9.If the velocity u of a body moving along a straight line with a constant retardation a is reduced by 75% in time ,then t=

solution:

initial velocity =u

final velocity =u-75% of u

=u-3/4u

v=u/4

we have the formula v=u+ at

retardation= -a

u/4=u+(-a)t

at=u-u/4

at=3u/4

t=3u/4a

if the velocity u of a body moving along a straight line with ,after time 3u/4a the velocity is reduced by 75% of u