$A$ $4$ -kilogram disk slides over level ice toward the east at a velocity of $1$ meter per second, as shown. The disk strikes a post and rebounds toward the north at the same speed. The change in the magnitude of the eastward component of the momentum of the disk is ............. $\mathrm{kg}$ $ \cdot $ $\mathrm{m}/ \mathrm{s}$ 

State and establish principle of conservation of mechanical energy.

A body falls towards earth in air. Will its total mechanical energy be conserved during the fall ? Justify.

A bullet hits and gets embedded in a solid block resting on a horizontal frictionless table. What is conserved ?

The inclined surfaces of two movable wedges of same mass $M$ are smoothly conjugated with the horizontal plane as shown in figure. $A$ washer of mass $m$ slides down the left wedge from a height $h$. To what maximum height will the washer rise along the right wedge? Neglect friction. 

A shell in free space initially at rest explodes into two pieces, $A$ and $B$ , which then move in opposite directions. Piece $A$ has less mass than piece $B$ . Ignore all external forces. Identify correct statement