Hi everyone Thanks for viewing this thread. Hope it's in the right forum. I have some questions on basic mechanics that could really do with some clarifications, so I'd really appreciate any help you could provide. 1. Consider a ball with sufficient initial speed such that it moves in a vertical circular track. At 90 degrees from the base of the track, its downward gravitational force is directed tangentially along the track, which is vertical at that point. But there seems to be no interaction between the ball and the track wall. So what is the normal force and thus the centripetal force on the ball at this point? 2. Does the elasticity of a collision depend solely on the mechanical properties of the colliding object? 3. On a related note, when a ball collides with and deforms a surface of putty, is there potential energy stored in the deformed state? If so, when one reverses the deformation by performing work on the putty surface (which would be positive since the putty moves in the direction of the applied force), does this energy dissipate into heat? And what happens when this ball collides with and deforms a surface of sand? Is all the kinetic energy converted into thermal energy? 4. Consider a rigid object free to rotate without any specific axis defined by the presence of axles. Forces are applied to provide a net torque. Does the object rotate about an axis through the center of mass due to it being associated the lowest moment of inertia for the object? If so, how does this change when there is an axle through an arbitrary point which may not be at its center of mass? 5. . View attachment 81845 I'm having some problem with understanding why the static friction in the second case must necessarily be directed to the right. My understanding is that the frictional force would act such that it opposes the slipping of the contact point and that if the magnitudes of the force and the torque were in the correct ratio such that the acceleration of center of mass were equal to the product of the radius of the yo-yo and its angular acceleration, a static frictional force would not even exist. What is erroneous with my interpretation> 6. Lastly, with the typical example of a figure skater demonstrating the conservation of angular momentum, how is energy conserved when the skater extends and retracts his/her arms? I could see that it would have something to do with the work performed by the action of his/her arms but I'm not sure how this would be able to decrease rotational kinetic energy when the skater extends the arms. The work still seems to be positive as it moves in the direction of the force. Thanks for your time.