A) \[\frac{2v{{ }_{d}}{{v}_{u}}}{{{v}_{d}}+{{v}_{u}}}\]
B) \[\sqrt{v{{ }_{u}}{{v}_{d}}}\]
C) \[\frac{v{{ }_{d}}{{v}_{u}}}{{{v}_{d}}+{{v}_{u}}}\]
D) \[\frac{v{{ }_{u}}+{{v}_{d}}}{2}\]
Correct Answer: A
Solution :
Key Idea Average speed of a body in a given time interval is defined as the ratio of distance travelled to the time taken. \[Average\text{ }speed=\frac{Distance\text{ }travelled}{Time\text{ }taken}\] Let \[{{t}_{1}}\] and \[{{t}_{2}}\] be times taken by the car to go from X to Y and then from Y to X respectively. Then, \[{{t}_{1}}+{{t}_{2}}=\frac{XY}{{{v}_{u}}}+\frac{XY}{{{v}_{d}}}=XY\left( \frac{{{v}_{u}}+{{v}_{d}}}{{{v}_{u}}{{v}_{d}}} \right)\] Total distance travelled \[=XY+XY=2XY\] Therefore, average speed of the car for this round trip is \[{{v}_{av}}=\frac{2XY}{XY\left( \frac{{{v}_{u}}+{{v}_{d}}}{{{v}_{u}}{{v}_{d}}} \right)}\] or \[{{v}_{av}}=\frac{2{{v}_{u}}{{v}_{d}}}{{{v}_{u}}+{{v}_{d}}}\]
You need to login to perform this action.
You will be redirected in
3 sec
