Torque & Moment of Inertia

Professional golfer Belen Mozo's top-notch swing relies on two key physics concepts to hit the ball in a straight line, torque and moment of inertia. "Science of Golf" is produced in partnership with the United States Golf Association and Chevron.

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DAN HICKS reporting: The grip, the stance, the posture. Most golfers obsess over each part of their swing.

The slightest change in mechanics can often be the difference between reaching the green or landing in a hazard. 

BELEN MOZO (LPGA Pro Golfer): To be able to hit the ball straight with a square head through impact, that’s probably the main goal for all golfers. And it’s something that doesn’t come really easy.

HICKS: Professional golfer Belen Mozo has competed in more than 40 LPGA Tour tournaments since her rookie year in 2011. The secrets to her top-notch swing - and the swing of any golfer - are two key physics concepts: torque and moment of inertia.

Matt Pringle is the Technical Director for Equipment Standards at the United States Golf Association. He says torque is applied by the golfers body to the club and the club head itself.

MATT PRINGLE (United States Golf Association): Anytime you see something moving not in a straight line, then something must be applying a moment to that or a torque to that. The golfer’s body is applying a torque to their own body as well as the club and eventually to the club head.

HICKS: Torque, or force times distance, is a turning force that changes the rate of rotation in an object. During the golf swing, Mozo applies torque to the club with her hips and shoulders as the club moves towards the ball.

PRINGLE: We call it the torque or the moment that we’re applying to the club.

HICKS: Imagine a wrench tightening a bolt. The longer the wrench, the more torque it creates. Similarly, the more turning force Mozo applies to her swing with her body, the more force her swing will generate.

MOZO: You get the power from up to down, so starting with your feet up the legs and the glutes, and then – smash it.

HICKS: Beyond the torque she produces with her body, the impact between the club head and the ball also produces a torque. If the club head is off center on impact torque will twist the club face, making the ball fly at an angle instead of in a straight line. To control torque in the club head, Belen Mozo takes advantage of the second physics concept: moment of inertia.

PRINGLE: The more moment of inertia a body has the more difficult it is to twist it.

HICKS: Moment of inertia, sometimes called rotational inertia, is the resistance an object has to torque, like the long pole that tight rope walkers use to maintain balance.

PRINGLE: The very long pole creates a tremendous amount of moment of inertia for the tightrope walker. It’s very difficult to move something so long.

HICKS: In addition, a wider head on the golf club means that the mass is distributed more widely across its surface, making it easier to drive the ball in a straight line, even if the shot is slightly off center.

PRINGLE: If I apply the same moment to two bodies and one had a larger moment of inertia than another, the larger moment of inertia would twist less.

HICKS: A large moment of inertia can give golfers a big advantage by making it easier to hit a straight shot. As a result, the USGA has placed limits on the size of a golf club head so that ultimately the golfer, not the club, decides the outcome of the shot.

PRINGLE: We put caps and measures and rules on the performance of the club. We let the designers create the clubs that they want to.

HICKS: In every swing a golfer has to use the right amount of torque and rotational inertia in order to hit the ball in a straight line. Something that takes physical coordination and years of practice.

MOZO: Every day you have to wake up and keep practicing and competing to become as good as you can be.

HICKS: Precision, mechanics and physics are the essential tools Belen Mozo - and all golfers - need to line up a great shot and go for the flagstick.

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