Converting rotational speed to angular velocity If a body rotates at N revolutions per minute (rpm), what is its angular velocity ω in radians per second?
Mechanical Engineering
Engineering Mechanics
Difficulty: Easy
Choose an option
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AπN/60
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BπN/180
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C2πN/60
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D2πN/180
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EN/2π
Answer
Correct Answer: 2πN/60
Explanation
Introduction / Context:Angular velocity is commonly required in radians per second for dynamics, power, and torque calculations. This question checks the fundamental conversion from rpm to rad/s.
Given Data / Assumptions:
- N is given in revolutions per minute (rpm).
- 1 revolution corresponds to 2π radians.
- 1 minute equals 60 seconds.
Concept / Approach:Use unit conversion: multiply by 2π radians per revolution and divide by 60 seconds per minute to convert rpm to rad/s.
Step-by-Step Solution:
Start with N rev/min.Convert revolutions to radians: N * 2π rad/min.Convert minutes to seconds: (N * 2π) / 60 rad/s.Therefore, ω = 2πN / 60 rad/s.Verification / Alternative check:For N = 60 rpm, ω should be 2π rad/s (one revolution per second). Substituting N = 60 gives ω = 2π, which matches.
Why Other Options Are Wrong:
- πN/60 or πN/180: Miss the factor of 2 for a full revolution (2π).
- 2πN/180: Divides by 180 instead of 60, underestimating by a factor of 3.
- N/2π: Inverts the correct relationship and has wrong units.
Common Pitfalls:Forgetting that 1 rev = 2π rad, or failing to convert minutes to seconds.
Final Answer:2πN/60