A ball of mass 50 g is thrown upwards. It rises to a maximum height of 100m. At what height its KE is reduced to 70% ?

Options

A. 30 m
B. 40 m
C. 60 m
D. 70 m

Answer

A

Explanation

(a) Initial kinetic energy, `K_i = mgh, "initial", U_i = 0`
Final kinetic energy `K_f = (70)/(100)K_i= 0.7K_i = 0.7 mgh`
Final potential energy `U_f = mgh',` where h' is to be found out.
According to the law of conservation of mechanical energy.
`K_i + U_i = K_f + U_f or mgh + 0 = 0.7 mgh + mgh'`
or `h = 0.7 h + h' or h' = h - 0.7 h = 0.3 h = 0.3 xx 100m= 30m`