# Hewitt Drew-It! PHYSICS Screencasts Energy

 28. Work and Potential Energy (Paul illustrates work and potential energy via a barbell and other vertical lifts, and the energy states of a simple pendulum.) End Question: If our block of ice weighed 500 N instead of 400 N, how much force would the little guy on the ramp have to exert to slide it up the ramp at constant speed? 29. Potential and Kinetic Energy (Paul derives kinetic energy from Newton’s second law, and illustrates energy transfers for a block of sliding ice.) End Question: Consider the identical blocks atop three ramps of the same height. All blocks have the same PE. When they topple from their elevated positions, they reach the bottom without encountering friction. Question: How does the speed of the blocks compare when reaching ground level? 30. Work-Energy Theorem (Paul enlists Nellie Newton to illustrate the work-energy theorem to solve a motion problem.) End Question: If the missing road surface were 10 m in front of Nellie, instead of 20+ m, what’s the maximum speed she can have to stop safely? In other words, v = what? 31. Conservation of Energy (Paul treats a block of ice on a ramp, a sliding bead, and balls on interesting tracks.) End Question: When balls are released on Tracks A and B, how will their KEs compare as they reach the ends of the tracks; how will their speeds compare, and which ball travels along the track in the shortest time? 32. Ballistic Pendulum (Paul explains how the classic ballistic-pendulum problem cannot be solved with energy conservation alone.) End Question: If the same-mass bullet were fired at twice the speed into the block, how much higher would the block swing? 33. Machines and Energy (Paul shows how a simple lever lifts a load, and how Nellie Newton lifts loads with pulley systems.) End Question: If 10 strands of rope support a load in a complex but ideal pulley system, how much force must Nellie supply to lift a 1000-N load? 36. Energy of Acrobats (A problem solution, nicely simplified, to acrobats Ari, Bari, and their dog Bo.) End Question: If Ari is replaced with circus dog Bo, who has one-fifth the mass of Bart, and the Bo’s trainer is positioned 20 m high for a hoped-for catch, will Bo’s propulsion be enough to reach the trainer?