Measurement of Length and Motion: 26 flashcards
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1. What is measurement of length fundamentally?
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A comparison of an unknown length against a fixed standard unit.
2. What is the SI base unit of length?
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The metre ($m$).
3. How is the metre defined today?
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As the distance light travels in vacuum in $\frac{1}{299,792,458}$ of a second.
4. Convert: $1\text{ km}$ = ___ $\text{m}$
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$1\text{ km} = 1000\text{ m}$
5. Convert: $1\text{ m}$ = ___ $\text{cm}$
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$1\text{ m} = 100\text{ cm}$
6. Convert: $1\text{ cm}$ = ___ $\text{mm}$
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$1\text{ cm} = 10\text{ mm}$
7. What is parallax error?
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An error that occurs when the eye is not directly above the reading mark on the scale.
8. Where should the object's end be aligned when using a ruler?
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With the zero mark of the ruler, not the ruler's edge.
9. What is the least count of an instrument?
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The smallest measurement the instrument can reliably read.
10. How is measurement uncertainty written?
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As $\pm$ the least count, e.g. $12.4 \pm 0.1\text{ cm}$.
11. Are rest and motion absolute?
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No — they are relative and depend on the chosen reference point.
12. What is rectilinear motion?
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Motion along a straight line.
13. What is circular motion?
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Motion along a circular path.
14. What is periodic motion?
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Motion that repeats at fixed time intervals.
15. What is random motion?
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Motion with no predictable path, e.g. dust particles in a sunbeam.
16. What is distance?
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The total path length travelled; it is a scalar quantity.
17. What is displacement?
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The straight-line change in position from start to finish; it is a vector.
18. How do distance and displacement compare?
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Distance $\geq$ displacement; displacement can be zero for a round trip.
19. Define speed.
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Speed $= \frac{\text{distance}}{\text{time}}$; it is a scalar.
20. Define velocity.
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Velocity $= \frac{\text{displacement}}{\text{time}}$; it is a vector.
21. What is average velocity for a round trip?
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Zero, because net displacement is zero — but average speed is not zero.
22. How is the second defined?
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By 9,192,631,770 oscillations of a caesium-133 atom.
23. How do you convert km/h to m/s?
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Multiply by $\frac{5}{18}$.
24. Why must units be consistent before using a formula?
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Mixing units (e.g. km with seconds) gives a meaningless number.
25. What does GPS combine to find your position?
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Length and time measurement, by timing radio signals from satellites.
26. Why is time measurement really motion measurement?
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Clocks measure a regularly repeating motion, such as a pendulum swing or crystal vibration.
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