[scone12]

Me and my flatmate were watching the Matrix to the other night and there's a scene where two characters are fighting on top of a lorry. This got into a bit of a debate about relativity. If you jumped (quite high) whilst on top of a fast moving lorry, would the lorry move away from under you or would you move relative to the lorry? Thanks.

[tickle]

Really ? The lorry would move away from under you and then you would be toast!

[massplumber2008]

The answer is relative to gravity and acceleration.

Since gravity equals 9.8 m/s^2 and the lorry is moving away from you you would be falling at a rate of 9.8 m/s the first second, 19.6 m/s the 2nd second, 29.4 m/s the third second, etc. until you reached the terminal velocity a free falling person can reach here on Earth, but by then you would have smashed into the lorry and the lorry would stop your fall.

Given enough distance/time as demonstrated by Felix Baumgartner, humans can reach incredible speeds... he just broke the sound barrier using his body while in a speed skydive.

Anyway, gravity and acceleration are your answer. If you know the speed of the lorry and the distance it is travelling and the weight of the person you could set up an equation that would tell you exactly when you would hit the lorry!

Good day!

Mark

[ebaines]

I'm not quirte following your question, so I'll rephrase it in hopes that this is what you meant to ask:

"If you jump straight upward while standing on top of a lorry that is moving at a high rate of speed, where do you end up?"

In classical physics if we ignore air fraction the person who jumps upward would land in the same spot on the top of the lorry that he jumped from - exactly as if the lorry was not moving. This is because his horizontal velocity is the same as the lorry's horizontal velocity, so during the time the perosn is in the aior both the person and the lorry travel the same horizontal distance, However, in reality air reistance is quite high when moving at 40 MPH or so, and hence the person would be blown backwards relative to the lorry a bit, and would actually land somwhat towards the rear of the lorry (or perhaps would fall completely off the back end). If you remember the scene in the first "Mission Impossible" movie where Tom Cruise is fighting the bad guy on top of the Chunnel train, that illustrates the effect quite well.

[tickle]

Thanks for the explanation ebaines. It was very enlightening. So in my practical mind, given I know nothing about physics, my answer was basically correct, yes?

[ebaines]

Hi Tick. Yes - in general if you weren't really careful you would indeed be toast. Like they say on TV... don't try this at home!

[Wondergirl]

You mean I can't be trapped in a falling elevator and at the last minute, before it hits bottom, jump up into the air so I won't smash onto the floor?

[ebaines]

You mean I can't be trapped in a falling elevator and at the last minute, before it hits bottom, jump up into the air so I won't smash onto the floor?

I think if you find yourself in a faling elevator that's exactly what you should do! Let us know how it turns out. I predict it would work great as long as the elevator is in a building that's about 3 feet tall.

[Wondergirl]

I think if you find yourself in a faling elevator that's exactly what you should do! Let us know how it turns out. I predict it would work great as long as the elevator is in a building that's about 3 feet tall.

Hmmmm, I was thinking of the Sears Tower in Chicago or a NYC skyscraper.

[ebaines]

I know this is all in good fun but it's interesting to run the numbers. If the elevator falls a distance of 500 feet it would hit the ground at about 122 MPH. Ouch. If you are athletic enough to be able to slam dunk a basketball (can you?) that means you can jump upward a height of about 3 feet, which means you are capable of generating an upward velocity by jumping of about 9 MPH. So if you timed the jump perfectly you would smash into the ground at only about 113 MPH instead of 122 MPH. Stated another way - it would be like falling from a height of "only" 425 ft. I think it would pretty much ruin your whole day. Oh, and of course the elevator would be smashed to smithereens and you'd be crushed to death anyway!

[Wondergirl]

Oh, and of course the elevator would be smashed to smithereens and you'd be crushed to death anyway!

Ha, ha! And as for slam dunking anything, I can barely stand up long enough to brush my teeth, so there!