1) Consider a car that slides to a stop. If the initial speed doubles, I can see the kinetic energy at the beginning is 4 times greater, but why does this mean that your new stopping distance will be 4 times the old one? 2) How would you demonstrate, using a pen and a sheet of paper, that static friction can do positive work on a pen? [If I roll the pen on the paper, is that static friction and is that positive work? How can friction do positive work? Does kinetic friciton always do negative work?] 3) A toboggan is initially moving at a constant velocity along a snowy horizontal surface where friiction is negligible. When a pulling force is applied parallel to the ground over a certain distance, the kinetic energy increases by 47%. By what percentage woud the kinetic energy have changed if the pulling force had been at an angle of 38 degrees above the horizontal? [The answer is 16% but I can't figure out how to do it...] 4) Can the Work-Energy Theorem (W=change in kinetic energy=Ek2-Ek1 only be used when the force and displacement are pointing in the same direction? (0 degrees to each other) Because when we derive this equation, we used theta = 0 degrees. For example, would the above equation work in a question like this? (not every force is in exactly the same direction as the displacement...) "A 61-kg skier, coasting down a hill that is at 23 degrees to the horizontal, experiences a force of kinetic friction of 72N. The skier's speed is 3.5m/s near the top of the slope. Find the speed after the skier has travelled 62m downhill." Thanks for helping! Please Register or Log in to view the hidden image!