Consider an object which has been given a speed $v$ on a rough horizontal surface. As time passes, the object covers a distance $l$ until it stops because of friction. Now, Initial kinetic energy = $\frac{1}2mv^2$ And final kinetic energy is zero. Therefore, work done by friction on the object i...

Lets say we have a frictionless and massless pulley with a massless string over it. There are two masses attached to it $m_1$ and $m_2$ where $m_2>m_1$. So $m_2$ would an experience an acceleration down while $m_1$ experiences an acceleration up and massless string would also move accordingly. Ho...