Question 30576
BASIS..START TIME ..0 SEC.......GROUND LEVEL =0 FT.ALL HEIGHTS MEASURED FROM GROUND LEVEL.
WE HAVE 2 BODIES B1 COMING DOWN FROM TOP TO GROUND...SAY.... U TO P.
B2 GOING UP FROM GROUND UPWARDS......SAY....P TO U....(OFCOURSE NOT CLASHING)
ATTACHMENTT:-
LET B1 COME DOWN FROM U TO R AND B2 GO UP FROM P TO Q WHEN THEY ARE AT THE 10 FEET SEPERATION ASKED FOR.THIS IS THE ATTACHMENT LET US SAY.LET THIS HAPPEN IN T1 SECS.FROM START.WE HAVE
PU=60 FEET...LET PQ=X FT.....QR=10 FT.....HENCE PR=X+10 FEET.
DISTANCE TRAVELLED IN T SECS.FROM START,MEASURED FROM GROUND LEVEL IS GIVEN BY
S=-16T^2+(V0)T+(S0).....WHERE V0 IS INITIAL VELOCITY AND S0 IS START DISTANCE FROM GROUND.
SO FOR B1 COMING DOWN WE HAVE   
X+10=-16T1^2+0*T1+60=-16T1^2+60...............................I
FOR B2 GOING UP ,WE HAVE...
X=-16T1^2+80T1+0=-16T1^2+80T1..................................II
SUBSTITUTING FOR X FROM EQN.II IN EQN.I,WE GET
-16T1^2+80T1+10=-16T1^2+60
80T1=50
T1=50/80=5/8 SECS.
SO THE ATTACHMENT TAKES PLACE T 5/8 SEC. FROM START.
DETACHMENT:-
NOW B1 CONTINUES TO COME DOWN AS B2 GOES UP,CROSSING EACH OTHER,WHEN THE DISTANE BETWEEN THEM COMES DOWN FROM 10 FT. TO ZERO.AS THEY FURTHER CONTINUE THEIR TRAVEL ,THEIR DISTANCE OF SEPERATION INCREASES FROM ZERO TO 10 FT.LET US CALL THIS DETACHMENT.LET THIS HAPPEN AFTER T2 SECS.FROM START WHEN B1 REACHES SAY S AND B2 REACHES T.WE HAVE..
LET PT=Y AND HENCE PS=Y-10...SO FOR THIS PART
FOR B1 COMING DOWN WE HAVE..
Y-10=-16T2^2+0*T2+60=-16T2^2+60...........................III
FOR B2 GOING UP,WE HAVE..
Y=-16T2^2+80T2+0=-16T2^2+80T2...........................IV
SUBSTITUTING FOR Y FROM EQN.III IN EQN.IV..WE GET
-16T2^2+80T2-10=-16T2^2+60
80T2=70
T2=7/8 SECS.
SO THE DETACHMENT TAKES PLACE AT 7/8 SECS.FROM START.
SO FOR 7/8-5/8=2/8=1/4 SECS.THE 2 BALLS ARE AT A DISTANCE OF 10 FEET OR LESS.