Objects around us are moving all the time. Cars move on the road, birds fly in the sky, and the hands of a clock move around the dial. When an object changes its position with respect to time and its surroundings, it is said to be in motion. In this chapter we study the different types of motion, how to measure time, and how to calculate the speed of a moving object.
Some objects move very fast while others move very slowly, and sometimes we need to compare the speed of two objects. To do this accurately, we need to measure both the distance travelled and the time taken. Scientists have developed standard units for measuring distance and time, and these units help us describe motion precisely.
Motion can be of different types depending on how the object moves. Linear or rectilinear motion is the motion of an object along a straight line, such as a car moving on a straight road or a train running on a straight track. Circular motion is the motion of an object along a circular path, such as the hands of a clock, the rotation of a wheel, or a stone tied to a string and whirled around. Periodic motion is motion that repeats itself after a fixed interval of time, such as the swinging of a pendulum, the revolution of the Earth around the sun, and the movement of the hands of a clock.
Some objects show a combination of motions. For example, when a car moves along a straight road, its wheels perform circular motion while the car itself performs linear motion. Similarly, the Earth rotates on its axis (circular motion) and also revolves around the sun (circular motion along an orbit). Understanding these types of motion helps us describe the movement of objects accurately.
Time is measured in seconds, minutes and hours. The basic unit of time is the second, which is denoted by 's'. The bigger units are minute (min), which equals 60 seconds, and hour (h), which equals 60 minutes. Historically, people measured time using natural events like the rising and setting of the sun, the phases of the moon and the changing seasons. Later, devices like the sundial and water clocks were used.
In 1656, Christiaan Huygens made the first pendulum clock. A simple pendulum consists of a small metallic ball called the bob suspended from a fixed point by a thread. When the bob is pulled aside and released, it swings to and fro. The time taken by the pendulum to complete one oscillation (one complete to-and-fro movement) is called its time period. The time period of a pendulum depends on its length, and the swinging of a pendulum is periodic motion. Modern clocks and watches, including digital watches and stopwatches, measure time accurately. A stopwatch is used to measure very short time intervals, like in races.
The distance travelled by a body in unit time is called speed. Speed tells us how fast or slow an object is moving. If an object travels a distance 'd' in time 't', then its speed is calculated as distance divided by time. The formula for speed is given below.
The SI unit of speed is metre per second (m/s), which is the speed of an object moving one metre in one second. Speed can also be expressed in kilometres per hour (km/h). To convert km/h into m/s, we multiply by 5/18, and to convert m/s into km/h, we multiply by 18/5. If a car travels 60 kilometres in 1 hour, its speed is 60 km/h. If a train covers 240 km in 3 hours, its speed is 240/3 = 80 km/h. Speed is an important concept used in transport, sports and many daily activities.
We often need to compare the speeds of different objects, and for this we must use the same unit. The SI unit of distance is metre (m) and the SI unit of time is second (s), so the SI unit of speed is m/s. In practice, km/h is also commonly used, especially for vehicles. To convert a speed given in km/h into m/s, we use the conversion factor 5/18; for example, 90 km/h = 90 × 5/18 = 25 m/s.
Different objects move at different speeds. A person walking moves at about 1.5 m/s, a running bicycle may move at 5 m/s, and a car may move at 25 m/s. Fast-moving objects like aeroplanes and trains have very high speeds. By measuring distance and time, we can calculate the speed and understand how fast an object is moving. Speed also helps us estimate travel time; if the distance and speed are known, the time taken can be found as distance divided by speed.
| Type of Motion | Path | Example |
|---|---|---|
| Linear (rectilinear) | Straight line | Car on a straight road |
| Circular | Circular path | Hands of a clock, wheel |
| Periodic | Repeats after fixed interval | Pendulum, Earth around sun |
| Unit | Conversion |
|---|---|
| 1 hour | 60 minutes |
| 1 minute | 60 seconds |
| 1 km | 1000 m |
| km/h → m/s | Multiply by 5/18 |
| m/s → km/h | Multiply by 18/5 |
Motion and time are fundamental concepts in science. Objects show linear, circular or periodic motion, and time is measured using devices that range from the ancient sundial to modern stopwatches and pendulum clocks. The swinging of a pendulum is a classic example of periodic motion, and its time period depends on its length. Speed, defined as the distance travelled per unit time, allows us to compare how fast different objects move, and its SI unit is m/s. By mastering these concepts and their units, we can describe motion in the world around us accurately.