Physics of Golf Ball Flight: Direction and Spin

January 8, 2008

The initial ball flight direction is determined mainly by the clubface.

A note about this article: there is important and relevant information at the end of the page. If you are not interested in the physics, but you are interested in knowing how they apply to you, then scroll down to the last two paragraphs.

I am currently in the process of researching the physics of golf ball flight, which is an area that really interests me. One thing about me is that I am never ready to accept conventional wisdom without a great deal of scrutiny - this time, it turns out that conventional wisdom was wrong.

Most articles I’ve read talk about swing path being the number one factor in shot direction, while clubface position being the variable that determines spin. This is not entirely true.

Basically, the way striking a golf ball works is that for a short period of time, the ball slides up the clubface. As soon as this slide begins, friction kicks in (due mostly to the grooves) and catches the ball, so the ball starts to rotate. When it leaves the club, the ball has already started this rotation.

In a frictionless collision, the ball would travel in the same direction as the clubface.

It is a tested and proven law of physics that for a spherical object, the linear velocity after the object gains enough rotational velocity so that it enters its roll is 5/7 the original velocity.

The equation 5/7*velocity*sinθ is used to calculate the horizontal vector of the velocity of the ball spinning on the clubface. That 2/7 decrease in horizontal velocity equates into a 2/7 decrease in angle. That is, the angle between the direction of the ball and the direction of the swing path is 5/7 of the angle between the direction of the clubface and the direction of the swing path. Remember, if there was no friction between the clubface and the ball, the angle between the direction of the ball and the direction of the swing path would be the same as the angle between the direction of the the clubface and direction of the swing path.

If you are well versed in physics and want to give a shot at understanding how the number 5/7 comes about, take a look at this post from physicsforums.com.

So the real question is, how do you use this information? Well, it’s clear the the common advice of “working the ball by aiming your body where you want the ball to start and your clubface where you want the ball to end up” doesn’t exactly work.

Instead, you should be gauging how much the ball is going to move based on the clubface in relation to the swing path, but not necessarily the target line. For example, say you want to hit a fade that starts left of your target and cuts towards it. Basically, you will need to aim your clubface only slighty right of where you want the ball to start and swing along a path that is left of the clubface. This means your path will be more severely left than your clubface.

You can also use this information to judge your bad shots with better feedback. Don’t forget: the best way to do it is still with a square clubface swinging down the target line.

Written by John at 7:26 pm. Theory