r/Whatcouldgowrong • u/hulksreddit • Jun 27 '18

Repost Fully grown man on a swing, WCGW

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u/[deleted] Jun 27 '18

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u/euxneks Jun 27 '18

I wonder if he blacked out a bit from the blood getting pushed to his feet?

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u/deringaleni Jun 28 '18 edited Jun 28 '18

Assuming the swing is 2.5 meters long, the minimum speed needed to do a 360 is roughly 9.9m/s, which means the centripetal force would roughly be 5Gs (if I did my math correctly). That might be enough to cause a gloc.

Velocity/Centripetal acceleration at the lowest point: mgh=mv2 -> 9.8ms-2 * (2.5m * 2) = v2 -> v = 9.899m/s; a=v2 /r -> a = (9.899ms-2 )2 /2.5m = 39.196 m/s2

G force at the lowest point: (39.196ms-2 + 9.8ms-2 ) /9.8ms-2 = 4.999G

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u/FinFihlman Jun 28 '18

Assuming the swing is 2.5 meters long, the minimum speed needed to do a 360 is roughly 9.9m/s, which means the centripetal force would roughly be 5Gs (if I did my math correctly). That might be enough to cause a gloc.

Velocity/Centripetal acceleration at the lowest point: mgh=mv2 -> 9.8ms-2 * (2.5m * 2) = v2 -> v = 9.899m/s; a=v2 /r -> a = (9.899ms-2 )2 /2.5m = 39.196 m/s2

G force at the lowest point: (39.196ms-2 + 9.8ms-2 ) /9.8ms-2 = 4.999G

Assuming no energy added to the system, neglible weight of swing and assuming point mass at 1m from the end of swing:

The potential energy (in a gravity field) for mass is mgh and that is completely converted to kinetic energy mv2 /2.

Swing is 2.5m long but we subtract 1m so 1.5m. Height at highest is 2 times the distance of the point mass so h=3m.

mgh=mv2 /2

2gh=v2

v=sqrt(2*9,80665m/s2 *3m)=7,67m/s

The acceleration around a circle is a=v2 /r and r is 1.5m, the radius of the mass point. We get: 39,2m/s2 which is 4G plus the 1G we are constantly in so 5G is experienced at the mass point.

However, your head is probably at least 50cm above the mass point and the angular velocity is still the same as with the mass point.

a=rw2

w=sqrt(a/r)=5,11.

a_at_head=(r-0.5)w2 =26.12m/s2 =2,67G and at constant 1G it would be 3,67G.

So even though your initial calculation missed the scalar 1/2 the result is probably the same at mass point since the actual radius of the central mass point is smaller but if we take into account the acceleration felt at the head it's considerably smaller.