I'll start with the "easy" one, a SAT question which I don't quite get:

Two objects move toward each other, collide, and separate. If there was no net external force acting on the objects, but some kinetic energy was lost, then

(A) the collision was elastic and total linear momentum was conserved.

(B) the collision was elastic and total linear momentum was not conserved.

(C) the collision was not elastic and total linear momentum was conserved.

(D) the collision was not elastic and total linear momentum was not conserved.

(E) None of the above.

The given answer to this question is (C). As I understand, in a collision where two objects separate, is an elastic collision rather than an inelastic one? Anyone care to explain why is this so?

As for the next question, a written A-Level Physics one:

A stone of mass m is attached to a string of length r, which will break if the tension in it exceeds Tmax. The stone is whirled in a vertical circle.

(a) draw diagrams showing the forces acting on the stoe when it is (i) at the top, (ii) at the bottom, of the circle.

The angular speed is increased very slowly.

(b) For what position of the stone, relative to the axis of the rotation, is the string most likely to break?

(c) What will be the angular speed when this occurs?

Okay, I have no problems drawing the diagrams for (a), where I have T (tension of the string), W (weight of the stone), N (normal force), and Fc (centripetal force) labeled correctly, I hope.

The problem comes to part (b). I can't actually see where do I start. Here's what I tried figuring out from: that in order for the string to break, the net force must be greater than the tension of the string? Where do I go from here?

As for (c), I think I'd know how to solve this one as I get part (b).

Any help would be appreciated. Thanks in advance.