How Does A Slinky Fall?

Key Takeaways

[Applause] [Music] now at some point growing up most of us have been captivated by one of these a slinky but recently I found out one of the most mesmerizing things about how it moves is something I'd never seen before how it falls so what's so surprising oh about a falling Slinky was what I expected well to help explain is phys just Rod cross the idea is that I hold the top end of the slinky like this and then let the bottom end dangle so the slinky is dangling freely and then I'm going to drop the slinky but I want you to predict what's going to happen will the top end fall first will the bottom end fall first will both ends fall together or will the two ends approach each other in the middle that is a tough question when I Let Go what does the bottom do shoot up it's going to fall it's grabing it's actually going to fall bottom goes up top goes down it might come up together you're going to see the top come down to the middle and the bottom come up to the top come to meet it and then drop the Top will accelerate faster than the bottom I reckon that bottom will stay there this will come down to there and then they go all right well why don't we give it a shot here I want you to to try to watch the whole Slinky as it falls to see what it's doing count it down right three 3 2 1 the problem is it's a bit hard to tell with the naked eye just what's happening no IDE think it came up you bottom come up I couldn't be sure it's all too fast yeah to really appreciate the physics involved you need to see it in slow [Music] motion oh oh gosh that's great that's weird that's unbelievable it does it does stay there just stays there like in midair it's suspended what yeah it doesn't move at all how does that work how does that work how do you explain that well you've got to look at what's happening at the bottom end gravity is pulling the bottom end down tension's pulling the bottom end up the two forces are equal and opposite so the bottom end remains at rest then I let go at the top end the tension in the spring changes but it propagates down the spring coil by coil until it reaches the bottom end and that takes about a quar of a second and then the bottom end Falls so the tension doesn't actually change at the bottom end until the rest of the slinky has collapsed correct the same principle applies to sporting equipment like tennis rackets or golf clubs when contact is made with a ball a wave travels up the shaft so the golfer's hands don't feel the hit until after the ball is already on its way to the hole now as a final extension on this experiment we've tied a tennis ball to the base of the slinky we're going to drop it and see what happens this time incredibly the same thing happens that's because the slinky has simply stretched further and reached a new equilibrium where the gravitational force down equals the tension force up it didn't make a difference is cing ch but that's what makes physics so interesting that's why I keep doing experiments like [Applause] [Music] this