Question

How is power calculated ?

Answer 1

Here, you are asked to find a way through which we can find power. In order to answer this question, you need to consider the definition of power. Here, you are expected to use the force, energy and their units as well, because power can be expressed in terms of these quantities. This question can be answered using the units of physical quantities as well. So, you have two ways to answer this question, one is to use the definition of power, the other one is to use the unit of power.

Complete answer:
Force is a quantity which is defined as the product of mass and acceleration of a system or body and is the cause for the motion of that system or body. As you know that acceleration is a vector quantity and mass is a scalar quantity, force is a vector quantity. Now, consider a block of mass $m$ lying on a smooth horizontal surface and a force equal to $F$ is applied horizontally and therefore the displacement of the block will happen in the same direction. Work is defined as the dot product of force and displacement and since in our case the angle is zero, we have work by force given as $dW = Fdr$.

Now, power is defined as the energy transferred per unit time. As you can see that if we divide work by time, we can get an expression for power. Therefore, we have $\dfrac{{dW}}{{dt}} = F\dfrac{{dr}}{{dt}}$ and we get power as $P = \dfrac{{dW}}{{dt}} = F\dfrac{{dr}}{{dt}}$. So, we can say that dividing work by time or multiplying force by velocity, we can calculate power supplied by the force. The other method is to go for the unit of power. The unit of power is watt and watt is defined as joules per second. From here also, we can calculate power as energy per unit time.

Therefore, power can be calculated by dividing energy or work by time.

Note: we have discussed in brief about power. You should remember that power is defined as energy per unit time and can be calculated by dividing energy or work done by time or multiplying force with the rate of change of displacement with respect to time, that is velocity. Also, keep in mind that the unit of power is watt and watt is joules per second.