Question

please explain problem 5

Unit 7. Work and Energy In Class Packet 5. Each of the diagrams below shows a displacement vector for an object. Draw and label a force Name: vector that will do work on the object with the sign indicated. a. b. C. A 0.2 kg plastic cart and a 20 kg lead cart both roll without friction on a horizontal surface. 6. Equal forces are used to push both carts forward a distance of 1 m, starting from rest. After traveling 1 m, is the kinetic energy of the plastic cart greater than, less than, or equal to the kinetic energy of the lead cart? Explain. 7. For each situation described below Draw a before and after diagram (like figures 10.8 and 10.12 in your textbook) Identify ALL forces acting on the object Determine if the work done by each of these forces is positive (+), negative (-), or zero (0) Make a table beside the figure showing EVERY force and the sign of its work. a. An elevator moves upward b. An elevator moves downward. .You slide down a steep hill. 4

Answer 1

(a) W > 0 (b) W < 0 (c) W = 0 Work done = F vector middot ds vector = vert F vector vert vert ds vector vert cos theta: theta elementof [0, 180 degree] Where theta is angle b/w force & displacement vert F vector vert, vert ds vector vert are magnitude (always + ve). So its cos theta which leads to work being [+ , - , 0] If both are acting [in case of zero work] WD is +ve if theta < 90 degree 0 if theta = 90 degree -ve if theta > 90 degree