To define the position of any moving object we need a reference point. This reference point itself could be attached to a moving object or to a stationary object. Here the orange block's position is being defined with reference to a point on the moving plank/wedge. The green arrow defines the position of the object on the moving surface relative to a reference point chosen on the moving surface. Notice that this vector is always parallel to the surface. That is, the motion of an object relative to a surface is always along the surface.

The orange block's position is also being defined with reference to a fixed point on the floor/wall. The blue arrow defines this. The red arrow defines the postion of the reference point chosen on the plank/wedge with reference to a stationary point on the floor/wall. You should be able to see yourself the relation between the three vectors.

The third option is interesting. Choose positive,negative and zero velocities for the block's x- velocity and you should be able to notice something interesting. You could have the block move vertically upward relative to floor or to the left or to the right. Observe this motion relative to plank.

The relation between various vectors is this. Position vector of plank relative to floor + position vector of block relative to plank= position vector of block relative to floor. In the applet red vector is position vector of plank relative to floor, the green vector is position vector of block relative to plank and the blue vector is the position vector of block relative to floor.

Surendranath. B.