[填空题]
A box half filled with sand is pulled by a motor at a constant speed of $2.0\,m\;s^{-1}$ on a frictionless surface. During the motion, sand is added to the box from above at a constant rate of $\sigma\;kg\;s^{-1}$
1.State the condition related to the mass of an object in order for the equation $F_{net}=ma$ to be equivalent to the equation $F_{net}=\frac{Δp}{t}$
2.The force applied by the motor on the box is $0.11\,N$. Determine $\sigma$. $kg\;s^{-1}$
3.The motor has an efficiency of 75%. Calculate the total power provided to the motor. $W$
