<< Chapter < Page Chapter >> Page >
This figure has two images. The first is labeled “a” and is a circle with radius r. The center of the circle is labeled 0. The circle also has the positive x-axis beginning at 0, extending through the circle. The second figure is labeled “b”. It has two concentric circles with center at 0 and the x-axis extending out from 0. The concentric circles form a washer. The width of the washer is from xsub(i-1) to xsubi and is labeled delta x.
(a) A thin disk in the xy -plane. (b) A representative washer.

We now approximate the density and area of the washer to calculate an approximate mass, m i . Note that the area of the washer is given by

A i = π ( x i ) 2 π ( x i 1 ) 2 = π [ x i 2 x i 1 2 ] = π ( x i + x i 1 ) ( x i x i 1 ) = π ( x i + x i 1 ) Δ x .

You may recall that we had an expression similar to this when we were computing volumes by shells. As we did there, we use x i * ( x i + x i 1 ) / 2 to approximate the average radius of the washer. We obtain

A i = π ( x i + x i 1 ) Δ x 2 π x i * Δ x .

Using ρ ( x i * ) to approximate the density of the washer, we approximate the mass of the washer by

m i 2 π x i * ρ ( x i * ) Δ x .

Adding up the masses of the washers, we see the mass m of the entire disk is approximated by

m = i = 1 n m i i = 1 n 2 π x i * ρ ( x i * ) Δ x .

We again recognize this as a Riemann sum, and take the limit as n . This gives us

m = lim n i = 1 n 2 π x i * ρ ( x i * ) Δ x = 0 r 2 π x ρ ( x ) d x .

We summarize these findings in the following theorem.

Mass–density formula of a circular object

Let ρ ( x ) be an integrable function representing the radial density of a disk of radius r . Then the mass of the disk is given by

m = 0 r 2 π x ρ ( x ) d x .

Calculating mass from radial density

Let ρ ( x ) = x represent the radial density of a disk. Calculate the mass of a disk of radius 4.

Applying the formula, we find

m = 0 r 2 π x ρ ( x ) d x = 0 4 2 π x x d x = 2 π 0 4 x 3 / 2 d x = 2 π 2 5 x 5 / 2 | 0 4 = 4 π 5 [ 32 ] = 128 π 5 .
Got questions? Get instant answers now!
Got questions? Get instant answers now!

Let ρ ( x ) = 3 x + 2 represent the radial density of a disk. Calculate the mass of a disk of radius 2.

24 π

Got questions? Get instant answers now!

Work done by a force

We now consider work. In physics, work is related to force, which is often intuitively defined as a push or pull on an object. When a force moves an object, we say the force does work on the object. In other words, work can be thought of as the amount of energy it takes to move an object. According to physics, when we have a constant force, work can be expressed as the product of force and distance.

In the English system, the unit of force is the pound and the unit of distance is the foot, so work is given in foot-pounds. In the metric system, kilograms and meters are used. One newton is the force needed to accelerate 1 kilogram of mass at the rate of 1 m/sec 2 . Thus, the most common unit of work is the newton-meter. This same unit is also called the joule . Both are defined as kilograms times meters squared over seconds squared ( kg · m 2 / s 2 ) .

When we have a constant force, things are pretty easy. It is rare, however, for a force to be constant. The work done to compress (or elongate) a spring, for example, varies depending on how far the spring has already been compressed (or stretched). We look at springs in more detail later in this section.

Suppose we have a variable force F ( x ) that moves an object in a positive direction along the x -axis from point a to point b . To calculate the work done, we partition the interval [ a , b ] and estimate the work done over each subinterval. So, for i = 0 , 1 , 2 ,… , n , let P = { x i } be a regular partition of the interval [ a , b ] , and for i = 1 , 2 ,… , n , choose an arbitrary point x i * [ x i 1 , x i ] . To calculate the work done to move an object from point x i 1 to point x i , we assume the force is roughly constant over the interval, and use F ( x i * ) to approximate the force. The work done over the interval [ x i 1 , x i ] , then, is given by

Questions & Answers

determine the area of the region enclosed by x²+y=1,2x-y+4=0
Gerald Reply
Hi
MP
Hi too
Vic
hello please anyone with calculus PDF should share
Adegoke
Which kind of pdf do you want bro?
Aftab
hi
Abdul
can I get calculus in pdf
Abdul
How to use it to slove fraction
Tricia Reply
Hello please can someone tell me the meaning of this group all about, yes I know is calculus group but yet nothing is showing up
Shodipo
You have downloaded the aplication Calculus Volume 1, tackling about lessons for (mostly) college freshmen, Calculus 1: Differential, and this group I think aims to let concerns and questions from students who want to clarify something about the subject. Well, this is what I guess so.
Jean
Im not in college but this will still help
nothing
how can we scatch a parabola graph
Dever Reply
Ok
Endalkachew
how can I solve differentiation?
Sir Reply
with the help of different formulas and Rules. we use formulas according to given condition or according to questions
CALCULUS
For example any questions...
CALCULUS
what is the procedures in solving number 1?
Vier Reply
review of funtion role?
Md Reply
for the function f(x)={x^2-7x+104 x<=7 7x+55 x>7' does limx7 f(x) exist?
find dy÷dx (y^2+2 sec)^2=4(x+1)^2
Rana Reply
Integral of e^x/(1+e^2x)tan^-1 (e^x)
naveen Reply
why might we use the shell method instead of slicing
Madni Reply
fg[[(45)]]²+45⅓x²=100
albert Reply
find the values of c such that the graph of f(x)=x^4+2x^3+cx^2+2x+2
Ramya Reply
anyone to explain some basic in calculus
Adegoke Reply
I can
Debdoot
A conical container of radius 10 ft and height 30 ft is filled with water to a depth of 15 ft. How much work is required to pump all the water out through a hole in the top of the container if the unit weight of the water is 62.4 lb/ft^3?
Milca Reply
hi am new here I really wants to know how the solve calculus
IBRAHIM
me too. I want to know calculation involved in calculus.
Katiba
evaluate triple integral xyz dx dy dz where the domain v is bounded by the plane x+y+z=a and the co-ordinate planes
BAGAM Reply
So how can this question be solved
Eddy
i m not sure but it could be xyz/2
Leo
someone should explain with a photo shot of the working pls
Adegoke
I think we should sort it out.
Eunice
Eunice Toe you can try it if you have the idea
Adegoke
how
Eunice
a^6÷8
Muzamil
i think a^6 ÷ 8
Muzamil
Practice Key Terms 4

Get the best Calculus volume 1 course in your pocket!





Source:  OpenStax, Calculus volume 1. OpenStax CNX. Feb 05, 2016 Download for free at http://cnx.org/content/col11964/1.2
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Calculus volume 1' conversation and receive update notifications?

Ask