta the night before. In this average case, we can say that a human has $S=.55m^2$, $C=1.16$, and that air has $\rho=1.225 kg\cdot m^{-3}$. ( These values are from a very well sourced school project)

Now, let's compare two situations: 1) as in your posed question, we can consider only the force of air resistance at various speeds to calculate energy expenditure, and then 2) use measured values from the literature for the power required for an average human to maintain certain speeds to calculate the energy expenditure.

1) First, let's consider a modest jog of 2.25 m/s. Using the quadratic drag force equation $F=\frac{1}{2}C\rho S V^2$, this yields a constant drag force of 1.98 if we assume there's no wind. The work done is $$W=\int_0^d{F\cdot dx}=\int_0^d{1.98dx}=1.98d$$ where d is the distance traveled. For a 5K run, this yields $$W=1.98\cdot 5000=9900J\approx 2.4\ kcal$$ since $1\ J\approx .00024\ kcal$.

If the runner was going at a 6 min/mile pace (~4.5 m/s), a similar calculation gives $F=7.81$ and $W\approx 39000\ J\approx 9.4\ kcal$. But notice that the kinetic energy of a runner of 70 kg moving at this rate is $T=\frac{1}{2}\cdot 70\cdot 4.5^2\approx 700\ J $. This is about 7% of the energy used to run the 5K itself!

2) Following this Harvard Medical School Table we can do some very rough obsevations that show the difference between running at different speeds. At 2.25 m/s, we see that a a 5K would take about 35 minutes to complete, so the energy expenditure would be somewhere around 350 kcal. At 4.5 m/s, a 5K would take about 18.5 minutes and expend about 380 calories. Now, the difference here may be small, but it of course scales as longer distances are ran.

Finally, we have to take into account the basal metabolic rate- the Harvard table does not separate this from its values. If we assume that the runner is a 5'8" man who is less than middle age and is 70 kg, he will have a BMR somewhere around 1700 kcal/day. This means that his body will spend about 40 kcal while jogging for ~35 minutes at 2.25 m/s, and about 20 kcal while running the 5K at 4.5 m/s. If we take this into account, the difference becomes $360\ kcal- 310\ kcal = 50\ kcal$.

In conclusion, we shouldn't rely upon knowledge of drag force to determine how much energy is being spent, and the mechanisms used by the human body to move our muscles become less efficient as we move faster.

$\endgroup$
3
  • $\begingroup$ So if we only consider the physical formula then the energy expiration rate be proportional to the square of velocity. If we add the metabolic then the expiration rate will be greater. Thank you. That's a excellent answer to my question! $\endgroup$
    – Erxin
    Commented Dec 25, 2015 at 6:52
  • $\begingroup$ Although the useful work is same but the total expiate energy is quite different. $\endgroup$
    – Erxin
    Commented Dec 25, 2015 at 6:58
  • $\begingroup$ Correct. The main complication is gravity. When we walk or run, we are basically bouncing up and down. This means that we spend lots of energy to get ourselves off the ground in order to move, and wind resistance contributes comparatively little resistive force. If we didn't have gravity but still had an atmosphere, the first situation would be true. $\endgroup$ Commented Dec 25, 2015 at 8:19
0
$\begingroup$

The absolutely simplest way to think about it concerns acceleration. The man has to expend metabolic energy accelerating to a given velocity, which is then dissipated as heat after the run and is not recovered into the metabolism. The higher the velocity, the more energy is expended. As other answers indicate, there are a whole slew of other factors concerning the specifics of physiology, so think of this as an answer featuring "a spherical human moving in a vacuum"!

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.