Ad Banner Placeholder

Cambridge IGCSE Physics · 0625

Chapter 1: Motion, forces and energy — Part 1

Topic 1.1 · Physical quantities and measurement techniques

Measuring length, volume and time

Physical quantities are measured with the correct instrument, then recorded with a unit. You need to describe these techniques:

Length
Use a ruler or metre rule. Place the zero of the scale at one end of the object. Keep your eye perpendicular to the scale to avoid a parallax error.
Volume of a liquid
Use a measuring cylinder. Read the volume at the bottom of the meniscus with the eye level with the liquid surface.
Time intervals
Use a clock or digital timer (stopwatch) for events such as a trolley run, a falling object, or a pendulum swing.
Measuring a length with a metre rule (eye perpendicular to the scale to avoid parallax) and a volume with a measuring cylinder, reading the bottom of the meniscus.
Diagram 1: Measuring a length with a metre rule (eye perpendicular to the scale to avoid parallax) and a volume with a measuring cylinder, reading the bottom of the meniscus.

Averages for small distances and short times

A single small length or a single short interval is hard to measure accurately. Measure multiples, then divide.

Small distance
Measure several identical objects placed end to end (or several gaps) and divide the total length by the number of objects.
Short time / pendulum
Time a large number of complete oscillations of a pendulum. Period T = (total time) / (number of oscillations). Start and stop the timer at the same point in the swing.

A student times 20 complete oscillations of a pendulum. The total time is 16.0 s. Calculate the period T.

Worked example: period T equals 16.0 seconds divided by 20 oscillations, so T is 0.80 seconds
Time many swings from the same point in the motion, then divide. T = 0.80 s.
Finding a small time interval by timing many pendulum oscillations, then dividing total time by the number of oscillations.
Diagram 2: Finding a small time interval by timing many pendulum oscillations, then dividing total time by the number of oscillations.

Scalars and vectors

A scalar has magnitude (size) only. A vector has magnitude and direction.

ScalarsVectors
distance, speed, time, mass, energy, temperatureforce, weight, velocity, acceleration, momentum, electric field strength, gravitational field strength

Learn this list. Mixing speed with velocity, or mass with weight, is a common Paper 2 / Paper 4 error.

Resultant of two perpendicular vectors

If two forces or two velocities act at right angles, their resultant can be found by calculation or by a scale drawing.

Calculation
Use Pythagoras: resultant magnitude R = √(a2 + b2). The direction is the angle whose tan is (opposite component) / (adjacent component).
Graphically
Draw the two vectors tip-to-tail (or as a parallelogram). The resultant is the diagonal from the start of the first to the end of the second. Use a stated scale (e.g. 1 cm = 5 N).

This applies to forces or velocities only, and only when they are at right angles.

Two forces of 3.0 N and 4.0 N act at right angles. Calculate the magnitude of the resultant.

Worked example: resultant of 3.0 N and 4.0 N at right angles is 5.0 N by Pythagoras
R = √(3.0² + 4.0²) = 5.0 N. State a direction as well if the question asks for it.
Two force (or velocity) vectors at right angles drawn tip-to-tail, with the resultant as the hypotenuse of the right-angled triangle, plus a scale parallelogram of the same two vectors.
Diagram 3: Two force (or velocity) vectors at right angles drawn tip-to-tail, with the resultant as the hypotenuse of the right-angled triangle, plus a scale parallelogram of the same two vectors.

0/10

Ad Banner Placeholder