Physics help

[Garren Jacobsen]

What would happen if say a person jumped off a 69 meter high damn and while falling gravity orientated differently. So the person is going straight down, fall through a portal, the exit portal opens going sideways (think that closet door scene in monsters inc).

In the alternative, what if they were falling down, went through a portal that opened on the ground, their momentum carrying them up, but the new gravity trying to pull them down?

How likely is survival in these scenarios?

 

[Russ]

Great stuff!

So when your person entered the portal here is how fast they would be going:

• Height in meters 69
• Mass in kg

• Speed at impact:36.77 m/s
• or132.39 km/h
• Time until impact: 3.75 s
• Energy at impact: 0.00 joules

So in scenario one, they would start bumping along the ground (almost instantly) at 132 kph or so. Kind of like jumping off of a moving train or car at the same speed. Good chance of lots of broken bones or a fatality. It would take a small miracle not to suffer at leas several fractures. Assuming normal physics.

The second scenario is a bit more complicated. So your person enters the new world travelling at 132 kph UP, but at least not longer accelerating. I guess we can assume gravity in the new place works the same as on earth. So you person would still travel upward for about 3.75 seconds before they hit v=0 (maximum height). My very, very rough math suggests that the person would travel about 70 meters up before they started falling again. A 70 meter fall is pretty serious.

My math might be a tad off, I am pretty out of practise at this sort of thing, but the formulas I used can be found here:

Gravity Velocity Equations for Objects Projected Upward by Ron Kurtus - Physics Lessons: School for Champions

 

[pmmg]

KPH is not my more familiar term but 132 is well over 40mph. A 40MPH impact usually results in death. Faster than that, and death becomes even more likely. This is about 80MPH. I think death is almost assured in either example. Course, stranger things have happened, so...

 

[Heliotrope]

Reader perspective

Height in meters 1.8
Mass in kg 56.9

Speed of reading: 220 words per minute
Time I spend pondering the physics of the scene: 0s
Energy spent wondering if earth physics relates to fictional portals: 0 joules.

Just saying

 

[pmmg]

Reader perspective

Height in meters 1.8
Mass in kg 56.9

Speed of reading: 220 words per minute
Time I spend pondering the physics of the scene: 0s
Energy spent wondering if earth physics relates to fictional portals: 0 joules.

So you say, but we all know those numbers are fudged a little. Speed reading is probably closer to 215...

 

[TheKillerBs]

Great stuff!

So when your person entered the portal here is how fast they would be going:

• Height in meters 69
• Mass in kg

• Speed at impact:36.77 m/s
• or132.39 km/h
• Time until impact: 3.75 s
• Energy at impact: 0.00 joules

So in scenario one, they would start bumping along the ground (almost instantly) at 132 kph or so. Kind of like jumping off of a moving train or car at the same speed. Good chance of lots of broken bones or a fatality. It would take a small miracle not to suffer at leas several fractures. Assuming normal physics.

The second scenario is a bit more complicated. So your person enters the new world travelling at 132 kph UP, but at least not longer accelerating. I guess we can assume gravity in the new place works the same as on earth. So you person would still travel upward for about 3.75 seconds before they hit v=0 (maximum height). My very, very rough math suggests that the person would travel about 70 meters up before they started falling again. A 70 meter fall is pretty serious.

My math might be a tad off, I am pretty out of practise at this sort of thing, but the formulas I used can be found here:

Gravity Velocity Equations for Objects Projected Upward by Ron Kurtus - Physics Lessons: School for Champions

Pretty much this, but with air resistance factored in, the upward travel (and the final velocity) would be less. How much less depends on a few factors (in this case, the variable factors are the position the person is falling in and the clothes they are wearing) and would be much more difficult to model. In any case, I wouldn't reckon survivability to be very high. Best case scenario would be falling into something like a wheat field, which could absorb some momentum and dampen the blow.

 

[Garren Jacobsen]

Pretty much this, but with air resistance factored in, the upward travel (and the final velocity) would be less. How much less depends on a few factors (in this case, the variable factors are the position the person is falling in and the clothes they are wearing) and would be much more difficult to model. In any case, I wouldn't reckon survivability to be very high. Best case scenario would be falling into something like a wheat field, which could absorb some momentum and dampen the blow.

What about water? Or a metric shit ton of fresh powder.

 

[TheKillerBs]

The problem with water is that at high speeds the surface tension acts somewhat like a solid, and a hit to the head could lead to drowning. I'm not sure about snow, but it might have a similar dampening effect as vegetation.

 

[Penpilot]

The basics of what Russ said is pretty spot on. Long time since first year physics, disclaimer.

In the first scenario, survivability depends on at what height, thus what speed, they enter the portal. Because that's the speed at which they will shoot out of it, travelling parallel to the ground when exiting.

In the second scenario, will have similar results, but because they're travelling up now, the danger is in the fall back down. If gravity at the exit point is similar to gravity at the entry point, you can probably expect the height achieved when exiting to be close enough to the starting height to be a significant danger, even with terminal velocity and friction, etc.

Best case in the second scenario would be for them to come out at a slight angle, and after they reach the peak of their ascent, they land on a high ledge or plateau before they gained any significant speed coming back down. Theoretically, the perfect landing would be if the ledge was at the exact peak of their ascent, where their velocity is 0.

 

[skip.knox]

All of which just goes to show that physics is stupid and magic is better!

 

[Russ]

Water is kind of hard to figure out from a pure physics perspective. The actual experiments from the US Navy did showed the if you entered the water properly you could survive without injury at greater heights than the raw calculations predicted. I had a friend who was a US Navy rescue diver/swimmer who has actually involved in that study. The data is out on the internet somewhere and I think we once discussed it on a thread on this site but I don't have a time to teach it down at the moment.

 

[Insolent Lad]

All this assumes an abrupt change from the one world to the next. If it were a more gradual changeover, everything would be a lot safer.

 

[Michael K. Eidson]

You can find an online calculator that takes into account air resistance at Free fall with air resistance (time and velocity) Calculator. There are related calculators accessible from this page. Fill in the mass of your falling object, the distance the object falls, and fudge the air resistance and gravity factors according to your world's needs. Click Execute. The calculator will show you the time it takes for the object to fall the stated distance and how fast it is falling at that moment. There are calculators available too for projection velocity, which might be of use to you.