Moment of inertia and rotational speed of a ball

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Discussion Overview

The discussion revolves around calculating the rotational speed of a ball in a simulation, focusing on the moment of inertia, torque, and the effects of friction during ball collisions with walls. Participants explore the relationships between linear and angular motion, as well as the implications of applying forces at different points on the ball.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Homework-related

Main Points Raised

  • One participant seeks to calculate rotational speed using the moment of inertia formula for a ball and expresses uncertainty about its application.
  • Another participant provides the kinetic energy formula for rotating objects and explains how to calculate torque using the force applied and the radius.
  • Questions arise about the nature of angular acceleration, specifically whether it is measured in radians and the meaning of the cross product in this context.
  • There is a discussion about calculating the normal force when the ball collides with the wall at an angle, suggesting the use of trigonometric components to find the effective force.
  • A participant expresses confusion regarding impulse and its calculation, particularly in the context of a simulation step.
  • One participant draws analogies between translational and rotational dynamics, suggesting that understanding these parallels could aid in grasping the concepts involved.
  • Another participant shares links to educational resources that may help deepen understanding of the relevant physics concepts.
  • There is a realization that pulling an object from different points (corner vs. center of mass) does not change the fundamental principles at play.
  • Participants express willingness to share and collaborate on the simulation project, with one offering to provide further assistance if needed.

Areas of Agreement / Disagreement

Participants generally agree on the relationships between linear and angular motion, but there remains some uncertainty regarding specific calculations and the application of concepts like impulse and friction in the context of the simulation.

Contextual Notes

Some participants express confusion about the definitions and calculations related to impulse and friction, indicating that further clarification may be necessary. The discussion also reflects varying levels of familiarity with the underlying physics concepts.

Who May Find This Useful

This discussion may be useful for individuals interested in physics simulations, particularly those focusing on rotational dynamics, collision mechanics, and the application of forces in game development.

deluksic
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Hey I'm making some kind of simulation... (in game maker) Ball collisions, ball to wall(friction)etc.

So I'm asking: how do i calculate rotational speed if i know r, mass, and distance to where is force being applyed?

I know that moment of inertia for a ball is 2/3*m*r2 and have no idea how to use it...

friction is also a problem.. i don't know how to calculate friction with wall if i know friction koeficient...

(sry for any mistake, I am not from england)
 
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The kinetic energy of a rotating object is KE=(Iw^2)/2, where I is the moment of inertia about the centroid of the object and w is the angular velocity. If you have a force being applied to the outside of the ball, you can get the torque by using T=r x F. You can then use T=I*a, where the I is the same I as before and a is the angular acceleration.

The frictional force is given as F=mu*N, where mu is the coefficient of friction and N is the force of the wall pushing back on the object.

Hope this gets you started.
 
hmm ill try to understand all that, put it in code than tell you...
 
few questions...

is that aceleration in radians or degrees?
x is cross product?

that frictional force is maximal friction and i need actuall friction when ball hit the wall with speed on an angle...
 
I believe the acceleration is in radians. And yes, the x is cross product.

AFAIK, you'll have to do an impulse force calculation or something to solve for the normal force of the wall because the force the ball is hitting with wall with is changing with time. The normal force is the force perpendicular to the wall, so if the ball is hitting the wall with a force at an angle, you can just get the component in the direction perpendicular to the wall using a F cos theta or F sin theta (however you're measuring your theta). That will be the normal force and you can sub that into the F=mu*N.
 
yes i know that... but that IMPULSE is confusing me... impulse is actually F=(m/\v)/t right?
t in my program is actually one step of simulation so force that will be applying is just mv?

also i managed to make it out with rotations now how do i calc that actuall speed of center of mass when force is being applyed? it can't be the same if u pull an object on a corner or in center of mass...
 
Rotating rigid bodies are world for it self. But here are some helpful tips.

I believe that u are familiar with dynamics, Newton etc. In translation-like motions you have mass (m) and speed (v).

Now think of it this way. Physics has to work in the same laws, little bit different and adjusted but same in their intention.

What is mass for translation, that is moment of inertia for rotating bodies. What is speed for translation(v) that is angular speed for rotating bodies (w). What is force for translation, that is torque for rotating bodies.

same for momentum, acceleration etc. These analogies could help you start.

if you want to study this, and really learn it here are some helpful links.

http://ocw.mit.edu/courses/physics/8-01-physics-i-classical-mechanics-fall-1999/video-lectures/lecture-19/"
http://ocw.mit.edu/courses/physics/8-01-physics-i-classical-mechanics-fall-1999/video-lectures/lecture-20/"
http://ocw.mit.edu/courses/physics/8-01-physics-i-classical-mechanics-fall-1999/video-lectures/lecture-21/"

After these 3 50 minutes lectures, i think you will have all you need for you project, there is more if you are interested
 
Last edited by a moderator:
now i see that it is totally the same if i pull something for a corner or in center of mass thanks! That MIT teacher is very good! (and funny xD..)
 
deluksic said:
now i see that it is totally the same if i pull something for a corner or in center of mass thanks! That MIT teacher is very good! (and funny xD..)

Pulling an object to make it move only depends of the angle of that force and the direction of the movement.
 
  • #11
deluksic said:
yes if ur interested u can download my first test of it to see what it looks like :D
here is my download:

https://sites.google.com/site/deluksicgames/contact-us

This is awesome ! If you need anything else, i am very familiar with classical physics and i know a little bit of programming, if you need any help further PM me.
 
  • #12
ok! I am glad u like it! :D
 

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