Motion and time are closely related. To describe motion we need to know how far an object has moved and how long it took. This chapter introduces standard units of time and distance, the concept of speed, and different types of motion.
Key Concepts and Definitions
Motion is the change in position of an object with respect to time and a reference point. If an object does not change its position, it is said to be at rest.
Distance is the total length of the path travelled by an object. The SI unit of distance is the metre (m).
Time is measured in seconds (s). Larger units include minutes, hours, and days.
Speed is the distance travelled per unit time.
Formula: Speed = Distance / Time
SI unit of speed: metres per second (m/s) or kilometres per hour (km/h).
Uniform motion: An object moves equal distances in equal intervals of time.
Non-uniform motion: An object moves unequal distances in equal intervals of time.
Simple pendulum: A bob (small heavy ball) suspended from a fixed point by a thread. It swings back and forth. One complete to-and-fro movement is called an oscillation. The time taken for one oscillation is the time period.
Odometer: Instrument in vehicles to measure distance travelled.
Speedometer: Instrument in vehicles to measure speed at a given instant.
Worked Examples
A car travels 150 km in 3 hours. What is its speed?
Speed = Distance / Time = 150 km / 3 h = 50 km/h
A cyclist travels at 12 m/s for 5 seconds. How far does she travel?
Distance = Speed x Time = 12 m/s x 5 s = 60 m
A train covers 360 km at a speed of 90 km/h. How long does the journey take?
Time = Distance / Speed = 360 km / 90 km/h = 4 hours
Convert 72 km/h into m/s.
72 km/h = 72 x 1000 m / 3600 s = 72000 / 3600 = 20 m/s
Rule: to convert km/h to m/s, multiply by 5/18.
A pendulum completes 30 oscillations in 60 seconds. What is its time period?
Time period = Total time / Number of oscillations = 60 s / 30 = 2 s per oscillation
A ball rolls 4 m in the first second, 4 m in the second second, and 4 m in the third second. Is this uniform or non-uniform motion?
The ball covers equal distances (4 m) in equal time intervals (1 s each), so this is uniform motion.
A bus travels 20 km in the first 30 min and 10 km in the next 30 min. Find the average speed for the full journey.
Total distance = 20 + 10 = 30 km; Total time = 30 + 30 = 60 min = 1 h
Average speed = 30 km / 1 h = 30 km/h
- Key Formulas
- Speed = Distance / Time
- Distance = Speed x Time
- Time = Distance / Speed
- Conversion: km/h to m/s → multiply by 5/18; m/s to km/h → multiply by 18/5
Common mistakes
Students often forget to use consistent units. Always convert all distances to the same unit (km or m) and all times to the same unit (h or s) before applying the formula. Also, average speed is total distance divided by total time, not the average of two speeds.
Summary
Motion is described using distance, time, and speed. Speed = Distance / Time. Uniform motion involves equal distances in equal times; non-uniform motion does not. The simple pendulum measures time using regular oscillations. Instruments like speedometers and odometers help measure speed and distance in vehicles.