a.

•$Wđ_{A}$ = $\dfrac{1}{2}$m$v_{A}^2$

=> $Wđ_{A}$ = $\dfrac{1}{2}$.5.$10^2$

=> $Wđ_{A}$ = 250 (J)

•$Wt_{A}$ = mg$z_{A}$ = 5.10.15 = 750 (J)

b.

$W_{A}$ = $Wđ_{A}$ + $Wt_{A}$ = 250+750 = 1000 (J)

Theo định luật bảo toàn cơ năng:

$W_{A}$ = $W_{B}$ = $Wđ_{B}$ + $Wt_{B}$

<=> 1000 = $\dfrac{1}{2}$m$v_{B}^2$ + mg$z_{B}$

<=> 1000 = $\dfrac{1}{2}$.5.$v_{B}^2$ + 5.10.10

=> $z_{B}$ = 10$\sqrt{2}$ (m/s)

c.

$W_{A}$ = $W_{C}$ = 3$Wt_{C}$ + $Wt_{C}$

<=> 1000 = 4.5.10.$z_{C}$

=> $z_{C}$ = 5 (m)

Đáp án:

\(\begin{array}{l}
a.\\
{W_t} = 750J\\
{W_d} = 250J\\
b.v' = 10\sqrt 2 m/s\\
c.h'' = 5m
\end{array}\)

Giải thích các bước giải:

\(\begin{array}{l}
a.\\
{W_t} = mgh = 5.10.15 = 750J\\
{W_d} = \frac{1}{2}m{v^2} = \frac{1}{2}{.5.10^2} = 250J\\
b.\\
W = 750 + 250 = 1000J\\
W = W' = mgh' + \frac{1}{2}mv{'^2}\\
1000 = 5.10.10 + \frac{1}{2}.5.v{'^2}\\
 \Rightarrow v' = 10\sqrt 2 m/s\\
c.\\
{W_d} = 3{W_t}\\
 \Rightarrow {W_t} = \frac{W}{4}\\
mgh = \frac{{1000}}{4}\\
5.10.h'' = \frac{{1000}}{4}\\
 \Rightarrow h'' = 5m
\end{array}\)