About vertical circular motion I have two cases here.
In the first case, we have a body moving at the speed shown. As we know the normal force on it due to the surface under it is just 9.8 * 10 = 98 Newtons.
Now, in the second case, the body is moving in a vertical circle. But at that instant at the top, the velocity vector is still pointing exactly to the right (just like in the first case). Why then would the normal force be different?
It seems to me that at that instant when the body is at the top of the circle, both cases are identical.

 A: Force and acceleration are vectors.  I assume the magnitude of the velocity is constant for both of your cases in the question.
For the first case (purely horizontal motion) the velocity vector is in the horizontal direction and does not change direction or magnitude, so the horizontal acceleration is zero. That means the net horizontal force to the right (applied force minus friction) is zero. (Of course some net force to the right had to be applied initially to accelerate the body from rest to the constant velocity.)  There is no motion in the vertical direction, since the vertical velocity is zero.  The net force upwards (normal minus gravity) is zero.  In this case the upward normal force merely counteracts gravity to keep the mass fixed in the vertical direction.
For the second case, uniform circular motion, the velocity vector has constant magnitude but changes direction as the mass moves around the circle. The motion is easier to visualize and evaluate using polar coordinates.  A change in the direction of the velocity vector is acceleration.  The (instantaneous) acceleration is radially inward to the center of the circle and has magnitude ${v^2 \over r}$ where $r$ is the radius of the circle.  Therefore, the net force in the inward radial direction has magnitude${mv^2 \over r}$.  In the absence of gravity, this force is supplied by a rope or the circular surface.  (With gravity, the net external force is more complicated; since the force of gravity is always vertically downward.)
In the first case, the normal force is vertically upwards and there is no vertical acceleration.  In the second case there is a net force radially inwards that provides acceleration.
