Rotational Derby
Name__________________________________________ Period_______
Purpose: To observe how objects of various shapes, sizes, and masses roll down an incline and to
determine what affects their speed.
Directions: Race objects with the same shape but different radii and mass after predicting the
outcome. Race different shape objects after predicting the outcome. Obtain the objects to race
from the bins your teacher has prepared. Make sure you return your objects to the bin after each
race so other groups can use them. It is OK to do the races in a different order depending on what
objects are in the bin when you are ready for a new race.
Race 1: Obtain a billiard ball and a steel ball. Predict the winner below.
Predicted Winner
Hold the balls at the top using a meter stick. Have another student be ready to catch them at the
bottom. Quickly flick the meter stick forward to release them at the same time. Record the
winner below. You may need to repeat the race a few times to be certain of what happened.
Actual Winner
Q1: What can you conclude about the time it takes 2 balls of different mass and radius to roll
down an incline?
Race 2: Obtain two hollow cylinders of different radii. Predict the winner below.
Predicted Winner
Hold the cylinders at the top using a meter stick. Have another student be ready to catch them at
the bottom. Quickly flick the meter stick forward to release them at the same time. Record the
winner below. You may need to repeat the race a few times to be certain of what happened.
Actual Winner
Q2: What can you conclude about the time it takes 2 hollow cylinders of different mass and
radius to roll down an incline?
Race 2: Obtain two solid cylinders of different radii. Predict the winner below.
Predicted Winner
Hold the cylinders at the top using a meter stick. Have another student be ready to catch them at
the bottom. Quickly flick the meter stick forward to release them at the same time. Record the
winner below. You may need to repeat the race a few times to be certain of what happened.
Actual Winner
Q3: What can you conclude about the time it takes 2 solid cylinders of different mass and radius
to roll down an incline?
Q4: How can you explain the results of your races between objects with the same shape but
different radii and mass?
Race 4: Obtain a hollow cylinder and a solid cylinder. Predict the winner below.
Predicted Winner
Race them and record the winner below.
Actual Winner
Q5: What can you conclude about the time it takes a hollow cylinder and a solid cylinder to roll
down an incline?
Race 5: Obtain a ball and a solid cylinder. Predict the winner below.
Predicted Winner
Race them and record the winner below.
Actual Winner
Q6: What can you conclude about the time it takes a ball and a solid cylinder to roll down an
incline?
Race 6: Obtain a ball and a hollow cylinder. Predict the winner below.
Predicted Winner
Race them and record the winner below.
Actual Winner
Q7: What can you conclude about the time it takes a ball and a hollow cylinder to roll down an
incline?
Q7: How can you explain the results of your races between objects with different shapes?
Race 7: Obtain a sloshy and non-sloshy soup can. Predict the winner below.
Predicted Winner
Race them and record the winner below. Make sure they don’t hit the floor!
Actual Winner
Q8: How can you explain the results of race 7?
Weighted Rods: Hold the PVC rod with weight in the center and gently rotate it back and forth,
being careful not to hit anything. Repeat with the other rod that has the weight on the ends.
Q9: Which shape is the rod with weight in the center most similar to? Which shape is the rod
with weight on the ends most similar to? This might help you answer Q7.
Q10: Rank all objects (ball, hollow cylinder, solid cylinder, sloshy soup, non-sloshy soup) from
fastest to slowest below. You may need to do some more races to do this.
Q11: If you were to include a hollow ball in the races, which object(s) would it be faster than?
Which object(s) would it be slower than?
Conclusion: The resistance to changes in motion is called inertia. The resistance to changes in
rotational motion is called rotational inertia or moment of inertia. The symbol for rotational
inertia is I. Based on your results, if you had a ball, a hollow cylinder, and a solid cylinder of equal
mass and radius, which has the largest I? Which has the least I?