IIT JEE , AIEEE Forum - Physics , Optics

prateek.agarwal

A plane mirror is lying on a horizontal X-Y plane. A bird is flying with a velocity (2i ? j + 4k) m/s above the mirror. If the mirror also starts moving with a velocity of (i + j + k) m/s, then calculate the velocity of image of the bird formed by the moving plane mirror.

(a) (? i ? j ? k) m/s
(b) (2i ? 2j ? 2k) m/s
(c) ( 2i ? j ? 2k) m/s
(d) ( i ? 2j ? k) m/s

prateek.agarwal

where ever there is question mark...its actually a minus sign...
(a) (? i ? j ? k) m/s
(b) (2i ? 2j ? 2k) m/s
(c) ( 2i ? j ? 2k) m/s
(d) ( i ? 2j ? k) m/s

vegeta1in

(2i+4j+k)=magnitude=root of 21 approximately=4.something

i+j+k=magnitude=root of 3=1.732

hence relative vel of bird wrt block=root of 21-root of 3

=approximately 2.something{21 is near to 25....hence root of 21 will be 4.5+}
hence answer is (d){only option with magnitude 2}
hence i go for (d) is dat da answer?!

computer001

im gettin C

computer001

consider wrt to mirror
vel of bird= i-2j+ 3k
now 1st consider z component:
the bird is movin awy from mirror upwards with 3m/s hence img moves dowwnwards with 3 m/s hence u get -3k
now when the bird move rite the img also moves rite(note many make a mistake here if u r in doubt try in fron of ur mirror)
so the i ,j comp of bird n img r same
so v have i -2j -3k
but this is wrt to mirror
hence ctual v= 2i -j -2k

computer001

@ anand:
if u r movin towards mirror perpendicularly yes v(apply vel of img=-vel of obj) can cuz if u go towards mirror the img comes 2wards u...but u have not considered side ways motion...stand in front of ur mirror n move left or rite..when u move left img also moves left no?? so its not directly replacing v by -v..

anandghegde

thanks gokul

computer001

cheers

Ask & Discuss Questions with Community & Experts