# Physical Quantities, Units and Measurement

### Questions Worth Pondering

1. Why do we need to invent units?
2. What makes a quantity a base quantity?
3. When do we express our answer/quantity in standard form?
4. What is the difference between precise and accurate?
5. What is the difference between systematic and random error?
6. When measuring the magnitude of a quantity (when there are multiple instruments available), how do we decide which instrument to use?
7. When a pendulum is oscillating, if we change the mass of the bob (still same size), will the period change? And if we perform the experiment on a different planet, will the results differ?

### Exam Tips

Examiner likes to ask questions that deal with the following:

1. Ability to give a good approximation of the magnitude and unit of a body (e.g length of a pen/chopstick, diameter of a football or length of soccer field).
2. Ability to read off from vernier calipers and micrometer screw gauge with zero errors. Actual reading = main scale reading + or – zero error.
3. know the order in operating and how to operate the measuring instruments, especially vernier calipers and micrometer screw gauge. E.g we use the ratchet (instead of the thimble) to tighten when the anvil, object to be measured and the spindle are in contact because we want to avoid over tightening which spoil the instrument and or the object and also results in inaccurate reading.
4. Watch out when reading off negative zero error. Being negative, you will regress or move backward!
5. You need to know the range and precision of (tape measure, metre rule, vernier and micrometer) common instruments as some question will ask you to decide the best instruments to do the job.
6. Period of a pendulum when changing mass of the bob, length of the string and gravitational field strength. Remembering the formula for period of a pendulum can come in handy! T = 2πl/g. l is the length of the string, g is gravitational field strength and T is the period.
7. State the difference between vector and scalar.
8. Need to know how to draw a vector or scale diagram (a.k.a graphical method) to deduce the magnitude of resultant vector and the angle it makes. There are two methods to add vectors together: parallelogram or tip-to-tail. Parallelogram method needs exactly two vectors. Tip-to-tail can handle any number of vectors.