Question

A car is moving along a straight line $OP$ as shown in the figure. It moves from $O$ to $P$ in $18\, s$ and returns from $P$ to $Q$ in $6\, s$. Which of the following statements is not correct regarding the motion of the car ?

• A. The average speed of the car in going from $O$ to $P$ and come back to $Q$ is $20\, m \,s^{-1}$
• B. The average velocity of the car in going from $O$ to $P$ and come back to $Q$ is $10 \,m \,s^{-1}$
• C. The average speed of the car in going from $O$ to $P$ and come back to $O$ is $20 \,m \,s^{-1}$
• D. The average velocity of the car in going from $O$ to $P$ and come back to $O$ is $20 \,m \,s^{-1}$

Answer 1

Answer: (D)

From $O$ to $P$ and come back to $Q$
Average speed $=\frac{\text{Path length}}{\text{Time interval}}=\frac{OP+PQ}{\Delta t}$
$=\frac{\left(360+120\right)m}{24\,s}$
$=20\,m\,s^{-1}$
Average velocity $=\frac{\text{Displacement}}{\text{Time interval}}=\frac{240\,m}{24\,s}$
$=10\,m\,s^{-1}$
From $O$ to $P$ and come back to $O$
Average speed $=\frac{360\,m+360\,m}{36\,s}=20\,m\,s^{-1}$
Average velocity $=\frac{0\,m}{36\,s}=$ zero