The Race of Two Balls (Physics Experiment for the Classroom)

This project was done for an intro to engineering class I am taking at National Park College taught by Professor Post. We were tasked with building anything of our choice and documenting the project and posting an Instructable on the project at its conclusion. I chose a physics demonstration from our book that Professor Post had stated that he wished he had a physical version of.

The demonstration involves to two balls with identical size and mass racing on parallel tracks. The difference is that the track in the foreground will descend lower than the track in the background and then re-ascend back to being parrel with the background track at the end of the race.

The experiment is intended to be a thought-provoking exercise for first year physics students involving velocity and acceleration. The students are supposed to guess which ball will reach the end first or if they will get to the end at the same time. The ball on the track in the forefront (the one that descends twice and ascends at the end of the race) will reach the end first. Even though it loses a lot of velocity at the end of the race it spends a greater amount of time at a greater speed.

This project went through an enormous amount of trial and error to perfect. Many different methods using different materials (as listed below) were attempted but did not work until the final iteration.

Included below are only the supplies used in the final build

2x4 wooden boards (Around 26 ft)

x2 1" Precision Chrome Steel Ball Bearings (G25) produced by PGN (Ordered off of Amazon)

White Paint

2 1/2in wood screws

1/2in wood screws

x2 PS 72-in Pilaster Strips (shelving rail available for purchase for $5.18ea at Lowes)

This build was made entirely made of 2x4s and wood screws. At this junction in the project the rail was made by cutting grooves into the longer side of the 2x4s with a series of cuts down the length of the board with a table saw. A groove was then carved out with a chisel and hammer. Pool balls (donated by the owner of Doc Penny's pool hall off of central in hot springs) were used as the balls in the experiment.

I ran some preliminary tests to determine what the best design for the groove would be. I had two main types of grooves (Beveled and Straight-cut). In the tests When the balls traveled down boards with the same length and at the same angle, The ball would reach the end in 1.3 sec when on the straight-groove rail and 1.8 sec when on the beveled groove rail. There is a relationship between the amount of surface area in contact of the board with the ball, and the straight-cut groove has less surface area in contact with the ball and allowed the ball to travel faster.

Some other tests I ran involved multiple straight-cut boards and I found that there was a correlation between how wide the track is and how fast the ball moves. The narrower the track the faster the ball travels. The trade-off being that the narrower the track becomes the ball loses stability and has a higher likelihood of jumping the track.

The issue with this build was its sharp corners where the change in angle happened. The pool balls would hit the corner hard and lose momentum. Because of this the balls would struggle to make it to the end of the track. It became clear the change in direction of the track would need to be more gradual. I elected to make the rail out of different materials and only use the wood only for support

The 2x4s in this build only worked as the supports for the rail. The rail for this iteration was made from 3/4inx3/4in Inside corner wall trim. The wall trim was flexible so that the ball could change direction more gradually. The issue with this design was that the wall trim could not bend enough for it to go down, level out, bend down again, level out again and then go up. The entire rail would have needed to be almost 14ft long in order for this design to work, which would be too long to conveniently be stored in a classroom.

I tried cutting the bottom of the wall trim and melting it with a lighter. This did allow the wall trim to bend more than normal, but it was still not enough to produce desirable results.

This Build was the prettiest, but sadly it did not work. I finally found rail that was flexible enough to suit my need. I used some aluminum shelving rail that I bent to shape. I cut some plywood sheets into specific curved shapes that I screwed to a long, wide plank. I then bent my rail to fit over my plywood shapes and secured the rail down with 1/2in screws through the small slits in the rail.

I also added a ramp to the opposite side of the build, which made it symmetrical. The purpose of this was so that the experiment could be attempted from either side and so that the ball would not roll of the side and onto the floor.

The issue with this design was simply that the angles were too extreme. The ball would build up too much speed and would jump the ramp at the apex of the incline. The next design was essentially the same just with less steep angles.

If you wish to build this demonstration yourself, here is where you should begin.

We need to cut our 2x4 planks

Supports

1. x4 11 1/8in (30-degree angle one side)
2. x2 26 7/8in (30-degree angle one side)
3. x1 72in (90 degrees on both sides)

Vertical Braces

1. x2 6in (90 degrees on both sides)
2. x2 5in (90 degrees on both sides)
3. x2 3in (90 degrees on both sides)

Base

1. x2 74in (90 degrees on both sides)
2. x3 7 1/2in (90 degrees on both sides)

Sand down the angled side of the plank so that the rail will run more smoothly over it.

First, we need to assemble the Base. Take the two 74in long boards and run them parrel to one another with the "4in" side facing down. Then take your three 7 1/2in boards and set them on top of the 74in boards perpendicular. Put one of you 7 1/2in boards at either end, then put the third board in the middle. Then use your 2 1/2in wood screws to attach them all. Your base is now assembled.

The rest of the piece can be assembled according to the blue print posted above.

From here you sand the entire piece until it is relatively smooth and paint the entire piece. You can use wood putty to fill in any unsightly holes or cover your screws.

You need to mark the midpoint of your rail and the midpoint the path on the wooden supports that the rail screws into. From here you line up both midpoints and slowly press down on the aluminum rail to bend it into shape, working your way out from the middle (this is easier to do with the help of a second person). You will need to do this twice (once for each rail). After the rail is bent to shape You can use 1/2in wood screws to attach the rail by screwing the rail down through the small slits on the interior of the rail. You need to insure that the screws are in deep enough so that the ball does not impact them when running over them.

Congratulations you are now finished!!!

You can use just about any ball for this experiment as long as they have consistent sizes and masses, but some are more forgiving than others. Pool balls work well but because of their high center of mass compared to the narrow rail, they have a tendency to jump the track if the track has any imperfections or if it's not quite level. For this reason I elected to use 1in Steel ball bearings which I have never had jump the track.