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082-Engineering-Mathematics-Anthony-Croft-Robert-Davison-Martin-Hargreaves-James-Flint-Edisi-5-2017

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868 Chapter 27 Line integrals and multiple integrals

A

s

m

M

N

ds

B

Figure27.1

An object ofmassmfallsfrom A to B.

vector called the acceleration due to gravity. Suppose we release the mass and allow

it to fall from point A in Figure 27.1. The vertical displacement measured downwards

from Aiss.

Workisbeingdonebythegravitationalforceinordertomakethemassaccelerate.We

wish to calculate the work done by the field in moving the mass from A to B. Suppose

we consider the amount of work done as the mass moves from point M to point N, a

distance δs.Elementaryphysicstellsusthattheworkdoneisequaltotheproductofthe

magnitudeoftheforceandthedistancemovedinthedirectionoftheforce.Inthiscase

themagnitudeoftheforceismg,andsothesmallamountofworkdone, δW,inmoving

from M toN,is

δW =mgδs

from which we have δW δs

=mg. As δs → 0 weobtain

δW

lim

δs→0 δs = dW ds =mg

To find the total work done as the mass falls from A to B we must add up, or integrate,

the contributions over the whole interval of interest,thatis

total work done =W =

∫ B

A

mgds

This is an elementary example of a line integral, so called because we are integrating

alongthelinefromAtoB.Inthiscaseitisstraightforwardtoevaluate.Sincebothgand

mare constants the integral becomes

W=mg

∫ B

A

ds

which equalsmg× (distancefrom AtoB).

Engineeringapplication27.1

Theworkdonebythegravitationalfield

AnobjectofmassmfallsverticallyfromAtoB.IfAisthepointwheres = 0andB

is the point wheres = 10 find the total work done by gravity as the mass falls from

AtoB.

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