Block and Hollow sphere move down a fricitonless inclined plane

[MikeMike]

ok, you guys complained enough.

here is the full question from my test.

A hollow cylinder of mass M and radius R rolls down an inclined plane. A block of mass M slides down an identical inclined plane. If both objects are released at the same time, (ignore friction)

A. the cylinder will reach bottom first
B. the block will reach the bottom first with the greater kinetic energy.
C the block will reach the bottom first.
D. the cylinder will reach the bottom with the greater kinetic energy
E. both the block and the cylinder will reach the bottom at the same time.

 

[five40]

the fat lady from trading spouses wins

 

[spidey07]

same time.

 

[dug777]

i guess the sphere would offer less air resistance...but since you say it's hollow it would have to be MUCH bigger, so that would be negated i guess...how hollow is it?

 

[dug777]

do you mean no air resistance as well?

 

[shuan24]

the sphere because it has less wind resistance

 

[Jzero]

An African sphere, or a European sphere?

 

[dug777]

for example it it's a huge hollow paper thin sphere of course it'll go slower, what with all that nasty air in the way and all...

 

[dugweb]

same time if there is no outside force other than gravity

 

[Demon-Xanth]

If there were absolutely NO friction, yes. They would both slide down the ramp.

Usually the question is a hoop and a disk of equal weight

 

[dug777]

this thread is stupid without more details...an why the ? in the summary?

 

[cKGunslinger]

Originally posted by: five40
the fat lady from trading spouses wins

Demons!

 

[glen]

Originally posted by: Demon-Xanth
If there were absolutely NO friction, yes. They would both slide down the ramp.

Yep.
And, normally you ignore air resistance,so same time.
With wind resistance, sphere first.

 

[thraashman]

it should be the same time because if two things of equal mass are dropped at the same time and there is nothing but gravity affecting them they should accelerate at the same rate.

 

[speg]

I guess no one took physics? Moment of inertia ftw!

 

[Armitage]

No friction at all, including air resitance - they reach the bottom at the same time. Or more precisely, the CG of each object will travel the same vertical distance in the same time.

 

[Armitage]

Originally posted by: speg
I guess no one took physics? Moment of inertia ftw!

If there is no friction at all the sphere will slide instead of rolling. no spin = no moment of inertia.

If you specify that the sphere rolls, then the block gets their first, but they both have the same kinetic energy as a function of time. The blocks KE is just translational, the spheres KE is translational & rotational.

 

[TuxDave]

Originally posted by: Armitage

Originally posted by: speg
I guess no one took physics? Moment of inertia ftw!

If there is no friction at all the sphere will slide instead of rolling. no spin = no moment of inertia.

Dammit.... I didn't catch that.

 

[mryellow2]

Originally posted by: speg
I guess no one took physics? Moment of inertia ftw!

How does that apply here? The cylinder will be sliding down an incline with zero friction aka, no rolling motion involved.


edit:Hrm, damn you fast posting people!!! /shakesfist

 

[speg]

that's very true, my bad I skipped that when I read the first part. That's a lame question then!

Edit: Then again, it doesn't say it's frictionless, it just says to ignore friction. So that means that there is frction? And that means the cylinder is going to roll?!

 

[hellokeith]

This was a test question on one of our physics exams, and it had to be tossed because the teacher told us to "ignore friction", which most of us did and deducted that both would slide down the slope simultaneously.

Physics, like any other science, is an approximation of the real world. Don't get too caught up in it.

 

[MikeMike]

The answer is C.

so says Purdue.

 

[wfbberzerker]

Originally posted by: Jzero
An African sphere, or a European sphere?

african spheres are non-migratory

 

[HardcoreRobot]

ignoring friction, they arrive at the same time

 

[Jzero]

Originally posted by: wfbberzerker

Originally posted by: Jzero
An African sphere, or a European sphere?

african spheres are non-migratory

That leads to the next question...how long is the inclined plane, and at what distance does the movement constitute a full-fledged migration? Is that actual distance, or horizontal distance?