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96 Making Things Move

ll things that move need some source of energy. This energy may be as simple as
Ausing the force of gravity to create movement (how apples fall from trees), or as
complex as the internal combustion engine in a gas-powered car. A person can
also supply power by cranking a handle or pedaling a bike. Our bodies turn the
chemical energy from the food we eat into mechanical energy so we can walk, run,
and jump. Motors turn electrical energy into mechanical energy so we can make
things move and spin.

In this chapter, we’ll discuss how power, work, and energy are related, identify sources
of power, and highlight some practical examples of putting these sources to work.

Mechanical Power

Mechanical energy is the sum of an object’s potential and kinetic energy. Potential
energy is how much energy is stored in an object at rest. Kinetic energy is the energy
an object has because of its motion.

For example, a ball stopped on top of a hill has potential energy equal to its weight
multiplied by the height of the hill.
Potential Energy = Weight × Height

If the ball weighs 2 lbs, and the hill is 20 ft high, the potential energy is 40 ft-lbs. If
you push the ball so it starts rolling down the hill, the potential energy is gradually
turned into kinetic energy.

Kinetic Energy = 1/2 × Mass × Velocity 2
While the ball is rolling down the hill, the potential energy is decreasing (because it’s
losing height), while the kinetic energy is increasing (because it’s going faster). At the
bottom of the hill, the ball no longer has any potential energy, because all of it was
converted to kinetic energy.
Let’s consider how roller coasters work. A motor drags you up that first hill, increasing
your potential energy, and then lets you go. On the other side of the hill, all this
potential energy is converted into kinetic energy while the roller coaster races down
the hill and makes your heart jump into your throat. When the motor pulls the cars up
the first hill on the roller coaster, it does mechanical work.
Mechanical Work (W) = Change in Energy (E)