The potential energy (in joule) of a body of mass $2 \mathrm{kg}$ moving in the $xy$ plane is given by $U=6x+8y$, where $x$ and $y$ are in metre. If the body is at rest at point $\left(6 \mathrm{m},4 \mathrm{m}\right)$ at time $t=0$, it will cross $y$-axis at time $t$ equal to

A block of mass $2 \mathrm{kg}$ is free to move along the $x$-axis. It is at rest and from $t=0$ onwards, it is subjected to a time-dependent force $F\left(t\right)$ in the $x$ direction. The force $F\left(t\right)$ varies with $t$ as shown in the figure. The kinetic energy of the block after $4.5 \mathrm{second}$ is