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For motion near the surface of the earth, we usually assume that the gravitational force on a mass m is

F=-mgk

but for motion involving an appreciable variation in distance r from the center of the earth, we must use

F=-Cr2er=Cr2r|r|=-Cr3r

where C is a constant. Show that both these F’s are conservative, and find the potential for each.

Short Answer

Expert verified

Both F’s are conservative

Step by step solution

01

Given Information

The force field are mentioned below.

F=-Cr2er=Cr2r|r|=-Cr3rF=-mgk

02

Definition of conservative force and scalar potential

A force is said to be conservative if ∇×F=0.

The scalar potential is independent of the path. The scalar potential is the sum of potential in all the 3 dimensions calculated separately.

The formula for the scalar potential isW=∫F.dr .

03

Use ∇×F to check conservative

The force is said to be conservative if∇×F=0.

ijk∂∂x∂∂y∂∂z00-mg

i0-0-j0-0+k0-00

Solve for other equation.

ijk∂∂x∂∂y∂∂z-Cxx2+y2+z232-Cyx2+y2+z232-Czx2+y2+z232

∇×Fi=3Cyzx2+y2+z2-32x2+y2+z2-3Cyzx2+y2+z2-32x2+y2+z2=0

∇×Fj=3Cxzx2+y2+z2-32x2+y2+z2-3Cxzx2+y2+z2-32x2+y2+z2=0

∇×Fk=3Cyzx2+y2+z2-32x2+y2+z2-3Cyzx2+y2+z2-32x2+y2+z2=0

Hence, the equations are conservative.

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Most popular questions from this chapter

∮(yi−xj+zk)−draround the circumference of the circle of radius 2, center at the origin, in the (x,y)plane.

A cylindrical capacitor consists of two long concentric metal cylinders. If there is a charge of k coulombs per meter on the inside cylinder of radius, R1and coulombs per meter on the outside cylinder of radius,R1find -k the electric field E between the cylinders. Hint: Use Gauss’s law and the method indicated in Figure 10.7. What is E inside the inner cylinder? Outside the outer cylinder? (Again use Gauss’s law.) Find, either by inspection or by direct integration, the potential role="math" localid="1659237306724" φsuch thatE=-∇φfor each of the three regions above. In each case E is not affected by adding an arbitrary constant toφ. Adjust the additive constant to makeφa continuous function for all space

Evaluate the line integral ∫xydx+xdyfrom(0,0)to(1,2) along the paths shown in the sketch.

Given

F1=-2yi+(z-2x)j+(y+z)kF2=yi+2xj:

(a) Is F1conservative? Is F2conservative?

(b) Find the work done by 2 on a particle that moves around the ellipse x=cosθ, y=2²õ¾±²Ôθfrom θ=0toθ=2Ï€

(c) For any conservative force in this problem find a potential function Vsuch

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(e) Use Green’s theorem and the result of Problem 9.7 to do Part (b) above.

Verify that the force field is conservative. Then find a scalar potential θ such that F=-∇φ,F=z2sinhyj+2zcoshyk,k=constant.

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