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## notes 1

by: Grace Arbanas

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# notes 1

Grace Arbanas
WSU
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COURSE
PROF.
No professor available
TYPE
Class Notes
PAGES
5
WORDS
KARMA
25 ?

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This 5 page Class Notes was uploaded by Grace Arbanas on Tuesday February 10, 2015. The Class Notes belongs to a course at Washington State University taught by a professor in Fall. Since its upload, it has received 19 views.

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Date Created: 02/10/15
PHYSICS 201 STUDY GUIDE CONSERVATION LAWS With a system of particles energy is conserved You can not make or destroy energy The types of energy in a system are kinetic energy and potential energy Kinetic energy is the energy due to the motion of the particles The potential energy which is thought of as stored energy is due to interactions between the particles For example two particles connected by a stretched spring have a potential energy The sum of kinetic and potential energy is the mechanical energy Emech K U The term mechanical designates this form of energy as being due to motion and mechanical effects such as stretched springs rather than chemical or heat effects Mechanical energy is energy of the object as a whole The total energy of the moving atoms in a system is called the thermal energy Esys Thermal energy is associated with the system s temperature The system energy Em is the sum of the mechanical energy of the object plus the thermal energy of the atoms inside the objects Esys E Eth KUEth Mech Kinetic and potential energy can be changed back and forth into each other or be changed into thermal energy but not the other way around These energy changes within the system are called energy transformations which do not change the value of Esys if they are within the system Unless completely isolated a system of particles is always surrounded by a larger environment so energy can be exchanged with the environment When energy is exchanged between the system and environment it is called an energy transfer There are two primary energy transfer processes The first one is due to forces exerted on the system by the environment For example you give a ball kinetic energy by pushing on it This mechanical transfer of energy is called work The second one is a process called heat WORK In order for work to be done three things are necessary There must be an applied force The force must act through a certain distance called displacement The force must have a component along the displacement Work is a scalar quantity W 2 Fr W F cos 9r the units are Nm or a Joule J If the force acting on an object varies in magnitude andor direction during the objects displacement graphical analysis can be used to determine the work done F is plotted on the yaxis and the distance through which the object moves is plotted on the xaxis The work done is the area under the curve CONSERVATIVE AND NONCONSERVATIVE FORCES The work done by a conservative force depends only on the initial and final position of the object acted upon An example of a conservative force is gravity The work done equals the change in potential energy and depends only on the initial and final positions above the ground and not on the path taken Friction is a nonconservative force and the work done in moving an object against a nonconservative force depends on the path For example the work done in sliding a box of books against friction from one end of a room to the other depends on the path taken LAW OF CONSERVATION OF ENERGY The law of conservation of energy states that Energy is neither created nor destroyed Energy can be transformed from one kind to another but the total amount remains constant For mechanical systems involving conservative forces the total mechanical energy equals the sum of the kinetic and potential energies of the objects that make up the system Example In an atwood machine the twoo masses are 800g and 700g The system is released from rest How fast is the 800g mass moving after it has fallen 120cm M g mg ZFy 898 798 ma 8a a 122532 0 S v2 vo2 2ax x0 122 2122512 V Msoogg v 05422E S POWER Is the rate at Which work is performed Units are Js Watt W time t t work P Fv ELASTIC POTENTIAL ENERGY Elastic potential energy is associated With elastic materials The force Fp applied to a spring to stretch it or to compress it an amount X is directly proportional to X That is F p kx Where k is a constant called the spring constant and is a measure of the stiffness of the particular spring The spring itself exerts a force in the opposite direction FS 2 kx Units are in Newtons N The force is sometimes called restoring force because the spring exerts its force in the direction opposite to the displacement This equation is known as the spring equation or Hooke s Law The elastic potential energy is given by PES 251062 units Joules J Example A dart of mass 0100kg is pressed against the spring of a toy dart gun The spring k 250 Nm is compressed 60cm and released If the dart detaches from the spring When the spring reaches its normal length What speed does the dart acquire F kAx k 5mv2 2 F 250 06m 15N 5 50100V m m v 21732 S IMPULSE AND MOMENTUM The impulse F At is a vector quantity equal in magnitude to the product of the force and the time interval in Which it acts Its direction is the same as that of the force Units Newton second Ns The momentum p of a particle is a vector quantity equal in magnitude to the product of its mass m and its velocity v p mv F At mAv Impulse change in momentum CONSERVATION OF LINEAR MOMENTUM According to the law of conservation of linear momentum When the vector sum of the external forces that act on a system of bodies equals zero the total linear momentum of the system remains constant no matter What momentum changes occur Within the system Although interactions Within the system may change the distribution of the total momentum among the various bodies in the system the total momentum does not change Such interactions can give rise to two general classes of events Explosions in Which an original single body ies apart into separate bodies and Collisions in which two or more bodies collide and either stick together or move apart in each case with a redistribution of the original linear momentum For two objects interacting with one another the conservation of momentum can be expressed as mlvli n12v2i mlv1 f 1112v2 f with initial and final velocities ELASTIC AND INELASTIC COLLISIONS If the K remains constant in a collision the collision is said to be completely elastic If the colliding bodies stick together and move off as a unit afterward the collision is said to be completely inelastic In inelastic collisions only the momentum is conserved Example A 12g bullet is fired into a 2kg block of wood suspended from a cord The impact of the bullet causes the block to swing 10cm above its original level Find the velocity of the bullet as it strikes the block Eg mgh Ek 5mv2 Eg 98201210 197 5012v2 Eg 197N v18128E S

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