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Unless otherwise noted, the content of this course material is licensed under a Creative Commons BY 3.0 License. http://creativecommons.org/licenses/by/3.0/ Copyright © 2009, August E. Evrard. You assume all responsibility for use and potential liability associated with any use of the material. Material contains copyrighted content, used in accordance with U.S. law. Copyright holders of content included in this material should contact [email protected] with any questions, corrections, or clarifications regarding the use of content. The Regents of the University of Michigan do not license the use of third party content posted to this site unless such a license is specifically granted in connection with particular content. Users of content are responsible for their compliance with applicable law. Mention of specific products in this material solely represents the opinion of the speaker and does not represent an endorsement by the University of Michigan. For more information about how to cite these materials visit http://open.umich.edu/education/about/terms-of-use Any medical information in this material is intended to inform and educate and is not a tool for self-diagnosis or a replacement for medical evaluation, advice, diagnosis or treatment by a healthcare professional. You should speak to your physician or make an appointment to be seen if you have questions or concerns about this information or your medical condition. Viewer discretion is advised: Material may contain medical images that may be disturbing to some viewers. Physics 140 – Fall 2007 lecture 5 : 18 Sep Mathematics is the language of precise thinking. – Richard W. Hamming (1915-1998) Ch 4 topics: • Newton’s laws of motion (I + II) • dynamics: force and acceleration Newton’s First Law: the Law of Inertia “Every body continues in a state of rest, or uniform motion in a straight line, unless it is compelled to change that state by outside forces impressed upon it.” In the “language of precise thinking”, we can say SF=0 Source: Undetermined velocity v is constant where S F represents the sum of all external forces acting on an object with velocity v. A valid inertial reference frame is one in which objects move at constant velocity unless forced to do otherwise. Why is there a minimum vertical distance (called B in the figure) when hanging a frame by wire? What will happen to the tension measured by the spring scale when I attach the opposite end of the string to the other, identical “salami”? A. B. C. D. ? The tension will stay the same. It will double. It will be halved. It will change by a factor different from two. Newton’s Second Law: Force and Acceleration “The change in the quantity of motion is proportional to the motive force impressed and is made in the direction of the line in which that force is impressed.” In short, SF=ma (Unit: 1 Newton (N) = 1 kg m/s2) The vector sum of forces SF acting on a body cause it to accelerate in the direction of SF. The magnitude of the body’s acceleration depends inversely on its inertial mass m. Mass is a measure of inertia (resistance to change in motion). Inertia F = ma density r = mass/volume google aerogel to find out what this funny stuff is Source: Undetermined Source: NASA ? mass ? Energy E = mc2 Source: The Scientific Monthly (1921) Higgs particle CC: BY University of Michigan some common forces Weight from near-Earth gravity, W • magnitude W=mg • directed to Earth’s center (defines downward) Normal/contact Force, N • occur at interfaces • act perpendicular to interface (come in pairs) • situation-dependent magnitude Tension in rope or string, T • acts at contact point • directed along rope/string • light (“massless”) strings have constant tension along their lengths (act as force conduits) N W T On a horizontal, frictionless surface, the blocks above are being acted upon by two opposing horizontal forces, as shown. What is the magnitude of the net force acting on the 3kg block? A. B. C. D. E. zero 2N 1.5 N 1N More information is needed.