Kinetic energy is the
energy of motion. An object which has motion - whether it be
vertical or horizontal motion - has kinetic energy. There
are many forms of kinetic energy - vibrational (the energy
due to vibrational motion), rotational (the energy due to
rotational motion), and translational (the energy due to
motion from one location to another). To keep matters
simple, we will focus upon translational kinetic energy. The
amount of translational kinetic energy (from here on, the
phrase kinetic energy will refer to translational kinetic
energy) which an object has depends upon two variables: the
mass (m) of the object and the speed (v) of the object. The
following equation is used to represent the kinetic energy
(KE) of an object.

where m = mass of object

v = speed of object

This
equation reveals that the kinetic energy of an object is
directly proportional to the square of its speed. That means
that for a twofold increase in speed, the kinetic energy
will increase by a factor of four. For a threefold increase
in speed, the kinetic energy will increase by a factor of
nine. And for a fourfold increase in speed, the kinetic
energy will increase by a factor of sixteen. The kinetic
energy is dependent upon the square of the speed. As it is
often said, an equation is not merely a recipe for algebraic
problem-solving, but also a guide to thinking about the
relationship between quantities.

Kinetic energy is a scalar
quantity; it does not have a direction. Unlike velocity,
acceleration, force,
and momentum, the
kinetic energy of an object is completely described by
magnitude alone. Like work and potential energy, the
standard metric unit of measurement for kinetic energy is
the Joule. As might be implied by the above equation, 1
Joule is equivalent to 1 kg*(m/s)^2.

Check
Your Understanding

Use your understanding of kinetic energy to answer the
following questions. Then click the button to view the
answers.

1. Determine the kinetic energy of a 625-kg roller
coaster car that is moving with a speed of 18.3 m/s.

2. If the roller coaster car in the above problem were
moving with twice the speed, then what would be its new
kinetic energy?

3. Missy Diwater, the former platform diver for the
Ringling Brother's Circus, had a kinetic energy of 12 000 J
just prior to hitting the bucket of water. If Missy's mass
is 40 kg, then what is her speed?

4. A 900-kg compact car moving at 60 mi/hr has
approximately 320 000 Joules of kinetic energy. Estimate its
new kinetic energy if it is moving at 30 mi/hr. (HINT: use
the kinetic energy equation as a "guide to
thinking.")