Matter & Energy

Matter is composed of atoms or groups of atoms called molecules. The arrangement of particles in a material depends on the physical state of the substance. In a solid, particles form a compact structure that resists flow. Particles in a liquid have more energy than those in a solid. They can flow past one another, but they remain close. Particles in a gas have the most energy. They move rapidly and are separated from one another by relatively large distances.


Power (physics), in physics and engineering, the rate of performing work or transferring energy. Work is equal to the force applied to move an object multiplied by the distance the object travels. Power measures how quickly the work is done. For example, suppose a person wants to push a heavy box across a room. To overcome the friction between the bottom of the box and the floor, the person must apply force to the box to keep it moving. Since friction is independent of an object's velocity along a sliding surface, no matter how quickly the person pushes the box across the floor, the force applied is the same. Now suppose the person pushes the box from one end of the room to the other in ten seconds, then pushes the box back to its original position in five seconds. In each trip across the room, the force applied and the distance the box is moved is the same, so the work done in each case is the same. But the second time the box is pushed across the room, the person has to apply more power than in the first trip because the same amount of work is done in five seconds rather than ten. Mathematically speaking, power is equal to the work done divided by the time interval over which the work is performed.

The concept of power does not apply only to situations where objects are moved around mechanically. For example, the concept of power is also useful when dealing with electricity. Imagine an electric circuit with a resistor. A certain amount of work must be done to move charge through the resistor. To move charge more quickly through the resistor—or, in other words, to increase the current flowing through the resistor—more power is required.

Power is always expressed in units of energy divided by units of time. Two units of power are the horsepower and the watt (see Electrical Units). One horsepower is equal to the amount of power required to lift 33,000 pounds a distance of 1 foot in 1 minute. One watt equals the power needed to do 1 joule of work per second. There are 746 watts in 1 horsepower.