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Gravitational potential and field strength

A change in gravitational potential (ΔV) is equal to the negative of work by gravity on a unit mass,

Δ V = - E Δ r

For infinitesimal change, we can write the equation,

V = - E r

E = V r

Thus, if we know potential function, we can find corresponding field strength. In words, gravitational field strength is equal to the negative potential gradient of the gravitational field. We should be slightly careful here. This is a relationship between a vector and scalar quantity. We have taken the advantage by considering field in one direction only and expressed the relation in scalar form, where sign indicates the direction with respect to assumed positive reference direction. In three dimensional region, the relation is written in terms of a special vector operator called “grad”.

Further, we can see here that gravitational field – a vector – is related to gravitational potential (scalar) and position in scalar form. We need to resolve this so that evaluation of the differentiation on the right yields the desired vector force. As a matter of fact, we handle this situation in a very unique way. Here, the differentiation in itself yields a vector. In three dimensions, we define an operator called “grad” as :

grad = x i + y j + z k

where " x ” is partial differentiation operator with respect to "x". This is same like normal differentiation except that it considers other dimensions (y,z) constant. In terms of “grad”,

E = - grad V

Gravitational potential and self energy of a rigid body

Gravitational potential energy of a particle of mass “m” is related to gravitational potential of the field by the equation,

U = m V

This relation is quite handy in calculating potential energy and hence “self energy” of a system of particles or a rigid body. If we recall, then we calculated “self energy” of a system of particles by a summation process of work in which particles are brought from infinity one by one. The important point was that the gravitational force working on the particle kept increasing as more and more particles were assembled. This necessitated to calculate work by gravitational forces due to each particle present in the region, where they are assembled.

Now, we can use the “known” expressions of gravitational potential to determine gravitational potential energy of a system, including rigid body. We shall derive expressions of potential energy for few regular geometric bodies in the next module. One of the important rigid body is spherical shell, whose gravitational potential is given as :

Gravitational potential due to spherical shell

Gravitational potential at points inside and outside a spherical shell.

For a point inside or on the shell of radius “a”,

V = - G M a

This means that potential inside the shell is constant and is equal to potential at the surface.

For a point outside shell of radius “a” (at a linear distance, “r” from the center of shell) :

V = - G M r

This means that shell behaves as a point mass for potential at a point outside the shell. These known expressions allow us to calculate gravitational potential energy of the spherical shell as explained in the section below.

Self energy of a spherical shell

The self potential energy is equal to work done by external force in assembling the shell bit by bit. Since zero gravitational potential energy is referred to infinity, the work needs to be calculated for a small mass at a time in bringing the same from infinity.

In order to calculate work, we draw a strategy in which we consider that some mass has already been placed symmetrically on the shell. As such, it has certain gravitational potential. When a small mass “dm” is brought, the change in potential energy is given by :

Self energy of a spherical shell

Self energy is equal to work in bringing particles one by one from the infinity.

U = V m = G m R m

We can determine total potential energy of the shell by integrating the expressions on either side of the equation,

U = G R m m

Taking constants out from the integral on the right side and taking into account the fact that initial potential energy of the shell is zero, we have :

U = G R [ m 2 2 ] 0 M

U = G M 2 2 R

This is total potential energy of the shell, which is equal to work done in bringing mass from infinity to form the shell. This expression, therefore, represents the self potential energy of the shell.

In the same manner, we can also find “self energy” of a solid sphere, if we know the expression for the gravitational potential due to a solid sphere.

Questions & Answers

How can we take advantage of our knowledge about motion?
Kenneth Reply
pls explain what is dimension of 1in length and -1 in time ,what's is there difference between them
Mercy Reply
what are scalars
Abdhool Reply
show that 1w= 10^7ergs^-1
Lawrence Reply
what's lamin's theorems and it's mathematics representative
Yusuf Reply
if the wavelength is double,what is the frequency of the wave
Ekanem Reply
What are the system of units
Jonah Reply
A stone propelled from a catapult with a speed of 50ms-1 attains a height of 100m. Calculate the time of flight, calculate the angle of projection, calculate the range attained
Samson Reply
58asagravitasnal firce
Amar
water boil at 100 and why
isaac Reply
what is upper limit of speed
Riya Reply
what temperature is 0 k
Riya
0k is the lower limit of the themordynamic scale which is equalt to -273 In celcius scale
Mustapha
How MKS system is the subset of SI system?
Clash Reply
which colour has the shortest wavelength in the white light spectrum
Mustapha Reply
how do we add
Jennifer Reply
if x=a-b, a=5.8cm b=3.22 cm find percentage error in x
Abhyanshu Reply
x=5.8-3.22 x=2.58
sajjad
what is the definition of resolution of forces
Atinuke Reply

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Source:  OpenStax, Physics for k-12. OpenStax CNX. Sep 07, 2009 Download for free at http://cnx.org/content/col10322/1.175
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