Ten lords are leaping

The science of how things spin and bounce...
29 December 2016

Interview with 

Hugh Hunt, University of Cambridge


The science of how things spin and bounce goes under the microscope for the tenth scientific day of Christmas. Hugh Hunt demonstrates for Graihagh Jackson...

Everyone sings: on the tenth day of Christmas my true love gave to me ten lords a leaping, nine ladies dancing, eight maids a milking, seven swans a swimming, six geese a laying, five gold rings, four calling birds, three French hens, two turtle doves, and a partridge in a pear tree...

Graihagh - So we’re up to the 10th day of Christmas - and our “lords a leaping” are bouncy balls today; engineer Hugh Hunt is an expert on how things bounce and roll. It seems like an interesting profession choice Hugh?

Hugh - Well, I don’t actually do much bouncing of balls but I do lots of stuff with spinning things.

Graihagh - So what are we going to be doing today. We’re getting down on our hand and knees onto the floor. So this is the name of science, this is a lot of dedication. Okay.

Hugh - And I’ve got various balls here - bouncy balls, and snooker balls, and billiard balls, and footballs, and ping pong balls. But what I want to do is to take one of these balls and roll it towards a wall. Just roll it along the floor just as a snooker ball might roll along the table and it’s going to hit the wall. And you might think it might just bounce back ordinarily, but what it does is it leaps up into the air and then lands a little way away from the wall, it just doesn’t roll back. Now you can do the experiment - I’m rolling the ball now… and you can hear it bounces twice. There’s one bounce where it hits the wall and another bounce where it hits the floor. You hear bump, bump - here we go.

Graihagh - Yeah it’s pretty subtle. They’re very close together, aren’t they?

Hugh - Once more - we’ll do it again. You’ll hear bump, bump…

Graihagh - Okay. And that’s because it’s rolling a little bit up the wall, banging the wall, and then bouncing back?

Hugh - When the ball hits the wall, because the ball is rolling for a very, very short time it sticks to the wall, and rolls up the wall, and then becomes airborne, bounces back and then lands just a short way away from the wall. Now this kind of rolling and spinning stuff isn’t all that important except in the 2010 football World Cup, there was a famous incident in the England/Germany game - Frank Lampard’s disallowed goal. And I can hear lots of people listening to this show jumping out of their seats in indignation that the goal was not awarded.

Graihagh - What happened then? I have to admit I don’t watch football, it’s not my deal.

Hugh - Well the ball came in towards the goal, it hit the crossbar and then it went down onto the grass, across the goal line, but then it bounced back up again, hit the crossbar again and came out. Because the ball came out of it’s own accord the goal was not allowed but it was the spinning of the ball that caused the ball to hit the ground and come back out again. So understanding how things spin and what happens about spinning things turned out to be pretty important in that game of football.

Graihagh - In your scientific opinion, should it have been allowed?

Hugh - Well yes, it should have been allowed, I guess I would like to think that referees in football should know all about conservation of angular momentum and the laws of physics, and would have been able to make that deduction for themselves on the field of play.

Graihagh - Other than football though, where are you studying this and using this spinning science?

Hugh - Well, I think once you get outside of the Earth and you go into space. If you think of the Philae Lander that landed on the comet last year and the New Horizon spacecraft that went out to Pluto. If you try and think about how they turn themselves around so that the legs of the spacecraft are pointing in the right direction to land, or that the camera is pointing in the right direction, or the antenna is pointing in the right direction, and it’s all about understanding the angular motion of spacecraft. If you ever watched these movies like The Martian or Interstellar and you can see what goes wrong when spacecraft are spinning out of control. And if you think of a balloon, you blow it up and then you let go of it, it randomly flies around the room, well that’s because it’s spinning out of control. So it’s really important to get control of spin and understand spin in space.


Add a comment