1 ) A ball at rest is dropped from a height of 12 m . It loses 25 % of its kinetic energy in striking the ground . Find the height to which it bounces . How do you account for the loss in kinetic energy ?
A ) 5 m , B
) 9 m ,
C ) 13 m , D
) None of these ,
Correct
Answer : - B ) 9 m ,
Solution
: -
So , K.E = ( 75 /
100 ) x mgh = ( 75 / 100 ) x 12 x mg = 9 mg
.
So ,
P.E = 9 mg ,
mgh = 9 mg ,
h = 9
meter , Answer .
Method
# 02 : -
Simply we can find 75 % of 12 m , because
25 % is loosed , so we will neglect it .
h = 12 x ( 75 % )
= 12 x ( 75 / 100 ) = 9 m
, Answer .
So it will bounce 9 m after its first strike with ground
.
2 )
A ball at rest is dropped from a height of 30 m . It loses 50 % of its kinetic energy in
striking the ground . Find the height to which it bounces . How do you account
for the loss in kinetic energy ?
A ) 9 m , B
) 15 m ,
C ) 21 m , D
) None of these ,
Correct
Answer : - B ) 15 m ,
Solution
: -
Simply we can find 50 % of 30 m , because
50 % is loosed , so we will neglect it .
h = 30 x ( 50 % )
= 30 x ( 50 / 100 ) = 15 m
, Answer .
So
it will bounce 15 m after its first strike with ground .
