Which attains more height and why?

1. Feb 2, 2010

Shahid0072

Hi..A disk A and disk b are rolling without slipping with same speed on a horizonal surface..Disc a rolls up an incline and disk b rolls up an incline except that it is fricitionless! Which disk attains more height nw why?

2. Feb 2, 2010

Staff: Mentor

Sounds like homework. What do you think?

3. Feb 2, 2010

curiousphoton

Try relating your Physics homework problems to your real life. Maybe you like to bowl. Imagine a bowling lane with an incline at the end instead of pins. Case A - You bowl your ball on a normal bowling lane at normal strength. Case B - You bowl your ball on asphalt at normal strength.

Once you've pictured a real world situation ask yourself simple questions:

(1) Which lane has more friction, Case A or Case B?
(2) Which ball will make it higher up the incline, Case A or Case B?

Put the two together and WahhhhLA. You understand a little bit of Mr. Newton

4. Feb 2, 2010

cesiumfrog

Cute problem... obviously the "without slipping" part is limited.

5. Feb 3, 2010

Shahid0072

Thanks for replying guys.Are you saying that in Case of disk A Both translational and rotational kinetic energies will be converted into potential energy..But in case of disk b,only translation kinetic energy will be converted to potential energy as there is no friction to support rotation!! But where is that rotational kinetic energy go? Will it be wasted? If yes,how? There is no dissipative force present!

6. Feb 3, 2010

Staff: Mentor

Without friction, the rotational energy doesn't go anywhere. The disk keeps rotating at whatever angular velocity it had when it entered the frictionless surface.

7. Feb 3, 2010

near

theoretically speaking based from the situation disc B would certainly the one that would gain the greater height because according to newton's 1st law of motion, "a body that stays at rest remains at rest and if it is moving, it will move at a constant speed." in this case disc B will experience no external forces that may cause it to accelerate nor to deccelerate like frictional force which is present on disc A. thus it will move at a constant speed and it will never stop until reaches the highest point.

8. Feb 3, 2010

Staff: Mentor

You forgot about gravity. And don't forget that in some situations--like this one--friction can actually help you go higher.

9. Feb 3, 2010

dE_logics

Under ideal situations both will attain the same height considering the fact that the disks are not accelerating/de accelerating by virtue of their rotating motion.

When A goes up that slope, it rolls and so there's no friction stopping it's motion...only gravity is doing that. Same is the case with disk B.

10. Feb 3, 2010

Staff: Mentor

When they hit the slope, A experiences friction but B does not. That makes a difference.

11. Feb 3, 2010

Shahid0072

thanks for replying all...doc is right ...friction helps in disc A in attatining more height as rotational kinetic energy is utilised and in case of disk B only translation kinetic energy is utilised....doc Al ..can u please correct me if i am wrong!!
when disk A of mass M goes up an incline,equations of motion are (assuming friction is enough for rolling without slipping)
translation:Mg-friction(f)=ma
rotation:fR=mR^2/2 * a/r

12. Feb 3, 2010

Shahid0072

and friction is acting upwards..

13. Feb 3, 2010

Staff: Mentor

You are correct.
The only change I'd make would be to account for the angle of the incline: Use mg sinÎ¸, instead of mg.
Absolutely.

14. Feb 3, 2010

Shahid0072

oh yea ..mg sinx ..m silly

15. Feb 3, 2010

Shahid0072

thank you so much...

16. Feb 3, 2010

Shahid0072

doc al..sir just in case you are not super busy...it will be great if you could reply to my angular momentum thread..