Question

a boy runs along a straight horizontal track. the graph shows how his speed v varies with time t. after 15 s the boy has run 50 m. what is his instantaneous speed and his average speed when t = 15 s?

instantaneous speed / m s⁻¹	average speed / m s⁻¹
b	3.3	5.0
c	5.0	3.3
d	5.0	5.0

Explanation:

Step1: Find instantaneous speed

Read the speed - time graph. At $t = 15\ s$, the value of speed $v$ on the graph is $5.0\ m/s$. So the instantaneous speed at $t=15\ s$ is $5.0\ m/s$.

Step2: Calculate average speed

The formula for average speed is $\bar{v}=\frac{\text{total distance}}{\text{total time}}$. Given that the total distance $d = 50\ m$ and the total time $t = 15\ s$. Then $\bar{v}=\frac{50}{15}\approx3.3\ m/s$.

Answer:

C. Instantaneous speed: $5.0\ m/s$, Average speed: $3.3\ m/s$