SC PHYS 201 - Review for Exam 3 (3 pages)

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Review for Exam 3



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Phys 201 1nd Edition Lecture 22 Outline of Last Lecture I Angular momentum and rotational kinetic energy Outline of Current Lecture II Review A Power B Impulse C Momentum D Gravitational Potential Energy E Torque F Rotational Kinetic Energy Current Lecture Power Power describes the speed at which work is done Power can be described calculated from the following equations P W t P F X t P FV Where W represents work t represents time x represents distance and V represent velocity Because power is energy over time the unit for power is J s which are also called Watts Most objects can only put out a limited amount of power This means that if a person continues to increase the force they are applying to something they will have to adjust their speed in order to stay within the parameters of their available power e g when riding a bike up a hill you have to pedal slower when you use more work to keep moving up Impulse The impulse helps relate actual force of a system to the average force The most common example of an impulse is in the case of a collision in which an equal and opposite force is experienced for a certain amount of time causing the momentum of the object to change This force in this amount of time experienced is the impulse and it is equal to the momentum of the object Conservation of Momentum 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 Momentum is used to describe a change in motion of an object The momentum p of a force is calculated by multiplying mass by velocity where momentum and velocity are both vectors If two objects collide with one another and the momentum lost by one object will be equal to the momentum gained by the other object then there is no change in in the momentum over time meaning that the final and initial momentums are the same This is called conservative momentum For objects with the same momentum the object with the fastest speed will have the greatest kinetic energy If one car collides with another car the most damage will come from meaning that the most energy will be lost when it collides with an object of the greatest velocity even if it has a smaller mass Conservative momentum is calculated from the equation m1V1 m2V2 m1V1 m2V2 in which the superscript indicates that we are looking at the final velocity after the collision when the objects are moving away from each other The velocities without a superscript indicate the initial velocity before the collision when the objects are moving towards each other Inelastic collisions conserve momentum but not kinetic energy An elastic collision on the other hand is one in which both momentum and kinetic energy are conserved throughout the collision Gravitational potential energy Gravity is related to the height of an object Gravitational potential energy PE mgh depends on h which is based on a subjective reference point and assumes that a constant force is used to lift something to any given height However because this is inaccurate and force does change with distance a more accurate measurement for the force of gravity is the equation Newton s universal law of gravitation F mMeG R2 Where m is the mass of the object Me is the mass of Earth G is the gravitational constant and R is the radius of the object in relation to the center of the Earth However an object changes potential energy as it moves considering its change in position This means that the radius R is not constant meaning that a more apt equation would be PE GmM 1 rf 1 ri Rotational Motion Rotational motion differs only slightly from linear motion We describe a rotating object in terms of angular acceleration which is just the change in angular velocity over the change in time Linear Motio n x V a Rotating Motion x x0 Vit 5at2 0 0t 5 t2 V V0 at 0 t V2 V02 2a x x0 2 02 2 0 Torque Torque is as a measure of how much the force acting on an object can change the motion of the object The simplest equation for torque T is T rF where r is the distance from pivot point that the force F is acting on the object For torque to work r and F must be perpendicular to one another The perpendicular position of these two vectors is called the lever arm If the force is working on the object at an angle the torque is measured as T rFsin The moment of inertia Tnet I determines the amount of torque required to rotate an object about an axis with a certain angular acceleration The moment of inertia depends on the mass of an object and the distribution of that mass Rotational kinetic energy Linear kinetic energy KE is equal to 5 time the mass m of the object times the square of the velocity V Rotational kinetic energy is similar except we use the moment of inertia I instead of mass and the angular velocity instead of velocity KE linear 5mV2 KE rotational 5I 2 Some rotating objects have both linear and rotational kinetic energy If you roll a ball across a surface the ball has rotational movement where it is spinning around its axis and linear movement as it moves across the ground To calculate the total kinetic energy of the ball both the rotational and linear kinetic energy need to be taken into consideration This means that KEtotal KErotational KElinear KEtotal 5 I 2 5 mV2


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