B Understanding Weight and Inertia: A Comparison of Mars and Earth

[russ_watters]

They are giving you forward momentum.

Momentum? Why would that matter to how fast you can run? (hint: it really doesn't).

My strength helps me to push the ground even harder.

What direction are you pushing on the ground?

Without friction we can’t run.

You mean friction between your feet and the ground? Does that mean you run slower if there is less friction?

What does Newton's 2nd Law tell us about the net force required to run at constant speed?

Speak clearly and finish thoughts completely.

 

[russ_watters]

You're really not engaging your brain here to think this through, you're just doing one-line thoughts.

Newon's 2nd (and 1st, really) law tells us that running at constant speed doesn't require net force. So, what are our legs primarily doing when we run? They're bouncing us up and down. That's what we use most of our strength for. Lower weight means less strength required to bounce up and down, which means people would be able to run faster/for longer.

So, what effect does lower friction have? Since net force is only required for acceleration, lower friction means slower acceleration.

 

[jbriggs444]

So, what are our legs primarily doing when we run? They're bouncing us up and down.

It might be helpful to think about how a kangaroo moves.

Edit: Imagine my surprise when trying to look up some experimental detail to find:

Kuehnegger maintains the “kangaroo hop” was best. “By performing a kangaroo-type leaping and jumping, you require the least amount of energy, and consume the least amount of oxygen.”

 

[rudransh verma]

Then, returning to the ground: when you are airborne there is only one force acting on you: the gravitational force, or your weight. This is lower on the moon, but your mass is the same, therefore your acceleration back to the surface is lower on the moon, giving you more hang-time.

Lower weight means less strength required to bounce up and down, which means people would be able to run faster

Don’t you think in a single jump when the person is going up he is moving faster than on Earth because there is no drag and less gravity but when coming down he slows down because of low gravity. So net result is that the person is not actually gaining any speed over on earth.

 

[sysprog]

Then, returning to the ground: when you are airborne there is only one force acting on you: the gravitational force, or your weight. This is lower on the moon, but your mass is the same, therefore your acceleration back to the surface is lower on the moon, giving you more hang-time.

Your movement still has its horizontal component.

Don’t you think in a single jump when the person is going up he is moving faster than on Earth because there is no drag and less gravity but when coming down he slows down because of low gravity. So net result is that the person is not actually gaining any speed over on earth.

The vertical component of a leap off the ground must overcome both inertia and gravity; the horizontal component must overcome inertia alone.

 

[Arjan82]

Don’t you think in a single jump when the person is going up he is moving faster than on Earth because there is no drag and less gravity but when coming down he slows down because of low gravity. So net result is that the person is not actually gaining any speed over on earth.

You need to disconnect the horizontal movement from the vertical movement. As I said your takeoff velocity vertically can be faster*), but that doesn't say anything about how fast you go horizontally.

This is what I meant when mentioning the car going from 0 to 60 in 4 secs (in vacuum, to not be concerned with drag). That is horizontal movement, and thus, as @sysprog also pointed out, only susceptible to inertia. The car will always go 0 to 60 in 4 secs on any planet(oid), earth, the moon, mars, jupiter, anywhere (given enough drag, equal engine performance etc).

*) Also your hang-time is higher, so the time between jumps is longer and you get to a higher point above the ground.

 

[phinds]

Don’t you think in a single jump when the person is going up he is moving faster than on Earth because there is no drag and less gravity but when coming down he slows down because of low gravity. So net result is that the person is not actually gaining any speed over on earth.

Personally, I have no interest at all in guessing, the way you do. If I actually wanted to figure out what was going on and what the answer definitely is, that's exactly what I would do ... FIGURE IT OUT.

The equation of motion is a parabola and gravity is involved. Assume a given mass for the runner/jumper, and assume a 45 degree takeoff with a specified force and then do the calculation given the different gravity on each planet.

You will, as you already understand, find that the jump is higher and longer under the lower gravity. As to whether it is faster or slower, I would leave that to the calculations and actually GET the answer rather than GUESS at the answer.

 

[Vanadium 50]

@rudransh verma you are not thinking about what other people are saying to you. The median time for you to respond is 14 minutes, with several being under 4. This is not enough time for you to consider what others are saying and trying to teach you.

Some people will find this rude, arrogant, snotty and/or disrespectful, and certainly not consistent with PF values. i won't speak to that and instead point out that it's ineffective. If you want to learn, you need to be engaged.

 

[rudransh verma]

The equation of motion is a parabola and gravity is involved. Assume a given mass for the runner/jumper, and assume a 45 degree takeoff with a specified force and then do the calculation given the different gravity on each planet.

O yes! You are right. I didn't realize i could do that sitting on my chair. By the way what will be the equation of parabola. The parabolas I know doesn't start at (0,0). Parabola should be in the first quadrant. If I know it I will be able to figure it out. $t^{2}=-x+1$ or $t^{2}=-x$ doesn't seem right.

 

[phinds]

O yes! You are right. I didn't realize i could do that sitting on my chair. By the way what will be the equation of parabola. The parabolas I know doesn't start at (0,0). Parabola should be in the first quadrant. If I know it I will be able to figure it out. $t^{2}=-x+1$ or $t^{2}=-x$ doesn't seem right.

You seem to have an extraordinarily poor understanding of math. I do not mean this as an insult, I mean it as a suggestion that perhaps you would be well-served to study a lot of basic math before you worry about physics problems since you can't do one without the other.

Parabolas don't "start" ANYWHERE. They have a particular shape but their equations are determined by, among other things, where they lie on an XY coordinate system and that can be anywhere and in any direction.

Take any specific parabolic shape and place it absolutely anywhere on an XY coordinate grid and oriented in any direction and you can write an equation that describes it.

The fact that you do not automatically realize that is the basis for my belief that you would be better off studying math than physics, until you understand math better.

 

[phinds]

@rudransh verma I see you have responded to my suggestion that you study math by tagging it indicating that you are skeptical. This is a perfect example of what I meant when I told you that you don't seem to believe that we here on PF know what we are talking about.

Continuing to rebuff our suggestions and/or treat them as invalid is not going to work well for you.

 

[rudransh verma]

Continuing to rebuff our suggestions and/or treat them as invalid is not going to work well for you.

I didn’t know the meaning of skeptical. I thought it means “I should think about it.”
As you said I am already going to study about parabolas and come back later.

 

[phinds]

I didn’t know the meaning of skeptical. I thought it means “I should think about it.”
As you said I am already going to study about parabolas and come back later.

Ah. Well, as I hope you have found out by now, it means "I think you are wrong".

You should be more careful that you know what you are saying.

 

[phinds]

Also, @rudransh verma parabolas are a very small part of math. You can't understand physics without all of basic math, so I continue to suggest that you make a systematic study of ALL of math up through at least calculus. Then you can decide if you want to go on to differential equations and beyond.

 

[Vanadium 50]

I didn’t know the meaning of skeptical.

And yet you neither apologized for nor removed the reaction,
I an skeptical of your explanation.

 

[sysprog]

O yes! You are right. I didn't realize i could do that sitting on my chair. By the way what will be the equation of parabola. The parabolas I know doesn't start at (0,0). Parabola should be in the first quadrant. If I know it I will be able to figure it out. $t^{2}=-x+1$ or $t^{2}=-x$ doesn't seem right.

I suggest that you should look up projectile motion (under gravity) $-$ here's an introductory-level example in a SciAm article: https://www.scientificamerican.com/article/football-projectile-motion

From that article:

The only choice he has to make to maximize distance, then, is the angle at which he kicks the ball. You can see from the equation above that the distance traveled by the ball will be greatest when sin(2θ) is greatest. The sine function reaches its largest output value, 1, with an input angle of 90 degrees, so we can see that for the longest-range punts 2θ = 90 degrees and, therefore, θ = 45 degrees. A projectile, in other words, travels the farthest when it is launched at an angle of 45 degrees.​

​

That's why @phinds suggested trying 45 degrees $-$ it produces the greatest distance for a single ballistic launch $-$ what is the greatest speed attainable for a specified distance is a different question.

 

[Doc Al]

Another useful site to explore: Trajectories - HyperPhysics