# Can a golf ball gain speed when it goes round the hole and doesn't drop?

1. Sep 5, 2010

### julesnash

When a golf ball goes round the hole and doesn't drop (bit like an orbit), it often appears to leave the hole faster than it went in. My understanding of physics tell me that this is not possible, as any speed gained from gravity when it dips slightly into the hole, is lost when it gains height on its' way out of the hole. Can anyone confirm this theory in physics terms? (I'm thinking Newtons Laws maybe?)

Thanks for you help.

2. Sep 5, 2010

### pallidin

Good question. I've seen that too.
I'll leave it to the experts to explain both to you and I.

3. Sep 5, 2010

### Pengwuino

You have it right. I'm a golfer so I think I know why it appears to come out faster. It's probably a matter of perspective. When you hit the ball and it's getting closer to the hole, the angular speed (that is, how quickly the ball is moving across your field of vision as opposed to how fast it's actually going) seems to be pretty low. That's because you're typically right behind the ball or very nearly right behind it. When it flies out, it probably will leave in a direction quite different than what you're looking at giving it a higher angular speed even though that doesn't mean it has a higher total speed.

When I watch someone ELSE putt and they have a ball fall in and out, I'm usually watching from a different area than the person putting and the ball will basically come out with the same speed as it went in.

4. Sep 5, 2010

### julesnash

Good point Penguino. I think that is probably the right explanation.

One of my friends has just suggested one possible way a ball could actually gain speed - if the ball had a lot of sideways spin, it could covert the rotational energy into straight line speed as it strikes the edge of the hole. But as the ball wouldn't have much (if any) sideways spin when rolling across a green, I don't accept this as a valid explanation.

I'm still interested if anyone can explain it in raw physics terms. (I'm trying to explain to my brother that the ball may momentarily accelerate as it dips into the hole due to gravity, but would decelerate by exactly the same amount as it rises again on the way out, and therefore the net results could not equal increased speed).

5. Sep 5, 2010

### pallidin

Or, is this a terrestrial variation of the much loved "sling-shot" effect utilized in some space exploration maneuvers?

So, I guess the question stands... Does the golf ball exit the circumstance with a higher velocity than it entered into?

6. Sep 5, 2010

### Hurkyl

Staff Emeritus
Sure, I suppose it's possible to sling-shot to gain speed if you can find a rapidly moving hole in the ground.

7. Sep 5, 2010

8. Sep 5, 2010

### pallidin

My gut feeling is that this accelerative effect WOULD work IF the plane of exit is LOWER than the plane of entry. "Gravity assisted acceleration" as it were.
I don't perceive this happening with a golf ball entering a hole with "level" ground immediately surrounding the hole.
However, if the exit surface is physically lower than the entry surface, I could see that happening.

Just some thoughts...

9. Sep 5, 2010

### Pengwuino

Ah yes, I've played one of those par 8 holes before. Tricky.

10. Sep 6, 2010

### julesnash

Thanks to everyone for your input. I have summarised my findings follows:

Due to Newton's first law, no object can gain speed unless energy is put into it. The hole gives no energy to the golf ball - it is a passive object, sitting still doing nothing. Therefore, a golf ball would not normally gain speed when it "lips out" (see exemptions below).

The best explanation of why the ball "appears" to accelerate is because of the perspective that you are looking at the ball. If you are the player, after you strike the ball, it is moving directly away from you and so appears not to be moving very fast at all (same as a star moving away from us at light years of speed but appears not to be moving - this is called "angular speed" and in this case is zero. Note that "angular speed" is not the same as "actual speed". Angular speed is about how fast something moves across your vision). When the balls comes off the hole at 90 degrees, it suddenly has a lot of angular speed (from the ball striker's perspective) but no more actual speed. The human eyes and brain confuse angular speed with actual speed.

However, there are few other interesting points.

There are two ways that a ball could actually gain speed:

1. If the hole was angled on a slope, and the ball left the side of the hole that was lower to the the side it entered. Gravity would then have acted on the ball as a force, and due to Newton's first Law, would have gained speed. However, the angle of a golf hole is normally so shallow, that this effect would have no noticeable difference on the speed of the ball. So it is highly unlikely that this effect would happen more the once every million shots.

2. If the ball had a lot of sideways spin (this is called rotational energy), and it hit the cup in the right direction, it could convert the rotational energy into kinetic energy (i.e. gain speed). A bit like in tennis, a heavy topspin shot kicks off the court - exactly this effect. However, a golf ball is very unlikely to have any sideways spin off the face of a putter, and having rolled along the grass which would have stopped all sideways spin by the time it hits the hole. So, again, it is highly unlikely that this effect would happen more the once every million shots.

About spaceships using planets to sling shot, this is different to the golf ball scenario. Spaceships do use the slingshot effect to gain speed, but the reason this is possible is because the planets are moving at massive speeds through the solar system relative to each other. When the rocket accelerates, it actually slows the planets orbit (very very fractionally). So the planet is more like a man's arm throwing a hammer in athletics - it puts energy into the hammer, thus accelerating it. If the planets in the solar system were completely static, there would be no sling-shotting of rockets around the solar system.

11. Sep 6, 2010

### pallidin

Well, I think you may have cracked it.
A right-handed, non-professional putt, tends to put a clock-wise spin on the golf ball. Not always, but tended.

In any event, that right-hand spin would have an effect when hitting the circular cup on the right-hand side, causing a small acceleration for the reasons you stated.

12. Sep 8, 2010

### misterb

I think it's a matter of perspective and spin. The golf pros are often practising giving the ball the right spin and spin can cause an acceleration. Actually a very easy but valid explanation. I have been golfing several times and know how difficult it is to give the ball the right spin, that needs a lot of training and experience.

13. Sep 8, 2010

### stewartcs

Wait...the holes move on a golf course?! That explains a lot about my score...LOL!

CS