Question 5
A cyclist travels a distance of 1 km in the first hour, 0.5 km in the second hour and 0.3 km in the third hour. Find the average speed of the cyclist in:
(i) km h-1
(ii) m s-1.
(i) total distance travelled = 1 km + 0.5 km + 0.3 km = 1.8 km
total time taken = 1 h + 1 h + 1 h = 3 h
So, average speed of the cyclist = 0.6 km h-1.
(ii) total distance travelled = 1 km + 0.5 km + 0.3 km = 1.8 km
1 km = 1000 m
1.8 km = 1.8 x 1000 = 1800 m
total time taken = 1 h + 1 h + 1 h = 3 h
1 h = 3600 sec
3 h = 3 x 3600 = 10800 sec
So, average speed of the cyclist = 0.167 m s-1.
ICSE Class 7 Physics — Motion: Complete Study Guide
Motion is one of the most important chapters in ICSE Class 7 Physics, carrying approximately 14 marks. It builds foundational concepts of rest, motion, speed, velocity, and graphical representation that are essential for understanding mechanics in higher classes. This chapter bridges everyday observations (cars moving, fans spinning, pendulums swinging) with scientific analysis and mathematical treatment.
Students begin by understanding that motion is relative — a passenger on a moving train is at rest relative to the train but in motion relative to the ground. The chapter covers seven types of motion: rectilinear, curvilinear, rotatory, oscillatory, circular, periodic, and random. Each type is explored with relatable real-world examples that help cement understanding.
The core quantitative concept is the speed formula (Speed = Distance / Time) and the ability to rearrange it, convert between km/h and m/s, and apply it to numerical problems. The distinction between speed (scalar) and velocity (vector), and between distance (total path) and displacement (shortest path with direction), introduces students to the important scalar-vector classification. Distance-time graphs provide a visual approach to understanding motion patterns — a skill that becomes increasingly important in higher physics.
| Formula | Details |
|---|---|
| Speed = Distance / Time | SI unit: m/s |
| Distance = Speed x Time | Find distance when speed and time are known |
| Time = Distance / Speed | Find time when distance and speed are known |
| Average Speed = Total Distance / Total Time | For non-uniform motion |
| km/h to m/s: multiply by 5/18 | Speed conversion |
| Acceleration = Change in velocity / Time | SI unit: m/s² |
Must-Know Concepts
- Motion is relative — depends on the observer's frame of reference
- Distance is always positive; displacement can be zero (if returning to start)
- Speed is scalar (no direction); velocity is vector (has direction)
- Uniform motion: equal distances in equal time intervals. Non-uniform: unequal distances
- D-T graph: flat line = at rest; straight slope = uniform speed; curve = changing speed
- Average speed ≠ arithmetic mean of speeds — always use Total Distance / Total Time
Common Mistakes
- Calculating average speed as (speed1 + speed2)/2 — this is almost always wrong
- Confusing speed and velocity in definitions and answers
- Misidentifying the slope of a d-t graph as distance instead of speed
- Wrong km/h to m/s conversion direction (multiply by 5/18, not 18/5)
Exam Tips
- Use the formula triangle (D/S/T) to quickly rearrange the speed formula
- In graph questions, always describe the motion type before calculating speed
- Label axes and scale clearly when plotting d-t graphs
- For types of motion, always pair each type with at least one example
What is the difference between speed and velocity?
Speed is the rate of change of distance and has only magnitude (scalar). Velocity is the rate of change of displacement and has both magnitude and direction (vector). A car moving at 60 km/h has a speed of 60 km/h. If we specify "60 km/h northward," that is velocity.
Can displacement be zero even if distance is not zero?
Yes. If an object moves along a path and returns to its starting point, the total distance covered is the length of the path, but the displacement is zero because the final position is the same as the initial position.
What does a horizontal line on a distance-time graph mean?
A horizontal line on a distance-time graph means the object is stationary (at rest). The distance is not changing with time, so the speed is zero during that interval.
Why is motion called relative?
Motion is relative because whether an object is at rest or in motion depends on the observer. A person in a moving bus is at rest with respect to the bus but in motion with respect to a person standing on the road. There is no absolute rest or absolute motion.