How Do You Calculate the Linear Speed of a Rolling Disc?

[public_enemy720]

One more problem on a tough worksheet...I have tried it for a while, but can't find an equation(s) suitable for the problem...

A constant force of 12N is applied to the axle of a disc rolling along a flat plane. The disc has mass m=22kg, diameter D=.50m, and rotational inertia I=.688kgm^2. What is the linear speed of the center of the disc, V, after it has rolled for 12m?

I drew a free body diagram, and I know that the sum of the moments is equal to I times alpha, and I know the good ol' F=ma. But I can't seem to be able to find an acceleration with what I am given. A hint would be wonderful.

 

[public_enemy720]

Oh boy...I went somewhere, but I don't know if it was right. Here is what I did:

I used m*a=f-f(friction) and friction*radius = I*alpha. Alpha is equal to a*r, so I plugged that into make friction times radius = I*a*radius.

Plugging that into m*a=F-f(friction) i got:

m*a=F-((I*alpha*radius)/(radius))

This is rather confusing, but I think it is somewhat correct. Any suggestions or gross errors on my part?

 

[OlderDan]

Oh boy...I went somewhere, but I don't know if it was right. Here is what I did:

I used m*a=f-f(friction) and friction*radius = I*alpha. Alpha is equal to a*r, so I plugged that into make friction times radius = I*a*radius.

Plugging that into m*a=F-f(friction) i got:

m*a=F-((I*alpha*radius)/(radius))

This is rather confusing, but I think it is somewhat correct. Any suggestions or gross errors on my part?

Right idea, but alpha does not equal a*r. alpha and a are certainly related and you need that relationship, but that is not it.

 

[vijay123]

it is...acceleration=radius x alpha

 

[radou]

it is...acceleration=radius x alpha

a_{T}=\alpha \cdot r, i.e. tangential acceleration equals angular acceleration times radius.

 

[minhtrietle]

I hope this can help you:

total K = (1/2)*m*v^2 + (1/2)*I*w^2

actually you don't have to use I = 0.688 which is given if you know the I of the disk is (1/2)*m*r^2.

plug all in, find out K = 3/4 * m * v^2

next,

the work does on the wheel also equals to the total K above. W = F*s = 12*12 = 144J

solve for v.

Good luck.

Minh T. Le