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Rotational inertia and moment of inertia

Before we can consider the rotation of anything other than a point mass like the one in [link] , we must extend the idea of rotational inertia to all types of objects. To expand our concept of rotational inertia, we define the moment of inertia     I size 12{I} {} of an object to be the sum of mr 2 size 12{ ital "mr" rSup { size 8{2} } } {} for all the point masses of which it is composed. That is, I = mr 2 size 12{I= Sum {} ital "mr" rSup { size 8{2} } } {} . Here I size 12{I} {} is analogous to m size 12{m} {} in translational motion. Because of the distance r size 12{r} {} , the moment of inertia for any object depends on the chosen axis. Actually, calculating I size 12{I} {} is beyond the scope of this text except for one simple case—that of a hoop, which has all its mass at the same distance from its axis. A hoop’s moment of inertia around its axis is therefore MR 2 size 12{ ital "MR" rSup { size 8{2} } } {} , where M size 12{M} {} is its total mass and R size 12{R} {} its radius. (We use M size 12{M} {} and R size 12{R} {} for an entire object to distinguish them from m size 12{m} {} and r size 12{r} {} for point masses.) In all other cases, we must consult [link] (note that the table is piece of artwork that has shapes as well as formulae) for formulas for I size 12{I} {} that have been derived from integration over the continuous body. Note that I size 12{I} {} has units of mass multiplied by distance squared ( kg m 2 size 12{"kg" cdot "m" rSup { size 8{2} } } {} ), as we might expect from its definition.

The general relationship among torque, moment of inertia, and angular acceleration is

net τ = size 12{τ=Iα} {}

or

α = net τ I , size 12{α= { { ital "net"τ} over {I} } ","} {}

where net τ size 12{τ} {} is the total torque from all forces relative to a chosen axis. For simplicity, we will only consider torques exerted by forces in the plane of the rotation. Such torques are either positive or negative and add like ordinary numbers. The relationship in τ = α = net τ I size 12{τ=Iα,`````α= { { ital "net"τ} over {I} } } {} is the rotational analog to Newton’s second law and is very generally applicable. This equation is actually valid for any torque, applied to any object, relative to any axis.

As we might expect, the larger the torque is, the larger the angular acceleration is. For example, the harder a child pushes on a merry-go-round, the faster it accelerates. Furthermore, the more massive a merry-go-round, the slower it accelerates for the same torque. The basic relationship between moment of inertia and angular acceleration is that the larger the moment of inertia, the smaller is the angular acceleration. But there is an additional twist. The moment of inertia depends not only on the mass of an object, but also on its distribution of mass relative to the axis around which it rotates. For example, it will be much easier to accelerate a merry-go-round full of children if they stand close to its axis than if they all stand at the outer edge. The mass is the same in both cases; but the moment of inertia is much larger when the children are at the edge.

Take-home experiment

Cut out a circle that has about a 10 cm radius from stiff cardboard. Near the edge of the circle, write numbers 1 to 12 like hours on a clock face. Position the circle so that it can rotate freely about a horizontal axis through its center, like a wheel. (You could loosely nail the circle to a wall.) Hold the circle stationary and with the number 12 positioned at the top, attach a lump of blue putty (sticky material used for fixing posters to walls) at the number 3. How large does the lump need to be to just rotate the circle? Describe how you can change the moment of inertia of the circle. How does this change affect the amount of blue putty needed at the number 3 to just rotate the circle? Change the circle’s moment of inertia and then try rotating the circle by using different amounts of blue putty. Repeat this process several times.

Questions & Answers

What is specific heat capacity?
hamidat Reply
Specific heat capacity is the amount of heat required to raise the temperature of one (Kg) of a substance through one Kelvin
Paluutar
formula for measuring Joules
Rowshan Reply
I don't understand, do you mean the S.I unit of work and energy?
hamidat
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ADAMS Reply
What limits the Magnification of an optical instrument?
Naeem Reply
Lithography is 2 micron
Venkateshwarlu
what is expression for energy possessed by water ripple
Prabesh Reply
what is hydrolic press
Mark Reply
An hydraulic press is a type of machine that is operated by different pressure of water on pistons.
hamidat
what is dimensional unite of mah
Patrock Reply
i want jamb related question on this asap🙏
sharon Reply
What is Boyles law
Pascal Reply
it can simple defined as constant temperature
Muhammad
Boyles law states that the volume of a fixed amount of a gas is inversely proportional to the pressure acting on in provided that the temperature is constant.that is V=k(1/p) or V=k/p
FADILAT
what is motion
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getting notifications for a dictionary word, smh
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the minimum thrust that an object must have in oder yo escape the gravitational pull
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Mua
what is a atom
ADAMS
how to calculate tension
Deena Reply
what are the laws of motion
Mua
what is force
Ugwu Reply
Force is any quantity or a change that produces motion on an object body.
albert
A force is a push or a pull that has the tendency of changing a body's uniform state of rest or uniform state of motion in a straight line.
nicholas
plsoo give me the gravitational motion formulas
Okoye
f=Gm/d²
FADILAT
What is the meaning of emf
Chinedu Reply
electro magnetic force
shafiu
Electromotive force (emf) is a measurement of the energy that causes current to flow through a circuit.
Darssini
tritium (gas, netrogen, cloud, lamp)
firdaus Reply
Continue - > tritium (gas, netrogen, cloud, lamp span, lamp light, cool)
firdaus
Practice Key Terms 3

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Source:  OpenStax, College physics. OpenStax CNX. Jul 27, 2015 Download for free at http://legacy.cnx.org/content/col11406/1.9
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