Energy in Collisions & Compressions

Professional golfer Mike Miller's drive is analyzed and used to explain the physics concepts of collisions and compressions. "Science of Golf" is produced in partnership with the United States Golf Association and Chevron.

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DAN HICKS, reporting: It's one of the most explosive hits in all of sports:  The collision between a driver and a golf ball that produces more than three quarters of a ton of force.

MIKE MILLER (Professional Golfer): There's definitely times when you know you hit it perfect, especially when it just feels great.

HICKS: Mike Miller is a professional golfer who won the Wilson Cup invitational in 2013, and is a two-time Metropolitan Golf Association player of the year.

MILLER: I find it that I’m able to drive the ball relatively straight and long, just because of the confidence that I have with my driver.

HICKS: Miller's drive is a good way to illustrate two key physics concepts: collisions and compressions.  A collision happens when two objects come into contact with one another, in this case, when the club head strikes the ball.

Compression occurs in the 500 microseconds that the driver is in contact with the ball, an impact that causes the ball and the club head to compress, especially the ball, that deforms like a coiled spring.

MATTHEW PRINGLE, (Equipment Standards, USGA): When the driver hits the golf ball there could be two or three thousand pounds of force between the two. When you put three thousand pounds of force on a golf ball, it can only do one thing and that's deform.

HICKS: Unlike kinetic energy, the energy of movement, potential energy is the energy an object possesses due to its position or configuration. The ball's compression produces elastic potential energy because of the way it compresses just before springing back.

PRINGLE: The golf shot is very dependent on the storage of energy in the golf ball during that impact.

HICKS: During impact, the ball deforms and stores some of the kinetic energy of the club as elastic potential energy in the ball. Because the ball's elastic energy is converted back into kinetic energy, the ball flies off the club.

STEVE QUINTAVALLA (Equipment Standards, USGA): Golf balls tend to lose energy in the process.  And in general they lose about forty percent of the energy that you put into them.

HICKS: Since it takes fractions of a second for the ball to deform and come back to its former shape after being hit, the USGA uses a machine called the universal tester that can put varying amounts of force on a golf ball to help examine how it compresses when it's struck.

QUINTAVALLA: This machine will, in a controlled way, load the golf ball up to 2000 lbs. and then let it restore to its original shape.

HICKS: To find out how much energy is transferred between two objects during a collision, scientists can calculate the Coefficient of Restitution, or COR.

This is done by measuring the difference in the speeds of the two objects after their collision and dividing the difference between the two speeds of the objects before their collision.  The answer will be expressed on a scale between zero, meaning the minimum of energy was transferred, and 1.0, meaning all the energy was transferred.

QUINTAVALLA: If two objects approach each other at one hundred meters per second and they collide and then they separate at eighty meters per second, then the coefficient of restitution would be eighty divided by a hundred, or 0.8.

HICKS: If a golf club can store more energy in the club head during a collision, less energy is lost, creating greater ball speed. Manufacturers work hard to develop club heads with greater flexibility.

HICKS: To ensure the game does not rely only on technology opposed to a player's abilities, the USGA puts limits on the performance of golf equipment.

PRINGLE: This is the pendulum test, it's got one purpose, and that's to find out how flexible the club head is.

HICKS: In the pendulum test, a steel ball attached to a hammer swings into the club face. The acceleration of the steel ball into the club face is measured, as well as how long the impact between the ball and the club face lasts.

PRINGLE: If it was hitting something rigid it would bounce off very quickly and if it was hitting something flexible, it would kind of catch it and release it quite slowly. And we've got some nice electronics that can measure that down to less than a millionth of a second, measure that timing and we've put a limit on that time, and that limits how flexible the club head can be.

HICKS: Even with the limits placed on technology, today's golf equipment still provides plenty of ways to improve a player's game.

MILLER: You gotta go out there with the attitude of it's a new day, and it's a new experience. That's why it's golf and that's why it's phenomenal.

HICKS: With the right practice and skill, golfers like Mike Miller can use a club to put compressions and collisions to their advantage, leading to success on the course.

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