If the scooter does not slip to the plane, path of both wheels must be parallel with its direction.

From the picture above, we can get

Hence, the radius of CM's path is

So the total friction force must equal to

Hence, the x and y component of the friction is

Now, draw the friction forces work on both wheels. Each friction has perpendicular direction to each wheel. Sum of these two frictions must equal to the total friction (both in x and y direction).

First, for y-direction,

From equation (5), we can get

Then for x-direction,

Substitute fx from equation (4) and f2 from equation (6),

From equation (6) and (7) above, we can see that f2 is greater than f1, so the coefficient of friction must equal to

For the normal forces, because the CM of the scooter is at the middle, we can say that

And the total normal forces must equal to its weight, so

Hence,

Back to coefficient of friction. From equation (6) and (8), we can get the coefficient of friction,

That's my solution.

If you have any question, correction, or comment, please leave a comment below.

Thank you.

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Excellent

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