# ch4 - Notes on Chapter 4 Overview Objects may interact with...

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John Ellison, UCR p.1 Notes on Chapter 4 Overview Objects may interact with each other and their velocities change as a result. The strength of an interaction depends on properties of the objects and their relative positions. Gravitational mass is responsible for gravitational interactions and electric charge is responsible for electric and magnetic interactions. To study interactions among objects we split the problem into two parts and say that each body exerts a force on the other and that the net force on a body changes its velocity. We find the force acting on a body from known force laws, i.e. equations which give the force in terms of the properties of the objects and their positions. There are four fundamental forces of nature - the gravitational force, the electromagnetic force, and the strong and weak nuclear forces. Each of these will be discussed in the Physics 40 series, but first we will study the relation between a force and the acceleration it produces on a body. This relationship was first understood by Isaac Newton (1642-1727) and the study of that relationship is called Newtonian mechanics . Newton’s First Law Newton’s first law states that if an object does not interact with any other objects (i.e. the net force acing on the body is zero), the body’s velocity cannot change: i.e. its acceleration is zero. Because the acceleration depends on the reference frame used to measure it, Newton’s first law can be true for only a select set of frames called inertial reference frames (or simply inertial frames ). Any frame moving with constant velocity with respect to an inertial frame is also an inertial frame (because the accelerations in the two frames are the same - see notes on chapter 3 . For most laboratory phenomena we can take a coordinate system attached to the ground to be an inertial frame. However, because of the motion of Earth, this is not sufficient for the description of ocean and wind currents, space probes, and astronomical phenomena. Newton’s Second Law If you place a box on a table and give it a shove it will move a certain distance and come to rest. This means it has an acceleration opposite to its initial velocity. The acceleration is produced by a force which acts on the body. In this case it is due to friction and acts in the direction opposite to the velocity of the object. Force is a vector with the same direction as acceleration. It is defined in terms of the acceleration imparted to a standard mass. A force of 1 Newton (abbreviated N) acting on an object of mass 1 kg produces an acceleration of 1 m/s 2 . Thus 1 N = 1 kg·m/s 2 . The mass of a body is characteristic property that relates a force on the body to the resulting acceleration. It is an intrinsic property of the body. Newton’s second law

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ch4 - Notes on Chapter 4 Overview Objects may interact with...

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