State the meaning of the work done by a force.
A block with mass $m$ is lifted vertically at constant speed. The vertical increase in height of the block is $\Delta h$, as shown in Fig. 3.1. Derive an expression, in terms of $m$ and $\Delta h$, for the block’s change in gravitational potential energy $\Delta E_p$. Define any other symbols you use.
An electric motor has an input power of $900\,\text{W}$. It needs $1.0\,\text{min}$ to raise a load with weight $240\,\text{N}$ at constant speed through a vertical distance of $150\,\text{m}$. Resistive forces are negligible. Show that the motor transfers $36\,\text{kJ}$ of work to the load in $1.0\,\text{min}$.
Find the useful output power of the motor.
Use your answer from (c)(ii) to determine the motor’s efficiency.
Some of the motor’s wasted power is dissipated by the resistance of its coil. This dissipated power is $280\,\text{W}$. The coil of the motor is made from wire with total length $23\,\text{m}$. The wire has a cross-sectional area of $2.6 \times 10^{-8}\,\text{m}^2$ and is made from a metal with resistivity $1.7 \times 10^{-8}\,\Omega\,\text{m}$. Calculate the current in the coil.