PHY 101 1st Edition Lecture 9 Outline of Last Lecture I Projectile Motion II Velocity Components III Displacement IV Direction of Velocity Outline of Current Lecture V Why do things move VI Aristotle vs Newton VII 4 1 Forces VIII 4 2 Newton s First Law Current Lecture Why and how do things move o Relationship between motion of object s and forces acting on them dynamics o Concepts Force mass acceleration action reaction o Laws Newton s three laws Newton s law of universal gravitation Aristotle vs Newton o Aristotle first theory of motion wrong theory Everything is in a state of rest until you give it a push force is the cause of motion Makes sense but is wrong widely accepted until 17th century o Newton Force is NOT a cause of motion but it causes CHANGE in motion acceleration What are forces 4 1 Forces o Ex Push pull Contact forces you need physical contact to apply force o Ex Gravitational magnetic and electric These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute Field forces don t need direct physical contact o A force is a vector SI Unit Newton N o Can be contact or field force Contact forces result from physical contact between objects Field forces occur between disconnected objects o Only field forces are fundamental At a microscopic level contact force is also field force o 4 fundamental forces in nature strongest to weakest Strong nuclear force subatomic particles Electromagnetic force between charges magnets Weak nuclear force radioactive decay Gravitational force between masses 4 2 Newton s First Law o First law of motion An object move with a velocity that is constant in magnitude and direction unless acted on by a nonzero net force o Before the 17th century people believed the natural state of matter was at rest o Galileo the natural state of matter is to continue its original state of motion o Newton formalized Galileo s idea into the first law of motion o things tend to stay where they are o If an object is at rest it will stay at rest until an external force is applied on it o If an object is moving it will keep on moving at a constant velocity until an external force is applied on it o Mass a measure of the resistance of an object to changes in its motion due to force o Net Force The vector sum of all the external forces exerted on the object o External Force Any force that results from an interaction with an object and it s environment o Internal Force Forces that originate within the object itself They cannot change the object s velocity you cannot pull yourself up
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