July 14 - Gravitational & Passive Forces 1 Name____________________________Date__________________Partners____________________________ GRAVITATIONAL AND PASSIVE FORCES And thus Nature will be very conformable to herself and very simple, performing all the great Motions of the heavenly Bodies by the attraction of gravity . . . –Isaac Newton OBJECTIVES • To explore the nature of motion along a vertical line near the Earth’s surface. • To extend the explanatory power of Newton’s laws by “inventing” an invisible force (the gravitational force) that correctly accounts for the falling motion of objects observed near the Earth’s surface. • To examine the magnitude of the acceleration of a falling object under the influence of the gravitational force near the Earth’s surface. • To examine the motion of an object along an inclined ramp under the influence of the gravitational force near the Earth’s surface. • To incorporate frictional forces into Newton’s first and second laws of motion. • To explore interaction forces between objects as described by Newton’s third law of motion. • To explore tension forces and understand their origin. • To apply Newton’s laws of motion to mechanical systems that include tension. OVERVIEW You started your study of Newtonian dynamics in the “Force and Motion” lab by developing the concept of force. Initially, when asked to define forces, most people think of a force as an obvious push or pull, such as a punch to the jaw or the tug of a rubber band. By studying the acceleration that results from a force when little friction is present, we came up with a second definition of force as that which causes acceleration. These two alternative definitions of force do not always appear to be the same. Pushing on a wall doesn’t seem to cause the wall to move. An object dropped close to the surface of the Earth accelerates and yet there is no visible push or pull on it. Modified from P. Laws, D. Sokoloff, R. Thornton Supported by National Science Foundation and the U.S. Dept. of Education (FIPSE), 1993-2000 PHYS 635, Summer 2005 University of Virginia Physics Department2 July 14 - Gravitational & Passive Forces The genius of Newton was to recognize that he could define net force or combined force as that which causes acceleration, and that if the obvious applied forces did not account for the degree of acceleration then there must be other “invisible” forces present. A prime example of an invisible force is the gravitational force – the attraction of the Earth for objects. When an object falls close to the surface of the Earth, there is no obvious force being applied to it. Whatever is causing the object to move is invisible. Most people rather casually refer to the cause of falling motions as the action of “gravity.” What is gravity? Can we describe gravity as just another force? Can we describe its effects mathematically? Can Newton’s laws be interpreted in such a way that they can be used for the mathematical prediction of motions that are influenced by gravity? It almost seems that we have to “invent” an invisible gravitational force to save Newton’s second law. Since objects near the surface of the Earth fall with a constant acceleration, we will use Newton’s second law to show that there must be a constant (gravitational) force acting on the object. Finding invisible forces (forces without an obvious agent to produce them) is often hard because some of them are not active forces. Rather, they are passive forces, such as normal forces, which crop up only in response to active ones. (In the case of normal forces, the active forces are ones like the push you exert on a wall or the gravitational pull on a book sitting on a table.) Frictional and tension forces are other examples of passive forces. The passive nature of friction is obvious when you think of an object like a block being pulled along a rough surface. There is an applied force (active) in one direction and a frictional force in the other direction that opposes the motion. If the applied force is discontinued, the block will slow down to rest but it will not start moving in the opposite direction due to friction. This is because the frictional force is passive and stops acting as soon as the block comes to rest. Likewise, tension forces, such as those exerted by a rope pulling on an object can exist only when there is an active force pulling on the other end of the rope. In this lab you will first study vertical motion and the gravitational force. Then you will examine the motion of an object along an inclined ramp. You will use Newton’s laws of motion as a working hypothesis to “invent” frictional and tension forces. Along the way you will examine Newton’s third law of motion. INVESTIGATION 1: MOTION AND GRAVITY Let’s begin the study of the phenomenon of gravity by examining the motion of an object such as a ball when it is allowed to fall vertically near the surface of the Earth. This study is not easy, because the motion happens very quickly! You can first predict what kind of motion the ball undergoes by tossing a ball in the laboratory several times and seeing what you think is going on. A falling motion is too fast to observe carefully by eye. You will need the aid of the motion detector and computer to examine the motion quantitatively. Fortunately, the motion detector can do measurements just fast enough to graph this motion. To carry out your measurements you will need • motion detector • basketball • 3 m tape measure • table clamps, rods, etc. to attach motion detector at least 2 m off floor. Modified from P. Laws, D. Sokoloff, R. Thornton Supported by National Science Foundation and the U.S. Dept. of Education (FIPSE), 1993-2000 PHYS 635, Summer 2005 University of Virginia Physics DepartmentJuly 14 - Gravitational & Passive Forces 3 Activity 1-1: Motion of a Falling Ball Prediction 1-1: Suppose that you drop the ball from height of about 2 m above the floor, releasing it from rest. On the axes that follow, sketch your predictions for the velocity—time and