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Disney World cast member hurt stopping 180-kg fake boulder from hitting audience (www.bbc.com)
submitted 10 months ago by to c/offbeat@lemmy.ca
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[–] 22 points 10 months ago (6 children)

I would have expected a 400lb rubber bolder moving that fast to NOT stop when hitting an average sized person. Are they exaggerating the weight of the boulder?

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  • [–] 13 points 10 months ago (2 children)

    It's the second power rule, plus extra needed thickness for strength that makes it confusing. A ball that is 10x larger in diameter than a soccer ball is 100x heavier.

    400 lbs vs 200 lbs means the 200 lbs person will be pushed with twice the speed if the ball stopped completely. Which is sort of what the video shows. mv=mv

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  • [–] 2 points 10 months ago (1 child)

    Ah good point, I assumed the rubber deflection would have altered the interaction. But when a larger bolder hits something there has to be some impulse calculation, Like when a bus hits a cyclist, the bus doesn't stop from firing the cyclist away fast

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  • [–] 1 point 10 months ago

    It's a partially elastic collision. So basically all the momentum is transferred into the person. Makes the calculation really simple:

    V_person = mass_ball / mass_person * V_ball

    For the bus and cyclist it would be some small amount of momentum removed from the bus by the cyclist because the bus doesn't stop. The relative masses are so different, which is why it appears the bus doesn't slow at all. It does though, but only a tiny bit.

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  • [–] 5 points 10 months ago (1 child)

    Assuming a 200lb man and a perfectly elastic collision, the ball would be reduced to ⅓ its original velocity, while the man would be accelerated to 1⅓ of the ball's original velocity, so 4× the end velocity of the ball.

    There's almost certainly some exchange between angular/linear momentum, but I'm approaching my wife's tolerance for math at the breakfast table XD

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