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Measuring the torsion constant of a string

A rod has a length of l = 0.30 m and a mass of 4.00 kg. A string is attached to the CM of the rod and the system is hung from the ceiling ( [link] ). The rod is displaced 10 degrees from the equilibrium position and released from rest. The rod oscillates with a period of 0.5 s. What is the torsion constant κ ?

Figure a shows a horizontal rod, length 30.0 centimeters and mass 4.00 kilograms, hanging by a string from the ceiling. The string attaches to the middle of the rod. The rod rotates with the string in the horizontal plane. Figure b shows the rod with the details needed for finding its moment of inertia. The rod’s length, end to end, is L and its total mass is M. It has linear mass density lambda equals d m d x which also equals M over L. A small segment of the rod that has length d x at a distance x from the center of the rod is highlighted. The string is attached to the rod at the center of the rod.
(a) A rod suspended by a string from the ceiling. (b) Finding the rod’s moment of inertia.


We are asked to find the torsion constant of the string. We first need to find the moment of inertia.


  1. Find the moment of inertia for the CM:
    I CM = x 2 d m = L / 2 + L / 2 x 2 λ d x = λ [ x 3 3 ] L / 2 + L / 2 = λ 2 L 3 24 = ( M L ) 2 L 3 24 = 1 12 M L 2 .
  2. Calculate the torsion constant using the equation for the period:
    T = 2 π I κ ; κ = I ( 2 π T ) 2 = ( 1 12 M L 2 ) ( 2 π T ) 2 ; = ( 1 12 ( 4.00 kg ) ( 0.30 m ) 2 ) ( 2 π 0.50 s ) 2 = 4.73 N · m .


Like the force constant of the system of a block and a spring, the larger the torsion constant, the shorter the period.

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  • A mass m suspended by a wire of length L and negligible mass is a simple pendulum and undergoes SHM for amplitudes less than about 15 ° . The period of a simple pendulum is T = 2 π L g , where L is the length of the string and g is the acceleration due to gravity.
  • The period of a physical pendulum T = 2 π I m g L can be found if the moment of inertia is known. The length between the point of rotation and the center of mass is L .
  • The period of a torsional pendulum T = 2 π I κ can be found if the moment of inertia and torsion constant are known.

Conceptual questions

Pendulum clocks are made to run at the correct rate by adjusting the pendulum’s length. Suppose you move from one city to another where the acceleration due to gravity is slightly greater, taking your pendulum clock with you, will you have to lengthen or shorten the pendulum to keep the correct time, other factors remaining constant? Explain your answer.

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A pendulum clock works by measuring the period of a pendulum. In the springtime the clock runs with perfect time, but in the summer and winter the length of the pendulum changes. When most materials are heated, they expand. Does the clock run too fast or too slow in the summer? What about the winter?

The period of the pendulum is T = 2 π L / g . In summer, the length increases, and the period increases. If the period should be one second, but period is longer than one second in the summer, it will oscillate fewer than 60 times a minute and clock will run slow. In the winter it will run fast.

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With the use of a phase shift, the position of an object may be modeled as a cosine or sine function. If given the option, which function would you choose? Assuming that the phase shift is zero, what are the initial conditions of function; that is, the initial position, velocity, and acceleration, when using a sine function? How about when a cosine function is used?

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What is the length of a pendulum that has a period of 0.500 s?

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Some people think a pendulum with a period of 1.00 s can be driven with “mental energy” or psycho kinetically, because its period is the same as an average heartbeat. True or not, what is the length of such a pendulum?

24.8 cm

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What is the period of a 1.00-m-long pendulum?

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How long does it take a child on a swing to complete one swing if her center of gravity is 4.00 m below the pivot?

4.01 s

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The pendulum on a cuckoo clock is 5.00-cm long. What is its frequency?

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Two parakeets sit on a swing with their combined CMs 10.0 cm below the pivot. At what frequency do they swing?

1.58 s

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(a) A pendulum that has a period of 3.00000 s and that is located where the acceleration due to gravity is 9.79 m/s 2 is moved to a location where the acceleration due to gravity is 9.82 m/s 2 . What is its new period? (b) Explain why so many digits are needed in the value for the period, based on the relation between the period and the acceleration due to gravity.

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A pendulum with a period of 2.00000 s in one location ( g = 9.80 m/s 2 ) is moved to a new location where the period is now 1.99796 s. What is the acceleration due to gravity at its new location?

9.82002 m/s 2

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(a) What is the effect on the period of a pendulum if you double its length? (b) What is the effect on the period of a pendulum if you decrease its length by 5.00%?

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Questions & Answers

what's the working difference between a dynamo and a pump?
Piyali Reply
a dynamo is basically a dc generator while pump is usually equipped with a motor
a dynamo converts mechanical energy to electrical while a pump is opposite to that
why sea water looks bluish?
Piyali Reply
cuz the sky is blue...
you see the reflexion of the "blue" sky in the water
somewhere sea water turns green why?
never seen bro... are u sure ?
ur answer was correct but due to the presence of phytoplankton color can be changed near the shore
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i need solutions of unuversity pbysucs volume 1
Vimla Reply
what is three dimensional coordinate system?
Sachindra Reply
considering the change of vectors in all three dimensions of space
direction co-sign of vector questions
Vipin Reply
opposite mean
what is the different between action and reaction?
Asali Reply
action is external force. Reaction exists because of the external action. Reaction is mostly an internal force
The difference is said in the word itself. There is no existance of reaction without an application of action. So action occur first then reaction. Reaction may produce in body itself or to other body i.e., it may be internal or external.
what is total energy
Israel Reply
kenetic energy + potential energy + rotational energy
any body or object can have it ..even in the molecular level
As you eat your way through a bag of chocolate chip cookies, you observe that each cookie is a circular disk of diameter 8.50 ± 0.02 cm and thickness 0.050 ± 0.005 cm. Find the average volume of a cookie and the uncertainty in the volume.
Mike Reply
how can proportionality sign can be replaced by = and propotionality constant aznd why?🙄🙄
Yanjeen Reply
because proportionality means linear relationship between the variables involved
When we say to quantities are proportional to each other, what we mean is that the variables maintain a constant ratio to each other.
let us consider an equation y= mx here y and x are variables and m is constant so whenever we divide y by x it maintains constant ratio called m , if m = 1 y=x if m doesn't equal 1 it may be other but for a closed function it is always same I.e the ratio remains constant
this applying for closed function proportionality sign can be replaced by = followed by a constant multiplier ratio
considering the propagation of a wave along the x direction on a vibrating string,how can one derive the one dimensional wave equation for the vibrating sting and the speed expression
Tebovanessapi@gmail. Reply
show whether or not the expression v^2= u^2 sin^2 d- 2gs is dimensionally constant
Oyetayo Reply
the period T of a pendulum depends on its mass m, length l and acceleration due to gravity g. using dimensional analysis, derive for T.
what is physics
Satyabrata Reply
Physics is the tool humans use to understand the properties characteristics and interactions of where they live - the universe. Thus making laws and theories about the universe in a mathematical way derived from empirical results yielded in tons of experiments.
This tool, the physics, also enhances their way of thinking. Evolving integrating and enhancing their critical logical rational and philosophical thinking since the greeks fired the first neurons of physics.
Physics is also under the category of Physical Science which deals with the behavior and properties of physical quantities around us.
Physical Science is under the category of Physics*... I prefer the most is Theoretical Physics where it deals with the philosophical view of our world.
what is unit
Satyabrata Reply
Metric unit
A unit is what comes after a number that gives a precise detail on what the number means. For example, 10 kilograms, 10 is the number while "kilogram" is the unit.
there are also different types of units, but metric is the most widely used. It is called the SI system. Please research this on google.
Unit? Bahay yon
How did you get the value as Dcd=0.2Dab
ebewele Reply
Why as Dcd=0.2Dab? where are you got this formula?...
since the distance Dcd=1.2 and the distance Dab=6.0 the ratio 1.2/6.0 gives the equation Dcd=0.2Dab
Well done.
how do we add or deduct zero errors from result gotten using vernier calliper?
Aina Reply
how can i understand if the function are odd or even or neither odd or even
I don't get... do you mean positive or negative@hamzaani
Verner calliper is an old calculator
Function is even if f(-x) =f(x)
Function is odd if f(-x) = - f(x)
Practice Key Terms 3

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Source:  OpenStax, University physics volume 1. OpenStax CNX. Sep 19, 2016 Download for free at http://cnx.org/content/col12031/1.5
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