MadSci Network: Physics

Re: Why is putting rotation on a basketball when you shoot good.

Date: Wed Jun 16 09:45:11 1999
Posted By: Tom Cull, Staff, Clinical Sciences MR Division, Picker International
Area of science: Physics
ID: 928271656.Ph

Hi Joe,
I looked around on the web for documentation but saw nothing worthwhile.  In fact, I found one site that had things completely backwards even though they were citing Physics of Basketball by Peter J. Brancazio in The Physics of Sports.

To get a feel for the effects from spin on a basketball bouncing off the floor or a backboard I recommend the following simple experiment that I have done for primary through junior high age students.  You need a basketball and a partner.  Bounce the basketball back and forth to each other.  Try putting different spin on the ball.  Notice that side spin tends to make the ball move in the direction it would go if it were rolling with that spin.  Topspin (forward spin) causes the ball to bounce away at a lower angle and if fast enough with more horizontal speed.   This effect comes from the ground "grabbing" the ball and providing a force in the forward direction.  Backspin causes the ball to bounce higher and softer because the "grab" force is directed back toward the thrower.   If you toss the ball with a lot of backspin it can actually bounce back to you.  This trick really impresses the younger kids who may not have seen it before.

I refer directly to the article Physics of Basketball by Peter J. Brancazio in The Physics of Sports.  Which was reprinted from
American Journal of Physics 49, 356-365 (1981).

The fundamentals of many sports are the accumulated wisdom of experience.  Shooting a basketball with backspin has proved to be a reliable method.   The aerodynamic forces that result from backspin of a basketball do not appreciably affect its flight.  The backspin is mostly a calibration for the shooter to produce and reproduce the same shot.  This is sometimes referred to by sports sciences and biosciences as muscle memory.  Muscle memory is always at work.  For example: a golfer gets his/her swing in a grove, or a pitcher learns to throw a slider over the inside part of the plate to a righthanded hitter, or everyday people know how to walk up and down stairs.  Of course muscle memory and human performance are not perfect and people make mistakes:  a golfer tops a ball, a pitcher hangs that slider out over the plate and the batter puts it over the fence, and occassionally people trip themselves up on the stairs. Muscle memory is part of basketball: some nights a player cannot miss and is bombing in three pointers from the suburbs, the next night the same player can't hit a layup.

With time, experimentation, and experience many sports techniques evolve.  For example, basketball before the late 1950's was a game of two hand set-shots and backboard banking.  Now basketball involves jumpers and hitting nothing but net.   A player who does not use a jump-shot nowadays will continually get rejected by opponents.  Backspin is a direct result of the common technique of jump shooting.  The common jump shot involves releasing the ball from about eye level or higher with the shooting arm bent at the elbow and the hand placed comfortably below the horizontal diameter of the ball.  The off hand is simply used as a guide for getting the ball into the shooting position (as well as to help keep defenders from slapping it away).   The release of the basketball is a complicated motion that involves at the very least shoulder rotation, elbow extension and wrist flection.

A ball shot with backspin will tend to be a softer shot with a higher arch than a ball that is pushed from the same position.
Red Auerbach was a firm believer in the control of the now standard shot because the fingertip shot gave the shooter more control, a softer shot, and luckier bounces.  An imperfect softer shot will deflect less when it hits the rim and is more likely to fall in than a harder/faster shot.  Occasionally one sees what is referred to as a "shooter's touch" or a "shooter's roll/bounce."  The "shooter bounce" is a shot that bounces off the rim and by some miracle (read collision physics) the falls in.  These types of bounces are the direct result of a soft (and admittedly lucky) touch.

The spin of the basketball is important when the ball strikes the backboard.   A ball with backspin hitting the backboard will lose considerable speed and will more softly approach the basket.   A fun experiment is to release a basketball with different sorts of spin at the backboard from right near the basket.  With the proper side spin a can literally look like it is crawling along the backboard into the basket.  If you put the opposite spin on the ball you can get a squeak and the ball will fall almost straight down to you.


Tom "Brick Master" Cull

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