Download Frictional Force - General Physics - Solved Past Paper and more Exams Physics in PDF only on Docsity! 1. (25 pts) a) (5 pts) When work is done on an object, must the energy transferred go into kinetic energy? If so, explain. If not, give an example. No, the energy need not go into kinetic energy. There are many other possibilities. For example, another force could be acting to take the energy back out (like friction). b) (5 pts) A block slides to the right across a desk. The frictional force on the block, due to the inter- action of the surface of the block with the surface of the desk, is to the left. In what direction is the frictional force exerted on the desk? The block and desktop are two interacting objects. The interaction force in question here is the friction between them. Friction obeys Newton’s 3rd law, so the frictional force on the desk is in the opposite direction, or to the right. Another way to look at this is to imagine standing on the book. To this observer, it appears that the desk is moving to the left. Thus the frictional force on the desk is, opposite to the motion, to the right. c) (5 pts) If there is no net work done on an object as it moves, can you definitely say that there are no forces acting on the object? No. That there are no forces is only one possibility. There could also be several forces acting such that the net force is zero. This would also result in no net work. d) (5 pts) Since µk < µs, is the force necessary to get an object started moving greater or lesser than the force necessary to maintain the object’s motion at a constant speed (once it has actually started moving)? The force necessary to maintain motion at a constant speed is therefore less than the force necessary to get the object started in the first place. e) (5 pts) The force between two electrically charged objects varies with the distance of separation. If one wanted to compute the work necessary to move one of the objects, could one use W = ~F · ~d or would one have to use that other method that involves adding an infinite number of infinitesimally small amounts of work as one object is moved in infinitesimally small steps further away from the other? Explain. You’d have to use that other method. W = ~F · ~d assumes that the force is constant. The problem clearly states that the force in question here, “varies with the distance of separation.” Since the force is not constant, you’d have to use that other method.
