On the school sports ground, a student walks along a straight 200 m track. He covers every 20 m section in exactly the same time, from start to finish. On the same ground, a 400 m race was held late in the afternoon. During the race, another student starts very fast, then slows down in the middle, and finally sprints again near the finish line. The coach notes the times for both students.

Answer the following :

(i) Which student shows uniform motion? Explain.

(ii) If one student covers 100 m in 20 s, calculate his speed.

(iii) What kind of motion does the student who runs in the race show? Give a reason.

(iv) How is average speed different from speed at a particular moment?

(i) The student walking along the 200 m track shows uniform motion because he covers equal distances (every 20 m) in equal intervals of time. Therefore, his speed remains constant throughout.

(ii) Given,

• Distance covered by the student = 100 m
• Time taken = 20 s

Speed of an object is given by,

$\text {Speed} = \dfrac {\text {Distance covered}}{\text {Time taken}} \\[1em] = \dfrac{100}{20} \\[1em] = 5 \text { m s}^{-1}$

So, the speed of the student is 5 m s^-1.

(iii) The student in the race shows non-uniform motion because his speed changes—he starts fast, slows down and then speeds up again.

Hence, he does not cover equal distances in equal intervals of time.

(iv) Average speed is the total distance travelled divided by the total time taken for the entire journey. Speed at a particular moment (instantaneous speed) is the speed of the object at a specific instant of time.