# The Naked Scientists Forum

### Author Topic: Does a golf ball speed up when it bounces out of the hole?  (Read 6089 times)

#### thedoc

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##### Does a golf ball speed up when it bounces out of the hole?
« on: 14/06/2012 19:30:01 »
Frazer Grundy  asked the Naked Scientists:

We were playing golf and my partners ball 'lipped out' of the hole - i.e. it caught the edge of the hole but didn't go in. Obviously in this situation the ball is deflected off line. But the questions is this - does it speed up or slow down?

He said that it speeds up, and therefore ends up further away from the hole than it would have done had it not hit the hole

I say that it slows down. More accurately, I think the ball drops as it hits the hole edge, and gravity speeds it up. Then it comes up and slows down, due to gravity plus the friction of the hole. The net effect that it has to slow down due to the friction of hitting the hole edge. THe only exception would be if it is deflected such that it changes direction from going uohill to downhill. THen it might end up going further from the hole

Who is right?

Thanks a million (no, that wasn't the value of the bet!)
Frazer

What do you think?
« Last Edit: 14/06/2012 19:30:01 by _system »

#### survivalist13

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##### Re: Does a golf ball speed up when it bounces out of the hole?
« Reply #1 on: 15/06/2012 11:55:16 »
Not being a golfer I'm not exactly sure what you mean but sounds a lot like restitution, when two things impact the speed of separation must ALWAYS be less than the speed of approach due to conservation of energy. Some energy is lost as heat and sound, no energy is put in from an outside source between the instant before two things collide and the instant after. This is why when you drop a tennis ball it will always bounce back to a lower height. Does this answer your question? The effects of gravity will cancel if the change in height is the same.
« Last Edit: 15/06/2012 11:57:44 by survivalist13 »

#### RD

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##### Re: Does a golf ball speed up when it bounces out of the hole?
« Reply #2 on: 15/06/2012 13:33:51 »
... when two things impact the speed of separation must ALWAYS be less than the speed of approach due to conservation of energy.

The energy of the ball also includes the kinetic energy of the ball spinning rapidly, (which is not obvious).
A spinning ball could jump out of a hole faster than it went in, as some of the spinning energy has now been converted into translational motion when the ball touches the surfaces of the hole.

A bit like those toys powered by a spinning flywheel inside them  ...
« Last Edit: 15/06/2012 23:45:30 by RD »

#### survivalist13

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##### Re: Does a golf ball speed up when it bounces out of the hole?
« Reply #3 on: 15/06/2012 13:40:14 »
... when two things impact the speed of separation must ALWAYS be less than the speed of approach due to conservation of energy.

The energy of the ball also includes the kinetic energy of the ball spinning rapidly, (which is not obvious).

If a spinning ball caught the edge of the hole it could jump out faster than it went in, as some of the spinning energy has now been converted into newbielink:https://en.wikipedia.org/wiki/Translation_%28geometry%29#Translations_in_physics [nonactive].

I hadn't thought of that. Yes in which case the ball could actually increase in velocity at the cost of a lose of rotational energy. The ball would have to be spinning in the correct direction though. (We don't deal with spinning balls impacting at A level). My original statement requires a qualifier.
« Last Edit: 15/06/2012 13:43:59 by survivalist13 »

#### RD

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##### Re: Does a golf ball speed up when it bounces out of the hole?
« Reply #4 on: 15/06/2012 13:56:42 »
apparently they can spin at over 7,500rpm ...

http://www.golfwrx.com/forums
« Last Edit: 15/06/2012 14:08:28 by RD »

#### survivalist13

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##### Re: Does a golf ball speed up when it bounces out of the hole?
« Reply #5 on: 15/06/2012 14:27:09 »
Presumably that's the spin off a drive (correct terminology?) and the OP is presumably referring to putting I think. In which case the spin will mostly be due to rolling along the ground and shouldn't play that much of a roll I'm thinking. However 8000rpm that's some spin. I wouldn't have thought you'd in part that much rotational motion to the ball when you hit it. If you have an equation for rotational energy you should be able to work out the potential speed increase from a impact, might be interesting :)

#### RD

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##### Re: Does a golf ball speed up when it bounces out of the hole?
« Reply #6 on: 15/06/2012 17:19:42 »
If you have an equation for rotational energy you should be able to work out the potential speed increase from a impact, might be interesting :)

Golf balls aren't uniform, so the above will be an approximation.

feature=related
« Last Edit: 15/06/2012 17:40:26 by RD »

#### survivalist13

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##### Re: Does a golf ball speed up when it bounces out of the hole?
« Reply #7 on: 15/06/2012 18:31:06 »
Ok for the sake of curiosity a golf ball is 0.045kg has a radius of 0.021m so moment of inertia (which is the bit of this I don't know about) = 7.94E-6
If it's spinning at 7000rpm that's 733 rad /s
KE due to spinning is therefore 2.05J
So if the golf ball under went a perfectly inelastic collision where 50% of the rotational energy was converted into translational energy the velocity of the golf ball would be, 6.7m/s or 24km/hour a not insignificant speed.

And before you know it you've revised for your mechanics exams :) and I keep righting gold ball :/

#### CliffordK

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##### Re: Does a golf ball speed up when it bounces out of the hole?
« Reply #8 on: 16/06/2012 02:36:49 »
Wouldn't the spin be somewhat dependent on whether one is putting, or T-ing?  Sorry I'm not too up on my golf terms.  But, I assume with a put, the ball is primarily rolling, so the spin would be dependent entirely on the velocity of the roll.

Whereas if the ball has been whacked  through the air, then it may be spinning quite a bit.

#### RD

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##### Re: Does a golf ball speed up when it bounces out of the hole?
« Reply #9 on: 16/06/2012 13:58:45 »
From the putter's perspective the angular velocity of a ball lipping the hole could increase,
but its linear velocity is constantly decreasing, (if on a flat surface).
« Last Edit: 16/06/2012 14:12:19 by RD »

#### Frazer

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##### Re: Does a golf ball speed up when it bounces out of the hole?
« Reply #10 on: 18/06/2012 09:59:49 »
Thanks for all the replies, very interesting. Just to clarify though, the driver spin thing is a red herring really.

I was talking about putting - you putt towards the hole and the ball catches the hole, yet stays out. If you see a slow mo of this (it's called a 'lip out'), the ball falls part way into the hole (sometimes even below ground level) but then comes up and out again.

With a drive, the ball can indeed spin up to 7 or 8,000 revs, but with a putt, the ball will be rolling end over end and hardly spinning at all.

So, by my logic, the ball will speed up as it falls down partway inside the hole, but will slow down as it comes up again. As friction has been applied to the ball, and no new energy added, it surely must end up going  slower that if it had not hit the hole, therefore must end up nearer to the hole?

The only exception I can think of is if the hole was on a slow and the ball got 'thrown' onto the downslope, in which case it may go further.

But can any of you clever scientist type guys prove this unequivocally and slightly more scientifically? If so, I win a bet (although I can't remember how much it was for!)

#### survivalist13

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##### Re: Does a golf ball speed up when it bounces out of the hole?
« Reply #11 on: 18/06/2012 12:24:24 »
Can you find a video of what you specifically mean, googling lip out comes up with a range of different scenarios. However in the of the day energy is conserved, and after you have hit a gold ball the total energy (kinetic plus gravitational potential) will only ever decrease due to friction. So I think you should win this bet. However for a specific proof of the case you'd need to know more precisely what's actually happening.
Assuming the ball is putted along the ground on a level green the only change in potential energy will be when it falls down the hole. The shape of the hole is also relevant. Assuming there is no back or front spin the rotational energy shouldn't really factor into it as it should be linked to the translational energy. There will always be friction due to the grass, air and in the case of a collision the compression and expansion of the two objects.

#### Frazer

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##### Re: Does a golf ball speed up when it bounces out of the hole?
« Reply #12 on: 22/06/2012 06:57:35 »
I'm not sure if I can find a video, but there will be loads on YouTube. I'm not sure if further description helps, but the scenario is this:

The player putts the ball towards the hole
The ball goes towards the very edge (rather than the middle of the hole)
The ball deviates into the hole, but doesn't drop
Instead, it comes back out of the hole but has been moved off line

So for instance if the ball catches teh right edge of the hole, the curve of the hole will pull the ball to the left, and it will exit the hole at an angle to the left of where it originally started

I'll try and find a video

#### Frazer

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##### Re: Does a golf ball speed up when it bounces out of the hole?
« Reply #13 on: 22/06/2012 07:14:29 »
here is a picture:

The ball travels along the white line shown , then catches the right edge of the hole, and will then be deflected left (at say 15 degrees). Does the deflections speed the ball up or slow it down?

#### imatfaal

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##### Re: Does a golf ball speed up when it bounces out of the hole?
« Reply #14 on: 23/06/2012 16:03:49 »
here is a picture:

http://english.chosun.com/site/data/img_dir/2011/03/29/2011032900540_0.jpg

The ball travels along the white line shown , then catches the right edge of the hole, and will then be deflected left (at say 15 degrees). Does the deflections speed the ball up or slow it down?

It's rolling when it hits and rolling when it leaves - there is no stored energy to be converted(unlike for instance the spin on a billiards ball), it will be slowed down by the process.  It might give the appearance of "leaving" faster because the section when it is falling into the hole will speed it up momentarily before it is slowed by climbing back out

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##### Re: Does a golf ball speed up when it bounces out of the hole?
« Reply #14 on: 23/06/2012 16:03:49 »