Rotational Dynamics of a Grindstone

[aw2693]

Homework Statement

A grindstone is used to sharpen knives and consists of a stone cylinder and a central axle There are bearings between the axle and the frame to reduce any friction.

This grindstone has a mass of 60 kg, a diameter of 0.600 m and has a moment of Inertia (I) of 4.50 kg.m2 . There is a constant friction torque of 5N.m at the bearings.

(a) You press a knife down on the rim of the grindstone with a normal force of 50N. The coefficient of friction between the knife and the stone is 0.60. How much force must be applied to the outer edge of the grindstone to maintain a constant angular velocity of 120 revolutions per minutes.[14]

(b) How much time does it take for the grindstone to go from 120 revolutions per minute to rest if it is acted up by the bearings friction alone? [6]

Homework Equations

I=1/2mR^2

F=ma

The Attempt at a Solution

Hi, could someone please tell me how to do section a). I have never done physics before and have no real idea where to start. My lecturer assumes everyone has a good standing in the subject but I have not. This is preparation for my exam coming up so any help would be highly appreciated.

 

[Nidum]

Before anyone answers this I think we would all be interested to know how you come to have a completed masters degree , an undergraduate degree in progress and no knowledge at all of high school physics ?

 

[aw2693]

Before anyone answers this I think we would all be interested to know how you come to have a completed masters degree , an undergraduate degree in progress and no knowledge at all of high school physics ?

My background is in chemistry and I have never had to do physics before, I have gone back to university to do chemical engineering and was told my background in chemistry was good enough to get in without any qualification in physics. However I have been put on a physics course and the lecturer assumes everyone has some background in physics but I don't and he's not very clear in his explanation of things, so I'm very stuck with the more complicated questions, such as the one above and would appreciate any help.

 

[RedDelicious]

The Attempt at a Solution

Hi, could someone please tell me how to do section a). I have never done physics before and have no real idea where to start. My lecturer assumes everyone has a good standing in the subject but I have not. This is preparation for my exam coming up so any help would be highly appreciated.

This is not a valid attempt at the solution. This part is not so much a suggestion as it is a requirement for the forum, so don't be surprised if your question gets deleted.

If you have no idea where to start, begin by looking up the terms in the question that you don't understand. Solve the example problems and become comfortable with the concepts. After you've done that, come back to this problem and apply what you've learned. You should now be able to make a meaningful attempt. It is at this stage that we help you sort things out.

 

[Nidum]

I think under these circumstances we can bend the rules a bit .

Draw me a diagram of the grindstone and show the forces that you think might be acting on it . Doesn't matter if it is not quite correct . It will be somewhere for us to start from .

 

[aw2693]

I think under these circumstances we can bend the rules a bit .

Draw me a diagram of the grindstone and show the forces that you think might be acting on it . Doesn't matter if it is not quite correct . It will be somewhere for us to start from .

I've read that the Torque = moment of inertia x angular acceleration, I have velocity but there is no change in acceleration so the equation would be 0 which can't be right. Also could I use torque = force x radius and the force would just be 50N? I know what I'm trying to achieve but I just don't know how to apply all the equations/rules to get there.

 

[Nidum]

Ok . Not the diagram that I was expecting but never mind .

Part (a) of your questions is solved using a balance of torques . Do you know the definition of torque ? If not then look it up .

Now try the diagram again . Draw a plain circle to represent the grindstone and mark in the centre where the axle is . There are three torques acting on the wheel . See if you can identify and mark in what they are .