OCR · A-Level · Physics
Course version: H556 · Version from Sept 2015
(b) period and frequency of an object in circular motion
Full specification wording
Kinematics of circular motion: (b) period and frequency of an object in circular motion
For an object moving in a circular path, the period T is the time taken to complete one full revolution, measured in seconds (s).
Checked against the OCR H556 specification (opens in a new tab) on .
- Point 2 of 3
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
For an object moving in a circular path, the period T is the time taken to complete one full revolution, measured in seconds (s). The frequency f is the number of complete revolutions per second, measured in hertz (Hz), where 1 Hz = 1 s⁻¹. Period and frequency are reciprocals: T = 1/f and f = 1/T. For example, a satellite completing an orbit in T = 120 s has f = 1/120 ≈ 8.3 × 10⁻³ Hz. If a wheel spins at f = 5.0 Hz, each revolution takes T = 1/5.0 = 0.20 s. Period and frequency describe the timing of the motion. They are related to speed v and radius r by the equation v = 2πr/T; therefore, knowing only the period or frequency does not uniquely determine the radius or speed, as a larger radius requires a proportionally greater speed to maintain the same period.
What earns the marks
- Period T is the time for one complete revolution, measured in seconds (s).
- Frequency f is the number of complete revolutions per second, measured in hertz (Hz), where 1 Hz = 1 s⁻¹.
- Period and frequency are reciprocals: T = 1/f and f = 1/T.
- A larger period corresponds to a smaller frequency, and vice versa.
- Period and frequency alone do not determine radius or speed, as they are related by v = 2πr/T.
Common mistakes and how to put them right
Confusing period with frequency.
Put it right: T is time per revolution in seconds while f is revolutions per second in hertz.
Inverting the reciprocal the wrong way.
Put it right: T = 1/f, so a frequency of 4.0 Hz gives T = 0.25 s.
Using degrees or radians per second for frequency.
Put it right: Frequency is measured in hertz (s⁻¹), while angular velocity in rad s⁻¹ is a different quantity.
Assuming period is independent of speed and radius.
Put it right: For a fixed period, speed must increase proportionally with radius.
Exam tips
- Check the unit: if the answer is in seconds it is a period; if in hertz or s⁻¹ it is a frequency.
- When a question gives revolutions per minute, convert to hertz by dividing by 60 before using T = 1/f.
- Sketch one full revolution and label the start and end of one period to avoid counting part-revolutions as whole ones.
Try the lesson
The start of the Kinematics of circular motion: (b) period and frequency of an object in circular motion lesson, made from checked teaching notes for this specification point.
Listen
For an object moving in a circular path, the period T is the time taken to complete one full revolution, measured in seconds (s). The frequency f is the number of complete revolutions per second, measured in hertz (Hz), where 1 Hz = 1 s⁻¹. Period and frequency are reciprocals: T = 1/f and f = 1/T. For example, a satellite completing an orbit in T = 120 s has f = 1/120 ≈ 8.3 × 10⁻³ Hz. If a wheel spins at f = 5.0 Hz, each revolution takes T = 1/5.0 = 0.20 s. Period and frequency describe the timing of the motion. They are related to speed v and radius r by the equation v = 2πr/T; therefore, knowing only the period or frequency does not uniquely determine the radius or speed, as a larger radius requires a proportionally greater speed to maintain the same period. A strong answer earns marks for these points. Period T is the time for one complete revolution, measured in seconds (s). Frequency f is the number of complete revolutions per second, measured in hertz (Hz), where 1 Hz = 1 s⁻¹. Period and frequency are reciprocals: T = 1/f and f = 1/T. A larger period corresponds to a smaller frequency, and vice versa. A common slip: Confusing period with frequency: the error is treating T and f as the same quantity. Instead, T is time per revolution in seconds while f is revolutions per second in hertz. Exam tip: Check the unit: if the answer is in seconds it is a period; if in hertz or s⁻¹ it is a frequency.
See it
Key words
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Period
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Frequency
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Reciprocals
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Relationship between timing quantities, speed and radius
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See every definition
- Period
- The time taken to complete one full revolution, measured in seconds (s).
- Frequency
- The number of complete revolutions per second, measured in hertz (Hz), where 1 Hz = 1 s⁻¹.
- Reciprocals
- Period and frequency are reciprocals: T = 1/f and f = 1/T, so a larger period corresponds to a smaller frequency, and vice versa.
- Relationship between timing quantities, speed and radius
- Period and frequency are related to speed v and radius r by the equation v = 2πr/T, so knowing only the period or frequency does not uniquely determine the radius or speed.
Label the diagram
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1Box 1
2Box 2
3Box 3
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