In radio astronomy, the word "beam" is used to refer to the response pattern of an antenna to a signal. In this context, astronomers speak of beam convolution,beam solid angle,beam width.

The more common use of the word "beam" is to refer to a bar (or rod, shaft, cantilever, etc.) under bending. A beam with one end clamped and the other end free is called a cantilever.
Let a beam have length L, width a, and thickness h. Also let the vertical displacement be
, the horizontal displacement be
,and the angular displacement of the beam from the original horizontal (called the neutral surface or axis), and set up a Cartesian coordinate system along the neutral surface. The angular displacement is then given by

The longitudinal strain is

and the general strain component is

If q(x) is the downward force per unit area and V(x) is the shearing force per unit length, then force balance gives

or

Similarly, let M[i/] be the torque (a.k.a. bending moment) per unit length and [i]P the horizontal force (i.e.,tension) per unit area. Then torque balance gives


Taking the derivative of (7) and plugging in (5) then gives

But M is given by

where
is the z-component of the longitudinal stress,and

where E is Young's modulus and
is the Poisson ratio.Plugging in,

where I is the geometrical moment of inertia, given by

The geometrical moment of inertia per unit width is then

so

where the flexural rigidity is defined by

Plugging (14) into (8) gives


Similarly, plugging (14) into (7) gives

so

If the tension (i.e., horizontal force) vanishes, then
so


If
is furthermore a constant (equal to the weight of the bar per unit length), then solving the fourth-order ordinary differential equation (20) for
gives

For a massless beam,
and the solution to (20) becomes

The following table summarizes the various constraints on
placed by various boundary conditions on the bar.
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The more common use of the word "beam" is to refer to a bar (or rod, shaft, cantilever, etc.) under bending. A beam with one end clamped and the other end free is called a cantilever.
Let a beam have length L, width a, and thickness h. Also let the vertical displacement be



(1)
The longitudinal strain is

(2)
and the general strain component is

(3)
If q(x) is the downward force per unit area and V(x) is the shearing force per unit length, then force balance gives

(4)
or

(5)
Similarly, let M[i/] be the torque (a.k.a. bending moment) per unit length and [i]P the horizontal force (i.e.,tension) per unit area. Then torque balance gives

(6)

(7)
Taking the derivative of (7) and plugging in (5) then gives

(8)
But M is given by

(9)
where


(10)
where E is Young's modulus and


(11)
where I is the geometrical moment of inertia, given by

(12)
The geometrical moment of inertia per unit width is then

(13)
so

(14)
where the flexural rigidity is defined by

(15)
Plugging (14) into (8) gives

(16)

(17)
Similarly, plugging (14) into (7) gives

(18)
so

(19)
If the tension (i.e., horizontal force) vanishes, then


(20)

(21)
If



(22)
For a massless beam,


(23)
The following table summarizes the various constraints on

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