Ch 1 · Physical Quantities, Measurement and Motion · IGCSE Physics 0625
Topic 1 of 5 in Motion, Forces and Energy — IGCSE Physics 0625 — 20 lessons.
Introduction
Every part of IGCSE Physics starts with a measurement: a length on a ruler, a reading on a stopwatch, a volume in a measuring cylinder. This lesson covers how to take those measurements carefully, how to tell scalar quantities from vector quantities, and how to describe motion with speed, velocity and acceleration. Motion graphs appear on almost every Paper 3 and Paper 4, so reading gradients and areas quickly and accurately is worth real marks.
Key ideas
1. Measuring length, volume and time. Use a ruler or tape for lengths (read at eye level to avoid parallax), a measuring cylinder for liquid volumes (read the bottom of the meniscus) and a stopwatch or digital timer for time intervals.
🧑🏫 Teacher's explanation
Imagine you are tracking a sprinter. To get an accurate time, you wouldn't just time one stride; you'd time ten strides and divide by ten to cancel out your reaction time error. This is the principle of 'averaging over multiples'.
📖 Definitions
🔍 Worked example — Easy
A student measures the time for 50 oscillations of a pendulum and finds it takes 120 seconds. Calculate the period of one oscillation.
🔍 Worked example — Standard
A boat travels 8.0 km due North and then 6.0 km due East. Calculate the magnitude of the resultant displacement.
🔍 Worked example — Exam-style
A car accelerates from 5.0 m/s to 25 m/s in 4.0 s. Calculate the acceleration.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Calculate the acceleration of the car. [2] (b) Calculate the distance travelled by the car during this 12 s interval. [2] [4 marks]
Practical notes
Worked example
Example 1 — reading a speed-time graph. Use the graph above. A cart accelerates from rest to 6.0 m/s in 3.0 s, travels at constant speed until t = 8.0 s, then decelerates uniformly to rest at t = 10.0 s.
🔍 Worked example — Easy
A cyclist accelerates from 0 m/s to 8.0 m/s in 4.0 s. Calculate the acceleration.
🔍 Worked example — Standard
A car travels at a constant speed of 15 m/s for 20 s, then decelerates uniformly to a stop in 5 s. Calculate the total distance travelled.
🔍 Worked example — Exam-style
A plane flies North at 200 km/h and then turns to fly East at 150 km/h. Calculate the magnitude of the resultant velocity.
⚠️ Common errors
Common mistakes and examiner tips
Practice
Answers: (1) s = 2400\ m, t = 8.0 × 60 = 480\ s, so v = 2400/480 = 5.0\ m/s. (2) T = 31.2/20 = 1.56\ s; timing many swings reduces the effect of reaction time on the value for one swing. (3) a = 30 - 12/6.0 = 3.0\ m/s^2. (4) Area of triangle = tfrac12 × 40 × 24 = 480\ m. (5) A straight sloping line, then a horizontal line (resting), then a straight line with a smaller gradient than the first. (6) v = √(60^2 + 11^2) = √(3721) = 61\ m/s; tanθ = 11/60, θ = 10^° east of north. (7) At first only weight acts, so she accelerates at 9.8\ m/s^2. As her speed increases, air resistance increases, so the resultant (downward) force decreases and so does her acceleration. When air resistance equals weight, the resultant force is zero and she falls at constant (terminal) velocity.