Question

The velocity$-$time plot for a particle moving on a straight line is shown in the figure:

Calculate the average speed of the particle in the time interval $10-20\text{s}$.

• A. $20\text{m/s}$
• B. $-20\text{m/s}$
• C. $10\text{m/s}$
• D. $-10\text{m/s}$

Answer 1

The correct option is C $10\text{m/s}$

Given:
To calculate the average speed between $10-20\text{s}$, we need to first make sure the acceleration of the body is constant.
Since it is a continuous straight-line from $0-20\text{s}$, the acceleration of the body is indeed constant and is equal to $\frac{-20-10}{20}=-1.5{\text{m/s}}^2$

At $t=10\text{s}$, the magnitude of the velocity = speed = $0\text{m/s}$

At $t=20\text{s}$, the magnitude of the velocity = speed = $20\text{m/s}$

The average speed is $\frac{0+20}{2}=10\text{m/s}$
[This method of obtaining the average speed works only when the acceleration of the body is constant in the given time interval.]