# I Relative speed when someone runs on the treadmill

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1. Nov 29, 2017

### canucks

We are using VR technology for people with spinal cord injury ( incomplete so they can walk with assistance) for a study. They will be walking on a treadmill wearing an oculus. We have a question regarding relative speed since we are updating the software view based on that
if person A runs at 5 m/s , treadmill runs at 7 m/s , opposite direction of the person . If person B is watching, at what speed do they appear..aka whats the relative speed? what if treadmill and peron run in the same direction as person A . What speed does person B see him?

2. Nov 29, 2017

### Staff: Mentor

I'm confused: how can the running and treadmill speeds be different? A diagram might help...

3. Nov 29, 2017

### Staff: Mentor

...moving to general physics.

4. Nov 29, 2017

### canucks

Imagine if you running really fast on the treadmill or grabbing on the parallel bar and walking slowly.

5. Nov 29, 2017

### Orodruin

Staff Emeritus
Assuming that speeds involved are much smaller than the speed of light in vacuum, they just add normally.

The person could change position relative to the treadmill? Of course, this would eventually lead to the person falling off the treadmill...

6. Nov 29, 2017

### Staff: Mentor

This doesn't help. If my speed and the treadmill speed don't match, don't I fall off?

7. Nov 29, 2017

### canucks

We are using Lokomat treadmill so he is wearing a harness and won't fall. Are we adding or deducting the speeds if they move on the same direction?

8. Nov 29, 2017

### Staff: Mentor

Yes, though I don't think I'd use the word "eventually" to describe a half second...

9. Nov 29, 2017

### Orodruin

Staff Emeritus
Depends on the size of the treadmill ;)

10. Nov 29, 2017

### Staff: Mentor

Sorry, I can't help you until you fully describe the scenario. A diagram is highly recommended.

11. Nov 29, 2017

### Staff: Mentor

Agreed.

12. Nov 29, 2017

### Ibix

Think about it this way - if you are walking on one of those travellator moving walkways, you do it to go faster, right? Does the belt move towards your destination or away from it?

13. Nov 29, 2017

### PeroK

If someone is on a moving walkway at the airport and that is going at, say, 2m/s. And you watch them, then they are going at 2m/s relative to you.

If they start to walk at 1m/s relative to the walkway then you see them move at 3m/s.

If they walk at 1m/s "the wrong way" on the walkway, then you see them move at 1m/s.

Isn't that all rather obvious?

14. Nov 29, 2017

### Orodruin

Staff Emeritus
I think it is not so relevant to the OPs question though. It seems the speed of the person relative to the ground is fixed by a harness so the treadmill speed needs to be exactly opposite to the treadmill speed or the person will likely get injured.

15. Nov 29, 2017

### jbriggs444

That is often accomplished with poured concrete. :-)

16. Nov 29, 2017

### Orodruin

Staff Emeritus
Oops. Meant to have one of those as the walking speed ...

17. Nov 29, 2017

### A.T.

Why are you updating the VR view based on what some person B sees, and not on what A would see running at 5 m/s?

18. Dec 2, 2017

### CWatters

So person A runs at 5 m/s relative to the treadmill belt.
Treadmill runs at 7 m/s relative to the ground the treadmill machine is standing on.

If person B is standing on the ground then it appear to him that Person A is moving backwards at 2m/s.

If the treadmill machine is say 2m long he will fall off the back of the treadmill in 1 second unless he runs faster.

If he is lightly suspended in a harness that attempts to fix his position relative to the ground his feet will be swept out from under him unless he runs faster (eg at the same speed as the belt).

I believe you need to match the speed of the belt to the runner and show the VR at the same speed.