功率

Power is a measurement of energy per time, measured in watts. An 100 Watt bulb would be using 100 Joules of energy per second.

You can also calculate power by work done per time, as change in energy is work. Here's a problem:

A 2kg box is lifted by a pulley (ignoring friction) 10 meters into the air. Assume 10$\frac{m}{s^2}$ for gravity. If the pulley does this in 4 seconds, what is its power output?

The answer is 50W. Using the formula for change in potential energy:

$PE=mgh$

$=2kg\cdot10\frac{m}{s^2}\cdot10m$ $=200J$

$P=\frac{W}{t}=\frac{200J}{4s}$

$P=50W$

In physics, power is the amount of energy transferred per unit time. It is a scalar quantity.

The equation of power is P = $\frac{w}{t}$
The unit of power is equal to the unit of work divided by the unit of time. So the unit of power is j/s. But it is well known as watt in honor of James Watt.

**Why the idea of power is important? **

Suppose, you are committed to break a wall by iron balls and you can through a ball with a kinetic energy of 200 joules at a time. Now, you have 100 balls and if you throw one by one, you can't break the wall. But if you through all of the balls simultaneously, there will be a large possibility for the cluster of balls to break the wall. Because you are giving a huge amount of energy on the wall at a time. This is why the idea of power is important. If you have 1000 joules of energy and you pass it through a bulb to illuminate for a very short period of time, you will get brighter light than if you pass the same amount energy through the bulb for a long period.