Friday, December 9, 2016

Walking on Cornstarch?

While looking on Instagram the other day, I came across a video captioned, ‘Walking on Water’. It showed multiple different people running across the surface of  a pool. The “water” was actually a putty made of cornstarch and water; liquid when you push it softly or stick your hand in, but solid when any pressure is applied. Intrigued, I googled it and came across another video, this time of the Ellen Show, where her guests were running across this material. This made me start to wonder: how much pressure does it take to turn a liquid mixture of cornstarch and water into a solid? What is the buoyant force pushing up on a person when they’re applying pressure down on it by running? I used the physics we’ve learned thus far in class to try to answer these questions. 

Pressure = Force / Area
Area of a foot: .03m2
Force = mg = (80kg)(9.8) = 784N

P = 784/.03 = 26133Pa

Because if you simply stand on the putty you will start to sink…
v (average speed of person) = 8 m/s
t = 3sec
v = v0 + at
8 = 0 + a(3)
a = 2.66 m/s2

Σ F = ma
FB - mg = ma
FB = ma + mg
FB = (80kg)(2.66m/s2) + (80kg)(9.8)
FB = 997N

Link to Ellen episode where they run across the cornstarch:

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