Đáp án:

\(\begin{array}{l}
a.\\
v' = 17,678m/s\\
h' = 15,625m\\
b.\\
h = 7,8125m\\
v = 21,65m/s\\
c.\\
v' = 12,5m/s\\
h' = 23,4375m
\end{array}\)

Giải thích các bước giải:

\(\begin{array}{l}
a.\\
W' = {W_d}' + {W_t}' = {W_d}' + {W_d}' = 2{W_{d'}}\\
W = W'\\
\frac{1}{2}mv_0^2 = 2.\frac{1}{2}mv{'^2}\\
{25^2} = 2.v{'^2}\\
v' = 17,678m/s\\
{W_t}' = {W_d}'\\
mgh' = \frac{1}{2}mv{'^2}\\
10h' = \frac{1}{2}17,{678^2}\\
h' = 15,625m\\
b.\\
W = {W_d} + {W_t} = 3{W_t}'' + {W_t} = 4{W_t}\\
W = W\\
\frac{1}{2}mv_0^2 = 4mgh\\
\frac{1}{2}{.25^2} = 4.10.h\\
h = 7,8125m\\
{W_d} = 3{W_t}\\
\frac{1}{2}mv{^2} = 3.mgh\\
\frac{1}{2}v{^2} = 3.10.7,8125\\
v = 21,65m/s\\
c.\\
W' = {W_d}' + {W_t}' = {W_d}' + 3{W_d}' = 4{W_d}'\\
W = W'\\
\frac{1}{2}mv_0^2 = 4.\frac{1}{2}.mv{'^2}\\
{25^2} = 4.v{'^2}\\
v' = 12,5m/s\\
{W_t}' = 3{W_d}'\\
mgh' = 3.\frac{1}{2}.mv{'^2}\\
10.h' = 3.\frac{1}{2}.12,{5^2}\\
h' = 23,4375m
\end{array}\)