# The Naked Scientists Forum

### Author Topic: Why does a bouncing object reverse its spin on the rebound?  (Read 1302 times)

#### thedoc

• Sr. Member
• Posts: 508
• Thanked: 11 times
##### Why does a bouncing object reverse its spin on the rebound?
« on: 25/07/2013 14:30:01 »
Tim DiGennaro  asked the Naked Scientists:

When I play fetch with my dog, sometimes we use a five pointed starfish toy. I have found that to get the most distance from the throw is to throw the toy with the arms vertical and putting a back spin on the toys thrown rotation, upon hitting the carpet the rotation reverses.

I understand it has to do something with that an object in motion will continue in motion. But, why does the rotation change?

Thanks,
Tim in Cleveland Tennessee

What do you think?
« Last Edit: 25/07/2013 14:30:01 by _system »

#### grizelda

• Hero Member
• Posts: 740
##### Re: Why does a bouncing object reverse its spin on the rebound?
« Reply #1 on: 25/07/2013 18:33:56 »
Some of the energy transferred to the carpet is transferred back to the toy, reversing its spin.

#### RD

• Neilep Level Member
• Posts: 8089
• Thanked: 51 times
##### Re: Why does a bouncing object reverse its spin on the rebound?
« Reply #2 on: 25/07/2013 20:36:22 »
... When I play fetch with my dog, sometimes we use a five pointed starfish toy. I have found that to get the most distance from the throw is to throw the toy with the arms vertical and putting a back spin on the toys thrown rotation, upon hitting the carpet the rotation reverses.

I understand it has to do something with that an object in motion will continue in motion. But, why does the rotation change?

When the toy hits the ground some of its translational motion is converted into rotational motion in the opposite direction to the backspin you originally applied to it , evidently sufficient to change the direction of the rotation. However this change direction is not inevitable : if you applied lots of backspin but threw the starfish straight upwards it could spin towards you when it landed rather than away from you.

... I have found that to get the most distance from the throw is to throw the toy with the arms vertical and putting a back spin on the toys thrown rotation

IMO that backspin enables an object to be thrown further is more interesting ... http://en.wikipedia.org/wiki/Magnus_effect
« Last Edit: 25/07/2013 20:39:41 by RD »

#### The Naked Scientists Forum

##### Re: Why does a bouncing object reverse its spin on the rebound?
« Reply #2 on: 25/07/2013 20:36:22 »