Workshops: Thrill Ride: Roller Derby
To roll a ball down a track so it comes to a stop at a predetermined point, and to analyze energy transformations experienced on a slide.
Yours truly, are designing a slide attraction for a traveling carnival. For a slide to be safe, the rider must slow to a stop at the end of the ride.
You cannot take any chances, so you build a scale model of the slide. Carnival officials want to see your design work successfully before they give you their approval to build the full-scale attraction. They are also interested in the energy transformations that occur. The longer the slide, the more the carnival officials will like it!
For each group:
- Strips of poster board, flexible white tag board, or other material to make a track
- Support materials (books, blocks, pieces of wood)
- A few different balls (such as a marble, table-tennis ball, and golf ball)
- Small cup
- Blank paper
- Use poster board or other materials to build a troughlike track. Connect several sections together with tape. Be sure the tape does not interfere with a rolling ball.
- Set up the track so the starting point is at least 50 centimeters above the floor. Mark off in centimeters the section of track where you will start the ball rolling. Lay track elements end-to-end and carefully tape the sections together.
- Position the track so the last segment of track runs uphill and the finish is a few centimeters above the floor. Place a cup under the end of the track.
- Between the start and finish, the track can carry the ball downhill, uphill, or along a level path. The track can even curve, but somewhere along the way, it must touch the floor.
- NOTE: Since this is a traveling carnival ride, the slide must come apart in several sections for transportation and storage. The longer your track, the more impressed the carnival officials will be.
- Hold the ball somewhere on the upper part of the incline, and not the starting point. Let the ball roll down the track toward the finish line. If the ball is going too fast, it will shoot off the end of the track and miss the cup. If it is going too slowly, the ball will not make it to the end of the track.
- Each time you roll the ball, record the starting position on the incline and note where the ball ends up. Keep adjusting until the ball falls into the cup at least twice in a row. Record the exact location where successful rolls began. Measure the height of this pint above the floor. Also measure the height of the end of the track above the cup.
- Try balls of different weights or sizes. Keep an organized data table.
- Now it is time to demonstrate your attraction to the carnival officials. During your demonstration, you cannot touch or move either the ball or the slide. If it works, you are on your way! If it does not work, go back to the drawing board. If you have difficulty meeting this standard, others in the class will give suggestions as to how to fine-tune your model. Then you can make modifications and try again.
Since the carnival officials are looking at designs from several manufacturers, they want a written record of your design.
Include in the report a sketch of a side view of your slide. Mark the approximate starting location of the ball. Label where the ball's velocity is increasing, where it is decreasing, and where it stays about the same.
Whenever the velocity of an object changes, a force must be acting on the object. On the sketch, draw arrows to show the direction of the force or forces acting on the ball in different sections along the track. Use different colors of arrows for the different forces (such as black for gravity, red for friction, and green for centripetal).
These questions will help you analyze changes in the ball's energy during the ride.
- Before you let the ball go down the incline, what was the ball's kinetic energy? (Kinetic energy is the energy of motion..)
When the ball reached the finish line, it was hardly moving. At that pint, what was the ball's kinetic energy (approximately)?
- What is the height of the starting point (Hs) above the floor? What is the height of the finish line (Hf) above the floor?
- What is the height of the starting point above the finish line (Hs - Hf = the vertical drop (Dv) of the slide.)
- Where is the potential energy of the ball greater, the start or the finish?
- If the kinetic energy of the ball is about the same at the start and finish, the loss of potential energy is due to friction. To find out the percentage of potential energy lost to friction, use this formula:
Percent Lost to Friction = 100 X
(or Percent lost to friction is equal to 100 times the vertical drop divided by the height of the starting point.)
- If you covered the track surface with strips of sandpaper, how do you think the sandpaper would affect the ball? Would you have to change the starting position of the ball?
- If you kept your slide as it is, but used a heavier ball, how would that affect the starting position of the ball?
- Would it be possible to design a slide with a hill in the middle higher than the starting position of the ball? Explain your answer.
- Small models of systems in physical science do not perfectly represent the real thing. In what ways is your model not similar to an actual slide at a carnival? Generate a list with your group, then write the ideas on a sheet of paper. Compare your ideas with those of other groups and expand your list.