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Fields: Forces and Energy
Objective 2: Describe what a field is, how it
relates to non-contact forces, and how it
relates to energy.
Forces as Vectors
• A vector is a value that
has both magnitude (size)
and direction.
• Usually a vector is drawn
as an arrow.
• Vectors are often used to
represent the size and
direction of a force acting
on an object.
Vector Field
• A vector field is a map of all
vectors over a space or area,
as shown here
• Vector fields can be used to
model many things that we
can observe:
– ocean currents
– weather patterns
– and most importantly,
non-contact forces!!
Fields
• For non-contact forces (when objects do
not touch), a field is the space around an
object where the force exists.
– Example: The Earth has a gravitational field where
other objects experience the pull from the Earth’s
gravity
• Fields never disappear, no matter how
far away from the object you are!
Examples – Vector Force Fields
Gravitational Fields
• Gravitational field lines show which way an object
within the field will move.
Gravitational field lines on the surface
of earth pull masses straight down.
Gravitational field lines in space
pull masses towards the earth
(which is still straight down).
Examples – Vector Force Fields
Electric Fields
• Electric field lines show which way a positive
charge within the field will move.
Positive charges push other positive
charges away (arrows point outward);
negative charges pull positive charges
in (arrows point inward).
Electric field lines also show
which pairs of charges will
attract (top) or repel (bottom).
Fields & Energy
• Vector force fields store potential energy.
• We know this because objects released within the
field will start to move!
– Objects released in the Earth’s gravitational field will fall.
• Because these objects had gravitational potential
energy!
– Charges released in an electric field will move towards or
away from each other, depending on their charge (positive
or negative).
• Because these objects had electric potential energy!
Maximizing Potential Energy
• An object has MORE potential energy when it has
farther distance it could potentially travel.
• When an object is released within a field and it
begins to move, its potential energy decreases.
The higher up an object
is lifted into the air, the
more gravitational
potential energy it has.
The amount of
gravitational potential
energy it has decreases as
it falls towards the Earth.