# A force is a push or a pull. We use forces all the time. When you lift a

```A force is a push or a pull. We use forces all the time. When you lift a book,
when you walk, when you breathe, you use force. In industry and sports
everything depends on force.
Cranes use force to lift containers.
High jumpers use force
to push off the ground.
In this book, we describe the effects of forces in four types of energy
systems. In mechanical systems, a force acting on an object can result in a
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C HAPTER 1
THE PRIME MOVERS
change in the object’s motion. For this reason, we call force the prime
mover in a mechanical system.
A fluid system is one in which a fluid such as air, steam, water, oil or
gasoline is moved—usually through pipes, lines or tubes. In a fluid system,
forces are distributed on a surface. Pressure is the force on a surface divided
by the area of the surface. Fluids move in a fluid system because of pressure.
When blood moves through arteries, it moves because of blood pressure.
When water travels through a fire hose, it does so because of water pressure.
A hot-air balloon rises or falls because of air pressure.
Air pressure causes a
hot-air balloon to rise or descend.
An electrical system is one in which electric charge (electrons) moves
through wires or through complex circuits. Charge moves in an electrical
system when electrons experience an electric potential difference, or
voltage. When you turn on a television set, voltage causes electric charge to
flow through the circuits in the set to create pictures on the screen. When
you turn the ignition key in a car, voltage causes electric charge to flow
through the starter motor, which then turns the engine.
A thermal system is one in which heat flows from one place (a hot object)
to another (a cold object.) Heat flows in a thermal system because of a
temperature difference. When a hot object is placed in contact with a cold
object, heat flows from the hot object (high temperature) to the cold object
(low temperature.) For example, if a welding torch increases the temperature
of one end of a metal rod, the temperature difference causes heat to flow
from the hot end of the rod to the cold end. When cool air flows over a car’s
radiator, temperature differences cause heat to flow from the hot water to the
metal in the radiator to the cooler air.
C HAPTER 1
THE PRIME MOVERS
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