Đáp án:
\(\begin{array}{l}
2.\\
a.\\
W = 250J\\
b.\\
h = 5m\\
c.\\
h = 1,25m\\
d.\\
v = 5m/s\\
3.\\
a.\\
h = 25m\\
b.\\
h = 45m\\
c.\\
v' = \sqrt {645} m/s
\end{array}\)
Giải thích các bước giải:
\(\begin{array}{l}
2.\\
a.\\
W = \frac{1}{2}m{v^2} = \frac{1}{2}{.5.10^2} = 250J\\
b.\\
W = W' = mgh\\
250 = 5.10.h\\
h = 5m\\
c.\\
W = W = {W_t} + {W_d} = {W_t} + 3{W_t} = 4{W_t} = 4mgh\\
250 = 4.5.10.h\\
h = 1,25m\\
d.\\
{W_d} = 3{W_t}\\
\frac{1}{2}mv{^2} = mgh\\
v = \sqrt {2gh} = \sqrt {2.10.1,25} = 5m/s\\
3.\\
a.\\
W = W'\\
mgh + \frac{1}{2}m{v^2} = \frac{1}{2}mv{'^2}\\
10.h + \frac{1}{2}{.20^2} = \frac{1}{2}{30^2}\\
h = 25m\\
b.\\
W' = W\\
\frac{1}{2}mv{'^2} = mgh\\
\frac{1}{2}{.30^2} = 10h\\
h = 45m\\
c.\\
W = W' = {W_d}' + {W_t}' = {W_d}' + \frac{1}{3}{W_d}' = \frac{4}{3}{W_d}'\\
mgh = \frac{4}{3}.\frac{1}{2}mv{'^2}\\
10.45 = \frac{2}{3}v{'^2}\\
v' = \sqrt {645} m/s
\end{array}\)