Hewitt Drew-It! PHYSICS Screencasts
Rotational Motion


34. Circular Motion
(Paul discusses his father working as a ticket collector in a merry-go-round and ties this to a Burl-Grey problem involving circular motion.)
End Question: Here we see four possible paths, A, B, C, and D. Whatís your take on this? When the marble rolls off the edge, will it follow path A ó directly outward from the rotating record? Will it follow a curve, something like that of path B? Will it follow a straight line as shown in C? Or will it follow a tangent line as shown in D?

35. RR Wheels
(Paul links the linear-rotational speed relation to an explanation of why railroad trains stay on tracks via tapered wheel rims.)
End Question: If the pain of cups were fastened, not at their wide ends, but by their narrow ends, how would motion along the tracks be affected?

37. Centripetal Force 
(Is the force that holds a can whirled overhead at the end of a string an outward-acting or inward-acting force ó and why?)
End Question: If instead of whirling a can through the air by a string, suppose we whirled it in a circular path on a horizontal friction-free lab table, in such a way that the string remains horizontal. Would the string tension alone, without vector components, be the centripetal force?

38. Centrifugal Force
(Paul discusses the forces experienced by bugs inside a horizontally-whirled can, and why centrifugal force is fictitious.)
End Question: Using the equation for centrifugal force as a guide, what must occur for occupants of the ISS to no longer endure a weightless condition? Defend your answer.

39. Torque
(We drop in on Paulís class as he discusses the concept of torque.)
End Question: If you use a pipe to extend the lever arm of the wrench, so your grip is 3 times as far from the bolt, and you pull twice as hard as before, at right angles to the pipe, by how much will the torque you produce increase?

40. Balanced Torques
(We drop in on Paulís class as he discusses balanced metersticks and seesaws.)
End Question: Suppose the boy succeeds by sitting on the far end after shifting the fulcrum beneath the one-quarter mark of the seesawís length as shown. At rotational equilibrium, how will the kidís weight compare with the weight of the seesaw?

41. Torques on a Plank
(Torques on horizontal planks, taking center of gravity into account, are examined.)
End Question: For the two-meterstick combination, at what centimeter mark of the horizontal meterstick should you place your finger for balance?

42. Skateboard Torques
(How does rotational physics explain how a skateboard is able to lift without external forces?)
End Question: How do the upward curved surfaces of the ends of the skateboard enhance lifting?

43. Angular Momentum
(Conservation of angular momentum, with examples, including Paul in the classroom, are examined.)
End Question: When you crawl toward the edge of the turntable, does its rotational rate increase, decrease, or remain unchanged? What physics principle supports your answer?