Physics

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In physics deflection is the event where an object collides and bounces against a plane surface.

In such collisions involving a sphere and a plane, the collision angle formed with the surface normal (the incidental angle α) must equal the bounce angle (the accidental angle β), $\backslash alpha\; =\; \backslash beta$

See also

• reflection
• impulse

Construction

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In engineering mechanics, deflection is a term to describe the degree to which a construction or structural element bends under a load. An example of the use of deflection in this context is in building construction. Architects and builders select materials for various applications. The beams used for frame work are selected on the basis of deflection, amongst other factors.

The elastic deflection f and angle of deflection φ (in radians) in the example image, a cantilever beam, can be calculated using:

f_B = F·L³ / (3·E·I)

φ_B = F·L² / (2·E·I)

where

F = force acting on the tip of the beam

L = length of the beam (span)

E = modulus of elasticity

I = area moment of inertia

From this formula it follows that the span L is the most determinating factor; if the span doubles, the deflection increases 2³ = 8 fold.

Building codes determine the maximum deflection, usually as a fraction of the span e.g. 1/400 or 1/600. Either (tensile) strength or deflection can determine the minimum dimension of the beam.

Automotive

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Deflection as an automotive term is similar to its use in engineering. In an automotive context, deflection is used as a measure of the tightness of car engine belts, which is measured using a belt gauge.

Building engineering | Structural engineering | Materials science