# The Naked Scientists Forum

### Author Topic: How does spin on balls affect trajectory?  (Read 3704 times)

#### thedoc

• Forum Admin
• Administrator
• Sr. Member
• Posts: 511
• Thanked: 12 times
##### How does spin on balls affect trajectory?
« on: 22/07/2012 10:30:02 »
Ian Wright asked the Naked Scientists:

Hello Naked Scientists

I am Engineer in the aerospace industry and have been in the profession for 25 years now. As such I am completely comfortable with the science of aerodynamics including Newton's first three laws of motion, aerofoils and Bernoulli's principle. However there has always been one aerodynamically effect that I have never been able to get a satisfactory answer to, as it seems to turn the aerodynamic laws on their head.

When a tennis ball is top spun (or turn the same question through 90 degrees for a "bend it like Beckham" football free kick) why does the ball dip downwards as oppose to upwards' as Bernoulli's principle would seem to suggest.

When a tennis ball is top spun the ball is spinning about an axis perpendicular to the flow of air across it. The air travels (relatively) faster across the top surface of the ball which is rotating in the opposite direction as the airflow. This increase in velocity, according to Bernouilli's principle, should reduce the pressure on the top of the ball, relatively increase the pressure on the underside....making it curve upwards not down.

However as we know from any ball sport, and a thousand sliced golf drives, it is actually the opposite effect happens. Why?

What do you think?
« Last Edit: 22/07/2012 10:30:02 by _system »

#### graham.d

• Neilep Level Member
• Posts: 2208
##### Re: How does spin on balls affect trajectory?
« Reply #1 on: 23/07/2012 07:40:51 »
I think there are competing mechanisms that affect the rotating ball. What wins in this case is the air resistance slowing the rotation and the angular momentum being conserved by changing the balls trajectory. However, I have not done the maths on this. I am sure you must have access to some fairly sophisticated simulator packages that could show this. Also, a bullet's rotation, imparted by rifling the barrel, results in a spiralling trajectory as its rotation slows.

#### RD

• Neilep Level Member
• Posts: 8128
• Thanked: 53 times
##### Re: How does spin on balls affect trajectory?
« Reply #2 on: 23/07/2012 08:39:52 »
The rotation acts somewhat like an aerofoil, accelerating the airflow on one side of the ball reducing the air pressure, decelerating the airflow on the other side increasing air pressure, creating a lift force at a right angle to the ball's axis of rotation.

Quote
The curved path of a golf ball known as slice or hook is due largely to the ball's spinning motion (about its vertical axis) and the Magnus effect
https://en.wikipedia.org/wiki/Magnus_effect

« Last Edit: 23/07/2012 09:03:33 by RD »

#### The Naked Scientists Forum

##### Re: How does spin on balls affect trajectory?
« Reply #2 on: 23/07/2012 08:39:52 »

Login
Login with username, password and session length