Re: [Phys-l] block sliding on a skateboard

[Bernard Cleyet <bernardcleyet@redshift.com>]

I forgot to include the mass of the scateboard.  If large then the mass will slide.   Now I read its mass is 10 kg


On 2012, Feb 03, , at 17:55, LaMontagne, Bob wrote:


bc needs to back to K to relearn reading, and then redo the problem.

p.s. So one assumes no rolling friction, which in my experience is a good approximation, especially for only an ~ 10N normal force.


> Bernard - 
>
> Unless I have screwed something up badly here - the 4N friction force would only accelerate the 10 kg skateboard to 0.4 m/s2. Unless I am really having a mental block here, the block would still slide. The skateboard only moves about 5 cm while the block moves somewhere around 50.
>
> Maybe I'm missing something obvious - it's happened before!!
>
> :-)
>
> Bob LaMontagne


p.s. My analysis is to treat it as an inelastic colision using p conservation.  If I'm correct that part is a plug and chug.

The combo goes at 2/11 m/s.    (forever, according to Galileo)  Yes it's a good prob.  I'm only as far as the initial acceleration of the cart is 0.4 m's^2  .

More later.


> ________________________________________
> From: phys-l-bounces@carnot.physics.buffalo.edu [phys-l-bounces@carnot.physics.buffalo.edu] on behalf of Bernard Cleyet [bernardcleyet@redshift.com]
> Sent: Friday, February 03, 2012 7:14 PM
> To: Forum for Physics Educators
> Subject: Re: [Phys-l] block sliding on a skateboard
>
> On 2012, Feb 03, , at 15:58, LaMontagne, Bob wrote:
>
> > Here is a simple problem I proposed to my general physics students that has generated a lot of discussion by them despite the problem's simplicity. I will throw it out here for your amusement.
> >
> > Lock the wheels of a long skateboard so it won't move. Now take a 1 kg mass and drop it from a short distance onto the skateboard with a horizontal speed of 2 m/s. The mass slides along the surface of the board and stops after a half meter.
> >
> > Now unlock the wheels and repeat the experiment. How far does the mass now slide relative to the surface of the skateboard before it stops sliding? Take the mass of the skateboard to be 10 kg. How far does it slide relative to the ground?
> >
> > Bob at PC
>
>
> I must be stupid, why would anyone think it'l slide?** -- the coefficient of friction mass--skate board is much greater than that of the skateboard--sidewalk.
>
> Assuming friction constant it's 4N (mass skateboard) and the coefficient is ~ 0.4
>
> bc worried
>
>
> ** unless the board is on an incline or gravel
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