# Rotational inertia & tangential force

## Homework Statement

A horizontal disc of diameter 10 cm is spinning freely about a vertical axis
through its centre at an angular speed of 80 revolutions per minute. A piece
of putty of mass 7.0 g drops on to and sticks to the disc a distance of 3.0 cm
from the centre. The angular speed reduces to 65 revolutions per minute.

Calculate the moment of inertia of the disc.

I= MR2

## The Attempt at a Solution

R= 0.03
M= 0.007

but if i use the formula I= MR2,,,, what about the informaton they have given of rev/ min this is the angular speed....

not sure the method to do this???

Doc Al
Mentor
conservation

When the putty is dropped onto the rotating disc, some things change and some things remain the same. What stays the same?

mass remains same, diamiter remains the same
angular velocity decreases

Doc Al
Mentor
what's conserved?

mass remains same, diamiter remains the same
angular velocity decreases
All true, but what important dynamical quantity is conserved?

SGT
mass remains same, diamiter remains the same
angular velocity decreases

Mass of the disk remains the same, but the mass and moment of inertia of disk + putty is not the same.

so calculating this....

is my formula corect?

hage567
Homework Helper
What formula are you referring to?? I=MR^2 is correct for the putty if that's what you mean. But you haven't yet taken into account the moment of inertia of the disk.

mass remain the same-

how do i calcualte the moment of inertia...with the informaiton given>>

Doc Al
Mentor
mass remain the same-
More importantly, angular momentum remains the same. Compare the angular momentum of the system before and after the putty is dropped.
how do i calcualte the moment of inertia...with the informaiton given>>
Since the moment of inertia of the disc is what you are trying to find, just call it I_d. But realize that the moment of inertia of "disc + putty" is the sum of the individual moments of inertia. You can calculate the moment of inertia of the putty using I_p = MR^2.

tangential force

## Homework Statement

A horizontal disc of diameter 10 cm is spinning freely about a vertical axis
through its centre at an angular speed of 80 revolutions per minute. A piece
of putty of mass 7.0 g drops on to and sticks to the disc a distance of 3.0 cm
from the centre. The angular speed reduces to 65 revolutions per minute.

## Homework Equations

A constant tangential force is now applied to the rim of the disc
which brings the disc to rest in 8.0 s. Calculate the magnitude of
this force.

## The Attempt at a Solution

need help..dont know how to start

cristo
Staff Emeritus
What do you know about circular motion? You are given the angular speed of the disc, and the time in which it is brought to rest. Can you see how this will help you find the force?

Please show some work, even if it's just relevant equations/ knowledge from your text or course notes. You must know how this forum works by now!

didnt have a clue wear to start,,thats for the tip...

f=m*r*w^2

= 7000*0.05*w^2

w= 80-65 / 2= 7.5

f= 3.5*7.5^2
= 196.875N

can anyone tell me if this correct?

hage567
Homework Helper
No, that doesn't look right.

How did you get 7000 for the mass? You shouldn't even be using mass here, you should be using moment of inertia, I. Why are you solving for w? You know that already. You're dealing with angular momentum and torque, right? How do you think you can relate things like force, torque, and angular acceleration?

Also, be careful of your units.

torque= force *lever arm

angular acceleration= a/r

a=acceleration

a= v-u/ t

change in velocity= 65-80= -15 rev/ min

-15 rev/min= -15 rev every 60 sec = -0.25 rev/ sec

-0.25/8= -0.03125 rev/sec^2

angular acceleration= -0.03125 rev/sec^2

tangential force= mass*angular acceleration
= 0.07 * -0.03125
0.0021875 N

hage567
Homework Helper
torque= force *lever arm

angular acceleration= a/r

a=acceleration

a= v-u/ t

change in velocity= 65-80= -15 rev/ min

-15 rev/min= -15 rev every 60 sec = -0.25 rev/ sec

-0.25/8= -0.03125 rev/sec^2

angular acceleration= -0.03125 rev/sec^2

tangential force= mass*angular acceleration
= 0.07 * -0.03125
0.0021875 N

OK first of all, the disc is coming to a STOP when the force is applied, so that means your final angular velocity is ZERO. Take the time to look at what the question is asking you!

It is best to convert angular velocities into rad/sec.

You STILL didn't convert 7g to kg properly, but you need to consider more than just the mass, you are dealing with rotational quantities.

Remember that torque can be expressed as

$$\tau = I\alpha$$

where I is the moment of inertia. That is what you need to consider here (I pointed this out in my last post as well). Use this along with the first equation you listed. I know there was a first part to this question that you posted before, so you should already know the moment of inertia you need to use to solve this.

7g= 7kg/1000
= 0.007kg

Torque= I*alpha..

what is alpha...

the quesiton also ask to calcualte tan...force...why do we need torque?

Hootenanny
Staff Emeritus
Gold Member
Torque= I*alpha..

what is alpha...
alpha is the angular acceleration.
the quesiton also ask to calcualte tan...force...why do we need torque?
What is torque... hage567
Homework Helper
alpha is the angular acceleration. The force is creating a torque on the disc that is bringing it to a stop.

is torque a force.......

if so,

then

torque= I* alpha

I= MR^2

angular accelereation= change in angular velocity/ time
= (65- 80) / 8= -1.875 rev/sec^2

Hootenanny
Staff Emeritus
Gold Member
is torque a force.......
Torque is not a force. A torque is produced when the applied force does not pass through the centre of mass of the body.
I= MR^2
This is the moment of inertia fo a disc; however, the disc has a point mass located 0.03m from the axis of rotation. You must take this into account and calculate the moment of inertia for the composite body.

hage567
Homework Helper
is torque a force.......

if so,

then

torque= I* alpha

I= MR^2

angular accelereation= change in angular velocity/ time
= (65- 80) / 8= -1.875 rev/sec^2

WHY are you STILL using 65 and 80?!!!

The question says:

A constant tangential force is now applied to the rim of the disc
which brings the disc to rest in 8.0 s. Calculate the magnitude of this force.

That means it goes from 65 rev/min to ZERO in 8 seconds!!
You are NOT supposed to be finding the deceleration for when the putty falls on the disc, if that's what you're doing.

Also realize that the units of the angular speed are given in rev/min. The time interval the disc take to stop is 8 seconds. Your units are not consistent and make no sense.

moment of intertia of putty= MR^2
moment of intertia of disc= 0.5MR^2

angular momentum is converved

disc-
M=?
R=0.12m

moment of interia of disc=0.0072M

putty-
M=0.005
R=0.4

moment of inertia of putty= 0.000008

--------------
need help if i can do more to calculate the moment of inertia of disc.

hage567
Homework Helper
moment of intertia of putty= MR^2
moment of intertia of disc= 0.5MR^2

angular momentum is converved

disc-
M=?
R=0.12m

moment of interia of disc=0.0072M

putty-
M=0.005
R=0.4

moment of inertia of putty= 0.000008

--------------
need help if i can do more to calculate the moment of inertia of disc.

You need to use conservation of angular momentum to find the moment of inertia of the disc. You still haven't done that. You don't have to find the mass of disc explicitly to solve this problem.

So if angular momentum is $$L = I\omega$$, for conservation you must have

$$L_i=L_f$$

so $$I_i\omega_i=I_f\omega_f$$

So initially, I_i only consists of the moment of inertia of the disc, I_d. Once the putty hits the disc, what will be the new total moment of inertia?

Read Doc Al's post carefully (#9).

Doc Al
Mentor
Note to mentors: Perhaps we could merge these two threads ...
Threads merged. Note to imy786: Don't start multiple threads on the same problem. Don't start a thread about part 2 of a problem when you haven't finished part 1 (especially when you need to solve part 1 first).

hage567
Homework Helper

It's wrong. None of the numbers you used in post #23 for mass or radius are given in the question, so where in the world did you get them from?

L= Iw

angular speed=w= 72 rev/ minute....need to change this rad/sec

72*2pi / min
144pi/ min
= 144pi/ 60 secs

I1W1= I2w2

I1 = MR^2

hage567
Homework Helper
L= Iw

angular speed=w= 72 rev/ minute....need to change this rad/sec

72*2pi / min
144pi/ min
= 144pi/ 60 secs

I1W1= I2w2