HOW

At the given moment the wheel can be assumed to be in a purely rotational motion about the axis perpendicular to the plane of the motion and passing through the contact point,which is also called as instantaneous axis of rotation,as suggested in figure 1. Due to this many students use to think that the lowermost point has zero acceleration and the topmost point has an acceleration of magnitude (w^2)(2R)[=2.(V^2)/R],which points towards the contact point, as it is moving in a circular path of radius 2R centred at P.
But this a is very common misconception. If it is true then the centre must be possessing an acceleration of the magnitude V^2/R pointing towards the contact point P which is not true because it moves in a straight path with uniform speed so it should not have a centripetal acceleration.

So what went wrong?

Try to figure it out on your own from figs 2 and 3.

Figure 2 represents the translational part of the motion and figure 3 represents the rotational part.

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VERY NICE EXPLAINATION!!!
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