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Ratio between two power quantities expressed in decibels

The decibel is a logarithmic unit used to express the ratio between two values of a physical quantity. The bel represents a ratio between two power ... more

Orbital Period - as a function of central body's density

The orbital period is the time taken for a given object to make one complete orbit around another object.

When mentioned without further ... more

Cryoscopic constant

Freezing-point depression describes the process in which adding a solute to a solvent decreases the freezing point of the solvent. freezing-point ... more

Numerical Aperture

In optics, the numerical aperture (NA) of an optical system is a dimensionless number that characterizes the range of angles over which the system can ... more

Distance between two points (three-space)

Distance is a numerical description of how far apart objects are. In analytic geometry, the distance between two points of the xyz-plane in three-space, ... more

Ellipse Circumference (Ramanujan formula)

Ellipse is a curve on a plane surrounding two focal points such that a straight line drawn from one of the focal points to any point on the curve and then ... more

Hooke's Law (spring)

Hooke’s Law of elasticity is an approximation that states that the amount by which a material body is deformed (the strain) is linearly related to ... more


In mathematics, the slope or gradient of a line is a number that describes both the direction and the steepness of the line. Slope is often denoted by the ... more

Worksheet 980

PPI can be calculated from knowing the diagonal size of the screen in inches and the resolution in pixels (width and height). This can be done in two steps

Using the Pythagorean theorem, for 3 different screen resolutions:

Diagonal Resolution - Pixels

Using the Diagonal Resolution from the previous formula we calculate the PPI for 3 corresponding screen sizes :

Pixels Per Inch (PPI)


10.1 inch tablet screen of resolution 1024×600 : 117.5PPI
21.5 inch PC monitor of 1080p resolution : 102.46PPI
27 inch PC monitor of 1440p resolution : 108.78PPI

Stokes' law

Stokes’ law is an expression for the frictional force – also called drag force – exerted on spherical objects with very small Reynolds numbers (e.g., ... more

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