Relative centrifugal force

Centrifugation is based on the fact that any object moving in a circle at a steady angular velocity is subject to an outward directed force, F. The 

F is frequently expressed in terms of the earth s gravitational force and is then referred to as the relative centrifugal force, RCF, or more commonly as the number times g.  

To be of use, however, these relationships must be expressed in terms of revolutions per minute, rpm, the common way in which the operating speed of a centrifuge is expressed. Since rpm values may be converted to radians using the equation 

The considerations used to calculate the RCF exerted on a sample spinning in a centrifuge rotor require that the sample be located at a fixed distance, r, from the center of rotation. Owing to rotor design, r varies from top to bottom of the sample holder. The problem is illustrated in the following example. 

As can be seen here the centrifugal force exerted at the top and bottom of 

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Relative centrifugal force is a measure or the intensity of centrifugal force compared with that due to gravity. 

The effect of centrifugation is therefore dependent upon the mass of the particle and the convenience of using the relative centrifugal force is that the value for the mass of the particle is eliminated from the equation, permitting more general comparisons of centrifuge performance ... 

In comparing the relative centrifugal force developed by different centrifuges it is important to appreciate that the value varies considerably from the top to the bottom of a centrifuge tube (Figure 3.39). 

Figure 3.39 Relative centrifugal force. The relative centrifugal force developed by any rotor can be calculated but will vary depending upon the distance from the centre of rotation.

Figwe 8. The effect of centrifugal force in the BPA treatment protocol on the clarification parameters. Results are expressed as a percentage of those for the untreated sample spun at 16,000 g. All treatments involved 2 min centrifugation. The KCl, BPA-1000, and BPA-1050 concentrations are the same as in Figure 7. A turbidity versus relative centrifugal force (RCF) B protein versus RCF C RNA versus RCF D DNA versus RCF E GUS activity versus RCF. Legend = untreated extract = BPA-1000 treated extract = BPA-1050 treated extract... 

The force on a sedimenting particle increases with the velocity of the rotation and the distance of the particle from the axis of rotation. A more common measurement of F, in terms of the earth s gravitation force, g, is relative centrifugal force, RCF, defined by Equation 7.2. 

B 9. Assume that a centrifuge is operating at 43,000 rpm. What is the relative centrifugal force at a distance from the central axis of 6 cm ... 

Depending on the available ultracentrifuge different relative centrifugation forces have been used, ranging from 120000 g up to 436000 g (Nakai et al. 2003). Depending on the affinity of the test substrate for nonspecific binding different material of ultracentrifuge tubes can be applied. 

The relative centrifugal force (RCF) value is thus related to rpm - a popular way of reporting the speed of a given centrifuge with a particular rotor. However, rpm is not a very scientihc way of expressing the experimental conditions used, as it relates to the use of a specihc rotor. So when a different rotor is used with the same rpm any differences in angular velocity and radius... 

Relative centrifugal force (RCF) is related to revolutions per minute (rpm) - a means of reporting the speed of a given centrifuge - where RCF is expressed as a number multiplied by gravity (g). 

The gravitational force in a centrifuge is dependent on two quantities the speed of rotation and the distance from the centre of rotation. Equation 4.12 expresses the relative centrifugal force (RCF) (in units of g - the Earth s gravitational field) in terms of these two parameters. 

Fig. 8. Formation of the pocket in the well of a tilted plate during centrifugation (direction left to right). The solid support (lower layer) is collected in the pocket, while the liquid (upper layer) is expelled from the well. The liquid surface angle is perpendicular to the resulting force vector of the relative centrifugal force (RCF) and gravity (G).

The relative centrifugal force (RCF) is defined as the force field relative to the Earth s gravitational field Equation 13.2 shows its definition ... 

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