Homework 4, Due Monday, September 30


1. A triangular wedge is laid out with vertices at the O(0,0), at B(40cm, 0) and at T(40cm, 30cm). (O, B and T stand for origin, base, and top). Think of the $x$ coordinate as measuring side-to-side motion, and the $y$ coordinate as measuring up-and-down. (The third coordinate will not come into this problem. ) A block of mass $m$ is sitting partway up the wedge.
(a) Find the coordinates of a unit vector that points in the direction from T to O. Call this vector v.
(b) Find the coordinates of a vector perpendicular to the wedge, pointing into the wedge from the mass. Call this vector n.
(c)The force of gravity is F$=-mg$ j, where $g$ is the constant acceleration of gravity (about 32 ft/sec${}^2$, or 9.8 meters/sec${}^2$ --- you can just leave it as $g$). Find the component of F in the v direction, and the component of F in the n direction.
[Physics note: the amount of friction is proportional to the component of F in the n direction, and the friction force points in the v direction, but we're going to ignore friction in this problem.]
(d)Find the vector projection of F in the v direction, and the vector projection of F in the n direction.
(e)To haul the mass up the slope, you need to apply a force in the -v direction that is equal (or greater) than the component of gravity in the v direction. How much work is required to haul the mass up the slope from O to T?
(f)How much work would be required to haul a mass straight up the side of the wedge from B to T?



The moral of the story is the same for almost all examples of leverage. You can apply a small force for a short distance, or a large force for a small distance, but you can't get around the total amount of work you need to do.

Stewart Section 12.2, page 799, problem 36.
Stewart Section 12.3, page 807, problems 48, 52
Stewart Section 12.4, pages 814-6, problems 16, 30, 38, 39,