Your roommate is working on his bicycle and has the bike upside down. He spins the 60-cm-diameter wheel, and you notice that a pebble stuck in the tread goes by three times every second. What are the pebble's speed and acceleration?
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Rotational Velocity & Acceleration
Problem 75
Textbook Question
A painted tooth on a spinning gear has angular position θ = (6.0 rad/s⁴)t⁴. What is the tooth's angular acceleration at the end of 10 revolutions?

1
Step 1: Understand the given angular position equation θ = (6.0 rad/s⁴)t⁴. Here, θ is the angular position as a function of time t, and the coefficient 6.0 rad/s⁴ represents the proportionality constant. Angular acceleration is the second derivative of angular position with respect to time.
Step 2: Differentiate θ with respect to time t to find the angular velocity ω. Using the formula ω = dθ/dt, calculate the first derivative of θ = (6.0 rad/s⁴)t⁴. This gives ω = 4(6.0 rad/s⁴)t³ = 24.0t³ rad/s.
Step 3: Differentiate ω with respect to time t to find the angular acceleration α. Using the formula α = dω/dt, calculate the second derivative of θ. This gives α = d(24.0t³)/dt = 72.0t² rad/s².
Step 4: Determine the time t corresponding to 10 revolutions. One revolution corresponds to an angular displacement of 2π radians. Therefore, 10 revolutions correspond to an angular displacement of θ = 10 × 2π = 20π radians. Solve for t in the equation θ = (6.0 rad/s⁴)t⁴ by substituting θ = 20π. This gives 20π = (6.0 rad/s⁴)t⁴. Solve for t.
Step 5: Substitute the value of t obtained in Step 4 into the angular acceleration formula α = 72.0t² rad/s² to find the angular acceleration at the end of 10 revolutions.

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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Angular Position
Angular position describes the orientation of a rotating object in terms of an angle, typically measured in radians. In this question, the angular position θ is given as a function of time, indicating how it changes as the gear spins. Understanding this concept is crucial for determining how the gear's rotation evolves over time.
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Angular Velocity
Angular velocity is the rate of change of angular position with respect to time, usually expressed in radians per second (rad/s). It can be derived from the angular position function by taking its first derivative. In this case, knowing the angular velocity at a specific time will help in calculating the angular acceleration.
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Intro to Angular Momentum
Angular Acceleration
Angular acceleration is the rate of change of angular velocity with respect to time, measured in radians per second squared (rad/s²). It can be found by taking the derivative of the angular velocity. In this problem, calculating the angular acceleration at the end of 10 revolutions requires understanding how the angular position function relates to both angular velocity and acceleration.
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Conservation of Angular Momentum
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