Motion, Forces and Energy
Physical Quantities and Measurement
Cambridge IGCSE Physics
Measuring length and volume
- A length: a metre rule. A volume of liquid: a measuring cylinder.
- Read the scale with the eye level with the reading, the line of sight at right angles to the scale.
- Read a measuring cylinder to the bottom of the meniscus.
Measuring time
- Measure a time interval with a clock or a digital timer.
- Use a stopwatch to time oscillations.
- Start and stop the timing as the object passes the same fixed reference point.
Measuring multiples
- A short time or a small distance: measure many together, then divide by the number: multiple oscillations and division to give the period.
- Timing many oscillations makes timing errors due to reaction time less significant.
- A thickness or circumference: wind string round the object several times, measure the length of the string and divide by the number of turns.
Method: measuring the period of a pendulum
- Measure the length with a metre rule close to the bob, using a set square as a horizontal aid.
- Use a fiducial point at the centre of the swing, where the pendulum passes the vertical.
- Start counting from zero as the pendulum passes the fiducial point, and start the stopwatch.
- Time 20 complete oscillations.
- Divide the time by 20 to give the period T.
Scalars and vectors
A scalar has magnitude only. A vector has magnitude and direction.
| Scalars | Vectors |
|---|---|
| distance | force |
| speed | weight |
| time | velocity |
| mass | acceleration |
| energy | momentum |
| temperature | electric field strength |
| gravitational field strength |
Resultant of two vectors at right angles
- Draw the two vectors tail to head at right angles. The resultant runs from the start of the first to the end of the second: a correct vector triangle.
- Draw the resultant vector including arrow.
- Magnitude: Pythagoras' theorem on the two vectors.
- Direction: tan θ = opposite / adjacent, giving the angle with the horizontal, marked on the diagram.
- Give the resultant as a magnitude and a direction.
By scale drawing
- State the scale, chosen so that the triangle is large.
- Draw both vectors to scale, tail to head, at right angles.
- Draw the resultant with its arrow.
- Measure the length of the resultant and convert it with the scale.
- Measure the angle with a protractor.
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