PHYS 1111 1nd Edition Exam 2 Study Guide Lectures 7 15 Chapter 5 Force A push or pull has a magnitude and direction so is a vector Newton s first law of motion o Inertia An object at rest remains at rest as long as no net force acts on it o An object moving with a constant velocity continues with the same direction and speed as long as no net force acts on it Newton s second law of motion F ma o If no net force acceleration is zero so velocity is constant Newton s third law of motion for every force that acts on an object there s a reaction fore acting on a different object that s equal in magnitude and opposite in direction If an elevator is at rest N mg o a 0 n mg Feel heavier o a 0 N mg Feel lighter o a g N 0 Weightlessness Chapter 3 section 6 If the reference frame is at rest or moving at a constant velocity there is a inertial reference frame If the reference from is acceleration then it is a non inertial from of reference the first and second laws of Newton fail There is never a 0 velocity in the universe because there is no center Chapter 6 Kinetic friction the friction encountered when surfaces slide against one another with finite relative speed ex Car skidding o f K k N a greater coefficient of friction more friction against object Static friction keeps 2 surfaces from moving relative to one another Ex Car rolling o f S S N As a force is applied static friction increases until it s max Once max of static friction obtained object begins to move and kinetic friction increases o Need more force to get object started than to keep moving Hooke s Law F x kx 2 Centripetal acceleration he net force required to keep an object moving in a circular path acp v r Chapter 12 sections 1 and 2 G 6 67 10 11 N m2 kg 2 Object heavier on Earth than on the moon Weight force exerted by Earth s gravity on an object Kepler s Laws of orbital motion 1 Plants follow elliptical orbits with sun at 1 focus of the ellipse 2 Radius vector from the sun to the planets sweeps out equal areas in equal time 3 Orbital period of a planet is proportional to the radius to the 3 2 power Chapter 7 W Fdcos Units of work is Joules 90 90 means work is positive and speed increases 90 270 means work is negative and speed decreases W mgh 1 1 W mv 2 mv 2 2 2 Fd P Fv measured on Watts t Chapter 8 Potential energy energy stored that can be converted to another kind of energy o PE mgh Conservation of mechanical energy E KE PE Conservative force a force in which work it does on an object doesn t depend on the path ex gravity Nonconservative force force for which work done depends on the path If both forces exist total work conservative work nonconservative work Total energy of a universe is always constant Energy is neither created nor destroyed only converted Nonconservative work increase in total mechanical energy Nonconservative work decrease in total mechanical energy Practice problems 1 If a car travels around a curve which has a radius of 316 m and static friction of 0 78 what s the maximum velocity the car can travel without skidding F ma N mg 0 2 v r Car wont skid of the static friction is less than the maximum static frictional force so 2 v m S mg r v S gr 0 78 9 81 316 49 1 2 A hockey puck slides across the ice Initial velocity is 45 m s and final velocity is 44 67 m s It slides 16 f S ma m m What is the coefficient of kinetic friction F f ma k mg W k mgd 1 1 1 W mv 2 mv 2 m 44 672 452 2 2 2 1 2 2 k m 9 81 16 m 44 67 45 2 k 0 094 3 The velocity of the conveyer belt is 2 ft s and the velocity of Ana with the conveyer is 1 ft s Whats the velocity of Anna as seen by Bob ft ft V ab V ac V cb 1 2 s s 4 A particle starting from A is projected down a curved path Upon leaving runway at point B the particle is traveling straight upward and reaches a height of 4 m above the floor before falling back down What s the velocity at A 1 m v 2 mgh 2 1 2 v 9 81 4 3 2 V 4 43 m s