# Thread: A question for the engineers in the audience

1. ## A question for the engineers in the audience

While the rest of you were getting ready for your Sunday morning ride, I was in my gym lifting and shooting hoops.

I finished with carrying/twisting/turning?tossing with what I call a "medicine ball", you know, those 6 or 8 lb SPRI balls. When I was about to head back and out the door, instead of carrying this 8 lb ball all the way across the gym floor and then all the way across the workout floor to its station, I just leaned over and gently threw it.

And it rolled and rolled and rolled if it hadn't hit the door it would have rolled all the way to its station.

So, it seemed as if I'd created energy because the gentle push I gave it surely had to be less than the energy I would have expended by carrying it that distance. How can one scenario of energy expended equal the other scenario? Same result---the ball got put back.

OK, some trail pics, from Three Sisters. I can't believe I used to ride most of that trail up.
That would be another mini-WOS challenge, who can clean it from bottom to top.

2. You burn a lot more calories running a mile than biking one - no energy *created*, the job (going a mile) just gets done more efficiently. Same thing.

3. Originally Posted by nhkrugb
You burn a lot more calories running a mile than biking one - no energy *created*, the job (going a mile) just gets done more efficiently. Same thing.
Not really. In both cases you move the same mass the same distance, so the energy expenditure (W=FxD) is about the same. There is a little difference because when you run your body is moving up and down more.
But when you run, you expend that energy over a shorter period of time. So power output. (Energy/time) is greater.
Conclusion: If your concern is just calorie burning, focus on how far, not how fast.

4. Originally Posted by smilinsteve
Not really. In both cases you move the same mass the same distance, so the energy expenditure (W=FxD) is about the same. There is a little difference because when you run your body is moving up and down more.
But when you run, you expend that energy over a shorter period of time. So power output. (Energy/time) is greater.
Conclusion: If your concern is just calorie burning, focus on how far, not how fast.
What about my SPRI ball rolling question guys

5. Originally Posted by xcguy

And it rolled and rolled and rolled if it hadn't hit the door it would have rolled all the way to its station.

So, it seemed as if I'd created energy because the gentle push I gave it surely had to be less than the energy I would have expended by carrying it that distance. How can one scenario of energy expended equal the other scenario? Same result---the ball got put back.
You have to remember what the Nike commercials say (or was it Isaac Newton?)
A body in motion tends to stay in motion.
It doesn't take energy to keep a body at constant velocity, if no forces are acting against it. In the real world, that ball had some friction and air resistance acting against it, and maybe gravity if the floor wasn't flat, which makes it stop rolling eventually.

If there was no air resistance or frictional forces (energy loss from the ball deforming etc), that sucker would roll forever with no new energy added.

6. Originally Posted by smilinsteve
Not really. In both cases you move the same mass the same distance, so the energy expenditure (W=FxD) is about the same. There is a little difference because when you run your body is moving up and down more.
But when you run, you expend that energy over a shorter period of time. So power output. (Energy/time) is greater.
Conclusion: If your concern is just calorie burning, focus on how far, not how fast.
I used a biking analogy since we are here at mtbr...

For argument's sake, biking a mile at 10 mph on a flat road is about 25 cal or so. Running it is about 110. http://davesbikeblog.blogspot.com/20...-calories.html Now, I don't intend to argue the specific numbers in that post, but the point is that they aren't even close. It takes a good deal of energy to run a mile at 10 mph, and biking it is very easy. I was just pointing out that using two wheels to move a person one mile is way more efficient. Just like rolling a ball across the floor is more efficient than carrying it.

Of course, it has to be rolled far enough to make up for the energy expended picking it up to its starting height at the end. I was just using a simple analogy to show that it isn't creating energy to roll it across the floor, it is just doing the job more efficiently by taking advantage of the fact that the ball is essentially a wheel in this situation.

7. Originally Posted by smilinsteve
You have to remember what the Nike commercials say (or was it Isaac Newton?)
A body in motion tends to stay in motion.
It doesn't take energy to keep a body at constant velocity, if no forces are acting against it. In the real world, that ball had some friction and air resistance acting against it, and maybe gravity if the floor wasn't flat, which makes it stop rolling eventually.

If there was no air resistance or frictional forces (energy loss from the ball deforming etc), that sucker would roll forever with no new energy added.
I think it was Isaac Hayes that sang "keep on keepin' on". Or was that Wayne Newton?

8. Originally Posted by nhkrugb
I used a biking analogy since we are here at mtbr...

For argument's sake, biking a mile at 10 mph on a flat road is about 25 cal or so. Running it is about 110. http://davesbikeblog.blogspot.com/20...-calories.html Now, I don't intend to argue the specific numbers in that post, but the point is that they aren't even close. It takes a good deal of energy to run a mile at 10 mph, and biking it is very easy. I was just pointing out that using two wheels to move a person one mile is way more efficient. Just like rolling a ball across the floor is more efficient than carrying it.

Of course, it has to be rolled far enough to make up for the energy expended picking it up to its starting height at the end. I was just using a simple analogy to show that it isn't creating energy to roll it across the floor, it is just doing the job more efficiently by taking advantage of the fact that the ball is essentially a wheel in this situation.
Sorry I misread your post. I was thinking walking vs. running, not biking vs running.

9. Don't need an engineer to explain the concept of potential vs. kinetic energy. Clearly the floor in your gym isn't level, and you were at a high point when you pushed the ball.

When you lift an object or carry it up a hill, you are converting your own kinetic energy (movement) into potential energy within the object, just like compressing a spring or twisting a rubber band. When you drop that object from a height, or when you roll it down a hill, that potential energy is converted back to kinetic energy via gravity. As your ball rolled downhill, its potential energy was converted back to kinetic energy (movement). You didn't create any energy. It was there all along. Had you carried the ball back to the rack, its potential energy would have been converted into the kinetic energy via your muscles lifting it, walking, etc. Yes, that act would have required the conversion of fuel to energy within your body, and it would have required more overall energy because you would also be moving your body's mass in addition to the ball, but again, its energy being converted, not created. You could say you saved energy by being lazy and rolling the ball, but it wasn't created.

10. Originally Posted by smilinsteve
Sorry I misread your post. I was thinking walking vs. running, not biking vs running.
No worries!

Now let's get back to quoting Isaac Hayes:

D*** WOMAN! I just gave you sweet loving five minutes ago. Are you trying to kill me?

11. what is this gym you speak of?

12. The ball is lifted against gravity. Work is done. You then expend energy holding against gravity, but no work is done. You then do work moving the ball horizontal when carrying it.

The floor is doing what you do holding the ball, you are not expending energy holding the ball against the acceleration of gravity when just rolling the ball.

The work done moving the ball horizontal the same distance should be the same plus or minus losses in the body or the floor whether one pushes or carries it.

Also the potential energy gained when lifting is lost or not in your favor as it returns to the floor. If one could use the energy gained when lifting and apply it to moving the ball further, that might change things. Think inclined plane.

Work needs displacement. Just cause one expends energy doesn't mean work is done.

My communication skills are and never were good but think this is the answer.

13. Originally Posted by KarateChicken
what is this gym you speak of?
You need to get out more, KC.

14. Originally Posted by 1niceride
The ball is lifted against gravity. Work is done. You then expend energy holding against gravity, but no work is done. You then do work moving the ball horizontal when carring it.

The floor is doing what you do holding the ball, you are not expending energy holding the ball against the acceleration of gravity.

The work done moving the ball horizontal the same distance should be the same minus losses in the body or the floor whether one pushes or carries it.

Also the potental energy gained when lifting is lost or not in your favor as it returns to the floor. If one could use the energy gained when lifting and apply it to moving the ball further, both ways might come close to being equal.

Work needs displacement. Just cause one expends energy doen't mean work is done.

My communication skills are and never were good but think this is the answer.
"Just cause one expends energy doesn't mean work is done". Sounds like John Wooden: "Don't confuse movement with accomplishment".

15. I was back in my gym this morning (actually, every morning, but this time with SPRI ball in hand). I positioned myself just inside the doors and rolled the ball the other direction. And it rolled and rolled and rolled...I felt like I was listening to the theme song from "Rawhide". It finally petered out about two feet from the opposite corner. And I'd consciously "pushed" it less than the original time. I'm calling the floor level.

I finally located an engineer in that gym. I posed the same question and he said "I'll get back to you". Kinetic energy? Potential energy? I'm thinking there's an alien energy at work in that gym. Voodoo physics. Weird stuff. Stay tuned.

16. Sorry I would have answered, but I've been busy in Crested Butte studying brake cooling physics.

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17. Normally that would be a tiny creek this time of year...

18. ## nice

Originally Posted by lidarman
Sorry I would have answered, but I've been busy in Crested Butte studying brake cooling physics.

<a href="http://photobucket.com" target="_blank"><img src="http://i6.photobucket.com/albums/y228/lidarman/Trips/Crested%20Butte%202011/P1060921b800.jpg" border="0" alt="Photobucket"></a>
really nice!

19. Jimi lives. After Hendrix died his father went to a show by Randy Hansen. After the show Father Hendrix walked up to him and called him "Jimi". I saw Hansen at The Glenn Miller Ballroom back in the 80s, and after seeing Hendrix live three times I thought this was the fourth. This is Randy keeping the spirit alive. Not much to do with mtn biking but all good.

<iframe width="425" height="349" src="http://www.youtube.com/embed/hlMlXZjlLNI" frameborder="0" allowfullscreen></iframe>

20. Originally Posted by xcguy
I was back in my gym this morning (actually, every morning, but this time with SPRI ball in hand). I positioned myself just inside the doors and rolled the ball the other direction. And it rolled and rolled and rolled...I felt like I was listening to the theme song from "Rawhide". It finally petered out about two feet from the opposite corner. And I'd consciously "pushed" it less than the original time. I'm calling the floor level.

I finally located an engineer in that gym. I posed the same question and he said "I'll get back to you". Kinetic energy? Potential energy? I'm thinking there's an alien energy at work in that gym. Voodoo physics. Weird stuff. Stay tuned.
I explained it in post 5. It has nothing to do with floor slope or potential energy.

It takes no energy to keep a body at constant velocity, if there are no forces acting against it.
For example, throw a football as far as you can; say it goes fifty yards. Now, go out into the middle of deep dark space and throw the same football with the same energy, and it goes 500 trillion miles. It didn't go farther because it had more energy, it went farther because there are no forces acting against it. It wont change velocity until it hits some space dust or gets pulled by the gravity of a nearby star.

Anyway, a rolling ball on a flat gym floor has a few forces acting against it. The tiny irregularities on the floor surface resist rolling. Air friction resists rolling. The ball flexes as its weight acts against the floor which absorbs energy and resists rolling. (The same types of things that effect bicycle tire rolling resistance).

I think you were impressed with how far the ball rolled because it rolled farther than a basketball or a beach ball would have. That is because the weight of the medicine ball means it takes more energy to move it at a given speed than it would for a lighter ball. It seemed like you didn't give it much energy, but you used more force than you would have for a beach ball, for example.
Sooooooo,

Those floor irregularities and air resistance are proportionately smaller compared to the total energy of the ball, so they have less success in slowing it.

21. Originally Posted by smilinsteve
I explained it in post 5. It has nothing to do with floor slope or potential energy.

It takes no energy to keep a body at constant velocity, if there are no forces acting against it.
For example, throw a football as far as you can; say it goes fifty yards. Now, go out into the middle of deep dark space and throw the same football with the same energy, and it goes 500 trillion miles. It didn't go farther because it had more energy, it went farther because there are no forces acting against it. It wont change velocity until it hits some space dust or gets pulled by the gravity of a nearby star.

Anyway, a rolling ball on a flat gym floor has a few forces acting against it. The tiny irregularities on the floor surface resist rolling. Air friction resists rolling. The ball flexes as its weight acts against the floor which absorbs energy and resists rolling. (The same types of things that effect bicycle tire rolling resistance).

I think you were impressed with how far the ball rolled because it rolled farther than a basketball or a beach ball would have. That is because the weight of the medicine ball means it takes more energy to move it at a given speed than it would for a lighter ball. It seemed like you didn't give it much energy, but you used more force than you would have for a beach ball, for example.
Sooooooo,

Those floor irregularities and air resistance are proportionately smaller compared to the total energy of the ball, so they have less success in slowing it.
We all 'want' to thank you for this.

22. Originally Posted by lidarman
We all 'want' to thank you for this.
You are "welcome".

Now click that little yellow thumbs up thing.

23. Originally Posted by lidarman
We all 'want' to thank you for this.
Ok, I might have gone a little out into left field, but that's what happens when you mix beer drinking and physics.

To try and be more direct, the concepts I was talking about boil down to inertia.
Inertia= a body's resistance to a change in velocity.
Inertia is related to an objects mass.
A medicine ball has much more inertia than other typical balls we play with, just because it is heavier. (29 inch wheels also have more inertia than 26, which explains some of the benefits, and detriments, of 29ers). It resists any change in velocity (deceleration) much more than a lighter ball would.

To relate this to Newtons second law: force = mass x acceleration
F=MA

rearranging:
A=F/M

So, as mass increases, acceleration decreases (for a given force).

In the case of a ball rolling,
F= The sum of forces trying to slow the ball
A= The amount that the ball decelerates due to the forces F.
M= Mass of the ball.
More mass, less deceleration.

24. Originally Posted by smilinsteve
Ok, I might have gone a little out into left field, but that's what happens when you mix beer drinking and physics.

To try and be more direct, the concepts I was talking about boil down to inertia.
Inertia= a body's resistance to a change in velocity.
Inertia is related to an objects mass.
A medicine ball has much more inertia than other typical balls we play with, just because it is heavier. (29 inch wheels also have more inertia than 26, which explains some of the benefits, and detriments, of 29ers). It resists any change in velocity (deceleration) much more than a lighter ball would.

To relate this to Newtons second law: force = mass x acceleration
F=MA

rearranging:
A=F/M

So, as mass increases, acceleration decreases (for a given force).

In the case of a ball rolling,
F= The sum of forces trying to slow the ball
A= The amount that the ball decelerates due to the forces F.
M= Mass of the ball.
More mass, less deceleration.
"Rollin' rollin' rollin' keep those ballies rollin' Rawhide". My initial concept of "creating" energy had mostly to do with how my slight push on the ball caused all the other forces to come into play. I knew that the mass of the SPRI ball would "force" it to keep rolling farther but I still couldn't grasp how my very light push could "unleash" so much forward motion. Of course, if I was on a hilltop and let go of a ball off a cliff, it would keep going down with no more input from me but I could blame gravity for that.

25. Originally Posted by xcguy
You need to get out more, KC.
irony, it's what's gym rats have for dinner

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