5. Calculate /* for a partly hollow ball with inner radius a and outer radius r. Express your answer in terms of b = a/r. What happens as a → 0 and as a → r?

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Chapter11: Angular Momentum
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APPLIED PROJECT
a
h
ROLLER DERBY
Suppose that a solid ball (a marble), a hollow ball (a squash ball), a solid cylinder (a steel bar),
and a hollow cylinder (a lead pipe) roll down a slope. Which of these objects reaches the bottom
first? (Make a guess before proceeding.)
To answer this question, we consider a ball or cylinder with mass m, radius r, and moment of
inertia 7 (about the axis of rotation). If the vertical drop is h, then the potential energy at the top is
mgh. Suppose the object reaches the bottom with velocity v and angular velocity w, so v = wr.
The kinetic energy at the bottom consists of two parts: mv² from translation (moving down the
slope) and Iw² from rotation. If we assume that energy loss from rolling friction is negligible,
then conservation of energy gives
mgh = mv² + Iw²
1. Show that
2gh
₂₂² =
1 + 1*
dy
dt
where I* =
2. If y(t) is the vertical distance traveled at time t, then the same reasoning as used in
Problem 1 shows that v² = 2gy/(1+ I*) at any time t. Use this result to show that y
satisfies the differential equation
T =
2g
1 + 1*
I
mr²
-(sin a) √y
where a is the angle of inclination of the plane.
3. By solving the differential equation in Problem 2, show that the total travel time is
2h(1 + 1*)
g sin²a
This shows that the object with the smallest value of I* wins the race.
4. Show that I* = for a solid cylinder and I* = 1 for a hollow cylinder.
5. Calculate /* for a partly hollow ball with inner radius a and outer radius r. Express your
answer in terms of b = a/r. What happens as a → 0 and as a → r?
6. Show that I* = for a solid ball and I* = for a hollow ball. Thus the objects finish in the
following order:solid ball, solid cylinder, hollow ball, hollow cylinder.
Transcribed Image Text:APPLIED PROJECT a h ROLLER DERBY Suppose that a solid ball (a marble), a hollow ball (a squash ball), a solid cylinder (a steel bar), and a hollow cylinder (a lead pipe) roll down a slope. Which of these objects reaches the bottom first? (Make a guess before proceeding.) To answer this question, we consider a ball or cylinder with mass m, radius r, and moment of inertia 7 (about the axis of rotation). If the vertical drop is h, then the potential energy at the top is mgh. Suppose the object reaches the bottom with velocity v and angular velocity w, so v = wr. The kinetic energy at the bottom consists of two parts: mv² from translation (moving down the slope) and Iw² from rotation. If we assume that energy loss from rolling friction is negligible, then conservation of energy gives mgh = mv² + Iw² 1. Show that 2gh ₂₂² = 1 + 1* dy dt where I* = 2. If y(t) is the vertical distance traveled at time t, then the same reasoning as used in Problem 1 shows that v² = 2gy/(1+ I*) at any time t. Use this result to show that y satisfies the differential equation T = 2g 1 + 1* I mr² -(sin a) √y where a is the angle of inclination of the plane. 3. By solving the differential equation in Problem 2, show that the total travel time is 2h(1 + 1*) g sin²a This shows that the object with the smallest value of I* wins the race. 4. Show that I* = for a solid cylinder and I* = 1 for a hollow cylinder. 5. Calculate /* for a partly hollow ball with inner radius a and outer radius r. Express your answer in terms of b = a/r. What happens as a → 0 and as a → r? 6. Show that I* = for a solid ball and I* = for a hollow ball. Thus the objects finish in the following order:solid ball, solid cylinder, hollow ball, hollow cylinder.
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