Đáp án:

\(\begin{array}{l}
1.h = \frac{{140}}{3}m\\
2.v = 8m/s\\
3.{W_t} = 0,02J
\end{array}\)

Giải thích các bước giải:

\(\begin{array}{l}
1.\\
{W_{t1}} = mg{h_1}\\
500 = 3.10{h_1}\\
 \Rightarrow {h_1} = \frac{{500}}{{3.10}} = \frac{{50}}{3}m\\
{W_{t2}} = mg{h_2}\\
 - 900 = 3.10{h_1}\\
 \Rightarrow {h_2} =  - 30m\\
h = \frac{{50}}{3} + 30 = \frac{{140}}{3}m\\
2.\\
{W_t} = {W_v}\\
\frac{1}{2}k\Delta {l^2} = \frac{1}{2}m{v^2}\\
\frac{1}{2}.400.0,{04^2} = \frac{1}{2}.0,01.{v^2}\\
 \Rightarrow v = 8m/s\\
3.\\
F = k\Delta l\\
 \Rightarrow k = \frac{F}{{\Delta l}} = \frac{4}{{0,04}} = 100N/m\\
{W_t} = \frac{1}{2}k\Delta l{'^2} = \frac{1}{2}.100.0,{02^2} = 0,02J
\end{array}\)

1.

$W_o=mgh_o→h_o=30m$

2.

$\dfrac{1}{2}mv^2=\dfrac{1}{2}kx^2→v=\sqrt{\dfrac{k}{m}} =8m/s.$

3. 

$W_{dh}=0,5kx^2=0,02J$