Key Concepts
Physics
Angles
Energy
Friction

Introduction
Have you ever played mini golf? It might seem like just a fun game, but there's a lot of science behind it! Learn about the physics behind a rolling, bouncing mini golf ball—and design your own mini golf course in this fun science activity.

Background
In most mini golf courses there is not a direct path from where you start the ball to the hole. You often have to get the ball around, over or under one or more obstacles. Your goal is to get the ball into the hole by hitting it as few times as possible—which can be difficult with all that stuff in the way. One way to accomplish this is to bounce the ball off walls or other obstacles. By carefully planning out the ball's path you could even get a hole-in-one!

If you watch the ball carefully when it hits a wall, you will probably notice that the angle at which the ball hits the wall (known as the angle of incidence) is nearly identical to the angle at which the ball bounces back off of the wall (known as the angle of reflection). There might be a slight change, due to the ball's spin, but if you bounce the ball off a wall at a certain angle, it should rebound at roughly the same angle. This allows you to get the ball around obstacles. You will also notice that eventually your ball slows down and stops. The kinetic energy of the ball as it rolls is gradually converted to heat due to friction with the ground.

Now that you know a little more about the physics behind mini golf you're prepared to design your own holes—complete with fun obstacles!

Materials

• Paper
• Scissors
• Pencil
• Ruler
• Tape
• Large table or counter
• Small rubber bouncy ball
• Assorted sturdy, small-but-heavy household items, such as cans of food, a metal stapler, a roll of duct tape, and so on. The objects should be heavy enough that the ball can bounce off them without moving them.
• Larger ball, such as a tennis ball (optional)
• Larger sturdy objects (optional)

Preparation

• Cut two circles out of paper. Label one "start" and one "finish." The smaller you make the "finish" circle, the more difficult the game will be.

Procedure

• Make a few sketches of different mini golf hole designs before you start building. Draw the start location, finish location and any obstacles you want to put in between them. Try to include at least one obstacle that blocks the direct path from start to finish.
• Using what you learned about angles of incidence and reflection in the background section, is it still possible to get a hole-in-one in your design by bouncing the ball off obstacles or walls? Use the ruler to draw a series of straight lines representing the ball's path from start to finish. How many times does the ball have to bounce to make it to the finish location?
• Now build one of your mini golf holes by assembling the obstacles and start/finish circles, and try it out! Put the ball on the start circle, and roll it or flick it with your finger. Can you get a hole-in-one by following the path you drew on paper?
• If you don't get a hole-in-one, keep rolling the ball until you get it to the "finish" spot. Then go back to the start and try again. Can you get a hole-in-one if you keep practicing? How closely does the actual path match your predicted path?
• Repeat the process for some of your other designs. Are some of them easier than others?
• Extra: Get a few friends to play, and keep score!
• Extra: Try a larger version of the game on the floor with a tennis ball or larger rubber ball. Use heavier objects, such as sturdy furniture or plastic bins filled with toys for your obstacles and walls.
• Extra: Instead of rolling the ball with your hand, build your own golf club. See the link in the "More to Explore" section below for instructions.

Observations and Results
You might have found that getting a hole-in-one was more difficult than it looked. On paper it might be pretty easy to draw some shapes and then draw a line representing the ball's path from start to finish. But in practice you have to worry about friction and a spinning ball. Your ball doesn't rebound perfectly off the obstacles—it slows down a bit and its spin can change, changing its direction. If you tried to design a very complex hole that required multiple bounces to get from start to finish in one shot, this might have proved very challenging in real life! With practice, however, you could probably get a hole-in-one on a hole that only required one or two bounces. Real golf course designers have to take these same principles into account. They don't want the course design to be too easy—hole-in-ones wouldn't be very exciting if they happened all the time. Conversely, they don't want the game to be too difficult and frustrating for players, or it wouldn't be any fun.

More to Explore
Make Your Own Sports Equipment, from Scientific American
We Swear There's a Reason to Model This Ball Bouncing Off a Wall, from Wired
Rolling Race, from Scientific American
STEM Activities for Kids, from Science Buddies

This activity brought to you in partnership with Science Buddies