As promised last time, we will be having a short midterm after class today. But, before the test I will be presenting a quick lecture.

Pay attention, you may see a couple questions on the midterm from today’s lesson.

We always hear about spin of a golf ball. Top, back, and side spin all play a factor. Today, I’m going to be nice though. Since we have a midterm we will only look at the physics of the backspin of a golf ball.

Let’s begin by examining why golfers wish to have their balls spin backwards. I’m not much of a golfer myself, so I’ve asked NIU women’s golf coach Pam Tyska for an answer to this question.

“Spin is needed not only to stop the ball, but also to carry it through the air for a long way,” Tyska said.

For today, Let’s focus on how the backspin of the ball helps to stop the movement of the ball as it hits the ground.

To begin, we first need to understand friction. Friction is a force that resists the movement of an object. It is determined by the substance the object is moving across and the force the surface puts back onto the object.

Remember from our past lectures that a force is something that influences the motion of an object — like pushing or pulling.

Think of it like this: If you’re pushed while sitting on a frozen pond, you slide along the ice. Now, if you’re pushed while sitting on the ground, you would most likely not budge, and may even be pushed over.

This is because you were on two different substances (the ice and the ground) and every substance has its own coefficient of friction. Some coefficients are very low, like ice, and some are higher, like the ground.

The backspin all begins when the golfer first strikes the ball.

“At impact, the grooves of the club face compress tremendously against the golf ball,” Tyska said. “The friction caused by this contact as the ball slightly slides up the club face produces spin in a microsecond of impact.”

When the spinning ball strikes the ground it will encounter two types of friction: A rolling friction and a slipping friction.

The more backspin the ball has, the higher the amount of rolling friction there will be. This higher amount of rolling friction causes the ball decrease the speed in the forward direction. This decrease in forward movement will also decrease the effects of sliding friction.

“The basic principle is the rotational energy will be taken away from the translational [non-rotating] energy,” NIU associate physics professor Larry Lurio said. “And, the ball won’t roll as far.”

Thus, if the ball is spinning fast enough, it can land in a situation where rolling friction balances the sliding friction so that the ball will come to a stop when it hits the ground. Even weirder, if the ball has enough spin it will move backwards instead of forwards.

I want to leave you plenty of time for that midterm, so let’s end lecture and begin. Please put your notes away and calculators should not be need, but are allowed.