There are all kinds of balls used in sports, with different sizes, shapes, and designs,
depending on the sport’s needs and history.
But there’s something especially weird about golf balls: their dimples.
There are more than 300 dimples on a standard golf ball
and they’re there for more than just aesthetics.
Dimpled golf balls can fly almost twice as far as they would if they were smooth.
In the mid-19th century, players noticed
that old-scratched up golf balls flew farther than new, polished spheres.
所以 他们为了让高尔夫球飞得更远 就在球上刻满凹痕
So they started carving grooves into their golf balls to make them fly better.
By 1930, after years of trial and error testing
the golf ball had evolved into the familiar dimpled shape we know today.
It might seem kind of strange that a rough surface would make golf balls fly better,
and at the time, they didn’t know why it worked.
They just knew that it did.
But these days, we know the dimples work because of a trade-off
即 增大一种空气阻力 导致另一种空气阻力减小
increasing one type of air-resistance leads to a huge decrease in another.
There are two main kinds of air resistance: surface friction and pressure drag.
Surface friction works a lot like friction between solid objects — air rubs against
the sides of an object, slowing it down.
一般来说 物体的表面积越大 其表面摩擦也越大
Generally, the more surface area something has, the more surface friction it creates,
so giving a golf ball a rough surface does increase surface friction.
But it also affects the second kind of air-resistance: pressure drag.
球飞起来时 会把空气推开 并在其背面形成尾迹
As the ball flies, it pushes air out of the way, creating a wake
a cone-shaped pocket of low pressure behind the ball
which sucks it backward and slows it down.
You can experiment with wakes if you paddle your hand through water at different angles.
If you lead with your palm, you’ll see a big triangular wake behind your hand, and
you’ll notice that it becomes much more difficult to move your hand through the water at higher speeds.
But if you turn your hand sideways, the wake becomes much smaller, and your hand passes
through the water much more easily.
In a golf ball
dimples reduce the size of the wake by creating a layer of what’s called turbulent flow.
The uneven surface messes up the smooth path of air around the ball, making it much harder
for the wake to form into a clean cone.
With its smaller wake, a dimpled golf ball doesn’t get sucked backward as much, so
it flies farther.
So, golf balls have dimples to mix up the air around the ball causing the wake to collapse
more quickly, which allows the ball to fly faster for longer.
Of course, culture is a big part of the equation too.
受这一作用的启发 网球变成了这样毛绒绒的如果有凹痕的话 橄榄球也会做的更好
Tennisballs get this effect by being fuzzy and Footballs would do better with dimples,
but have to settle for laces.
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