Question
An object is placed in front of a convex mirror at a distance of 60 m from its pole. Object starts moving towards the mirror at 20 ms−1. Find the average speed of the image for a time interval of 2 s if the focal length of the mirror is 20 m.
Answer: Option D
:
D
Given:
Initial object distance, u=−60m
Speed of object, vo=20ms−1
Focal length, f=20m
Time interval,t=2s
Applying mirror formula to find the initial position of the image v:
1v+1u=1f
1v+1−60=120
v=15m
Distance travelled by the object is the product of speed and time.
so=vot
so=20×2=40m
Therefore, now the object distance:
uf=u+so=−60+40=−20m
Applying mirror formula to find the final position of the image vf:
1vf+1uf=1f
1vf+1−20=120
vf=10m
Average speed is the ratio of total distance travelled to the total time taken.
Averagespeed of the image:
vi=v−vft
vi=15−102=2.5ms−1
Was this answer helpful ?
:
D
Given:
Initial object distance, u=−60m
Speed of object, vo=20ms−1
Focal length, f=20m
Time interval,t=2s
Applying mirror formula to find the initial position of the image v:
1v+1u=1f
1v+1−60=120
v=15m
Distance travelled by the object is the product of speed and time.
so=vot
so=20×2=40m
Therefore, now the object distance:
uf=u+so=−60+40=−20m
Applying mirror formula to find the final position of the image vf:
1vf+1uf=1f
1vf+1−20=120
vf=10m
Average speed is the ratio of total distance travelled to the total time taken.
Averagespeed of the image:
vi=v−vft
vi=15−102=2.5ms−1
Was this answer helpful ?
Submit Solution