Starting from rest, a DVD steadily accelerates to 500 rpm in 1.0 s, rotates at this angular speed for 3.0 s, then steadily decelerates to a halt in 2.0 s. How many revolutions does it make?
Ch 04: Kinematics in Two Dimensions
Knight Calc5th EditionPhysics for Scientists and EngineersISBN: 9780137344796Non è quello che usi tu?Cambia libro di testo
Capitolo 4, Problema 35
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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Determine the radius of the wheel. The diameter is given as 60 cm, so the radius \( r \) is half of the diameter: \( r = \frac{60}{2} = 30 \, \text{cm} = 0.30 \, \text{m} \).
Calculate the angular velocity \( \omega \) of the wheel. The pebble completes 3 revolutions per second, so the angular velocity is \( \omega = 2 \pi \times 3 = 6 \pi \, \text{rad/s} \).
Find the pebble's linear speed \( v \). The relationship between linear speed and angular velocity is \( v = r \omega \). Substitute \( r = 0.30 \, \text{m} \) and \( \omega = 6 \pi \): \( v = 0.30 \times 6 \pi \).
Determine the pebble's centripetal acceleration \( a_c \). The formula for centripetal acceleration is \( a_c = \frac{v^2}{r} \). Substitute \( v = 0.30 \times 6 \pi \) and \( r = 0.30 \): \( a_c = \frac{(0.30 \times 6 \pi)^2}{0.30} \).
Simplify the expressions for \( v \) and \( a_c \) to find the pebble's speed and acceleration. Ensure the units are consistent and the results are expressed in \( \text{m/s} \) for speed and \( \text{m/s}^2 \) for acceleration.

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Linear Speed
Linear speed refers to the distance traveled by an object per unit of time. In this scenario, the pebble's speed can be calculated by determining the circumference of the bicycle wheel and multiplying it by the number of revolutions per second. The formula for linear speed (v) is v = distance/time, where the distance is the circumference of the wheel.
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Centripetal Acceleration
Centripetal acceleration is the acceleration experienced by an object moving in a circular path, directed towards the center of the circle. It can be calculated using the formula a_c = v²/r, where v is the linear speed and r is the radius of the circular path. This concept is crucial for understanding how the pebble accelerates as it moves along the wheel's circular trajectory.
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Angular Velocity
Angular velocity is a measure of how quickly an object rotates around a central point, expressed in radians per second. It is related to the number of revolutions per second and can be calculated using the formula ω = 2πf, where f is the frequency of rotation. In this case, knowing the angular velocity helps in determining both the linear speed and the centripetal acceleration of the pebble.
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