The Spin of a Bouncing Ball

17 July 2012

Diana: I've got Hugh opposite me and he is holding a couple of balls as it goes. There's also a little table that's been setup so, what are we going to do with these two objects?

Hugh: Right. The bouncy ball, I've got here, I'll throw it under the table, and it's going to bounce three times - once on the floor, once under the table and then back on the floor again and here I go.

Diana: Ahh! So you threw that in my direction towards me and instead of coming out towards me, it's gone back to you.

Hugh: Absolutely.

Diana: So, what has happened? Why, this isn't right.

Hugh: It isn't right. It doesn't seem right, but it is in fact exactly what you would expect to happen if you sit down and think about it. The first thing is, imagine just what happens to the ball after it bounces on the ground. So, if you've got a friend, just bounce the ball towards that friend. I'm bouncing this ball towards...

Diana: Okay, so he's got a slightly larger ball here which hopefully will bounce a bit slower. Throw that towards me.

Hugh: Yeah and I'm bouncing it towards Diana.

Diana: Yup!

Hugh: And what did you notice, Diana?

Diana: Okay it was spinning as it sort of came to me.

Hugh: Alright. Well I didn't - it wasn't spinning when I threw it.

Diana: No.

Hugh: It's spinning after it hits the ground. It's coming in at an angle on the ground, it starts to spin. Now that's what you expect if a plane is coming to land and the wheels are not spinning. What the wheels do immediately as they touch the ground? They start to spin. Well why shouldn't the ball start to spin? Right, now I've got a spinning ball and it's heading up towards the underside of the table. Well we all know that if you've got a ball with a bit of spin on it, you know, Shane Warne and some of the best ever bowlers. I better not talk about cricket, but never mind.

Diana: Yeah, we've only got a few minutes on this.

Chris: Well talk about tennis because that's okay.

Hugh: Yeah, but there are athletes trying to play tennis aren't they? So you put some backspin on the ball and to see after backspin on the ball.

Diana: Yeah, it bounces back towards you.

Hugh: Yeah and in fact with top spin on the ball, then it - now it's heading towards you. So, the idea is you throw the ball on the floor. It starts to spin. Now that it's got some spin, it's backspin on the underside of the table, it's like coming back towards me. And it's one of these great things that if actually stop and think about it step at a time, it makes perfect sense.

Diana: I see, so what are the real world applications of this then?

Hugh: Well, applications are a bit hard to find but understanding spin is very important - gyroscopic effects and all that, but there was an episode in 2010 in the World Cup. Frank Lampard's gold was disallowed. It's exactly the same as this because if you look at the replays of it, the ball hit the crossbar, started to spin, went down, hit the ground inside the

goal, but because of the spin it came back out again.

Diana: So that was just enough to confuse the linesmen and the referee.

Hugh: Yeah, and maybe really go...

Chris: They're going to put the new technology into the stuff.

Hugh: Well, the new guideline technology should with any luck and pick that up.

Diana: Yeah.

Chris: Because I mean, the same science is manifest - I know I said tennis and you said, "Well, there's no (55:01)." But the thing is, when you see those balls go flying through the air and then they suddenly just nosedive just inside the touch line and the person has got it when (55;13) and the person then scores this point and then says, "I want to appeal this!" that's just the same thing, isn't it?

Hugh: Well, it's kind of different. I mean, the spin in the air is this Magnus effect which is the effect of aerodynamics, pushing the ball down if you've got topspin and hovering the ball up if you've got backspin. Now the impression is that it suddenly, but it's just that the ball is coming down at faster than G because of the topspin and down force due to Magnus effect. It's very deceptive and that's, well I guess what are good tennis player is doing is trying to create motions on the ball that deceive you.

Chris: But one other point I did here, some will make about this is that when the ball is traveling really fast, the air can't stick to the surface very well because of turbulence whereas when it slows down a little bit, then the air finds it easier to stick on and then this Magnus force kicks in and it pulls the ball down because of the spin and then it nosedives really quite dramatically and you've got 15 seconds to come back at me on that.

Hugh: It's certainly true that speed make a big difference and we know about swing on a cricket ball, reverse swing and so on. It's very sensitive to speed, humidity, all sorts of things.

Ingredients

A super bouncy rubber ball

A low table which is smooth underneath (with no structure getting in the way) standing on a solid non-slippery floor (carpet won't work).  Otherwise a wooden board supported 20-30cm over a table will work well.

Instructions

Take the ball and throw it so that it bounces off the floor, then onto the bottom of the table.

Do this gently, somewhere there aren't lots of delicate objects which could be easily broken.

Does the ball do what you would expect?

Result

You should find that the ball bounces back at you out from under the table.

Explanation

if you did the same experiment with a slippery ball, or on a slippery surface, you will find it will behave as you would expect, bouncing under the table and coming out the other side.

Simple path with no friction

When the ball bounces there is a large reaction force up from the table which pushes it back up and making it bounce. No other forces  act on it so it slips across the table, and keeps going at the same speed horizontally.

Simple bounce with no friction

However if you use a super bouncy rubber ball, it grips the surface where it hits. This means that there must be a large frictional force on the ball which starts it spinning, in fact because the ball is elastic its spin bounces and it starts to spin the other way.  This horizontal force causes the bounce to be more vertical than normal.

Friction collision with floor

The ball keeps spinning as it moves upwards and then hits the underside of the table, it isn't moving horizontally very fast now, but it is spinning rapidly, and in the collision the friction causes this spin to reverse, and actually pushes the ball in the opposite direction.

Fricitonal collision with table

The ball then hits the floor and the spin acts to speed it up horizontally and it come out from under the table.

Bounce path with friction

To find out more fascinating ways that spin can do entirely unpredictable things visit
Professor Hugh Hunt's website

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