Question

An object travels at $50{\text{m s}}^{-1}$ for the first $2$ seconds and travels at $70{\text{m s}}^{-1}$ for the next $2$ seconds. What is its average speed for the first $4$ seconds?

• A. $70{\text{m s}}^{-1}$
• B. $50{\text{m s}}^{-1}$
• C. $40{\text{m s}}^{-1}$
• D. $60{\text{m s}}^{-1}$

Answer 1

The correct option is D $60{\text{m s}}^{-1}$

Given:
Speed in first $2$ seconds = $50{\text{m s}}^{-1}$
Speed in the next $2$ seconds = $70{\text{m s}}^{-1}$
Distance travelled in the first $2$ seconds
= $50\times 2=100\text{m}$
Distance travelled in the next $2$ seconds
= $70\times 2=140\text{m}$

$\Rightarrow$Total distance travelled$=100+140=240\text{m}$
Total time taken $=4\text{s}$
$\therefore \text{Average speed}=\frac{\text{Total distance covered}}{\text{Total time taken}}$$=\frac{240}{4}=60{\text{m s}}^{-1}$