Lecture 12, 11/2/04 - Physics

5.7Exercise49. A 50.0-kg student climbs a 5.00-m-long rope and stops at thetop. (a) What must her average speed be in order to match thepower output of a 200-W light bulb? (b) How much work does shedo?Exercise52. A 1.50 ! 10 3 -kg car accelerates uniformly from rest to 10.0 m/sin 3.00 s. Find (a) the work done on the car in this time interval, (b)the average power delivered by the engine in this time interval, and(c) the instantaneous power delivered by the engine at t = 2.00 s.College Physics A04 Prof. Kinoshitawww5.8 work done by a varying force (read)W= F!x applies only for fixed FFor varying F:take small x-intervals where F~constantBottom line:(p. 143) the work done by a variable force acting on anobject that undergoes a displacement is equal to thearea under the graph of F x vs x.Exercise56. An object is subject to aforce F x that varies withposition as in the figure. Findthe work done by the force onthe object as it moves (a) fromx = 0 to x = 5.00 m, (b) fromx = 5.00 m to x = 10.0 m, and(c) from x = 10.0 m to x = 15.0 m. (d) What is the total work done bythe force over the distance x = 0 to x = 15.0 m?College Physics A04 Prof. KinoshitawwwChapter 6 Current and ResistanceMomentum & impulseConservation of momentumcollisions____________________________________________________________________________________________________________________________________6.1 momentum & impulse (read)• define momentum p:p=mv (eq. 6.1) Vector, SI unit:kg-m/sNOTE:impulse-> impulse-momentum theorem:6.1Exercise5. A pitcher claims he can throw a 0.145-kg baseball with as muchmomentum as a 3.00-g bullet moving with a speed of 1.50 ! 10 3m/s. (a) What must the baseball’s speed be if the pitcher’s claim isvalid? (b) Which has greater kinetic energy, the ball or the bullet?Exercise9. A car is stopped for a traffic signal. When the light turns green,the car accelerates, increasing its speed from 0 to 5.20 m/s in0.832 s. What are the magnitudes of the linear impulse and theaverage total force experienced by a 70.0-kg passenger in the carduring this time?Impulse imparted (to object) = area under F vs. t graph (read, p. 156)College Physics A04 Prof. KinoshitawwwCollege Physics A04 Prof. Kinoshitawww

6.2 conservation of pTotal momentum of system:Conservation of momentump = m 1 v 1 +m 2 v 2(p. 161)The principle of conservation of momentum states that, whenno external forces act on a system consisting of two objects thatcollide with each other, the total momentum of the system before thecollision is equal to the total6.2Quick Quiz 6.4An astronaut of mass M is floating in the environment of herspacecraft and is initially at rest. She decides to get to the other sideof the spacecraft by throwing a notebook of mass m in the directionopposite to that in which she wishes to move. If the speed of thenotebook is v, what is her recoil speed? (a) 0 (b) mv/(M–m)(c)!mv/(M+m) (d) mv/M (e) 2mv/(M–m)Write an equation:m 1 v 1i +m 2 v 2i = m 1 v 1f +m 2 v 2f (eq. 6.7)Read examples, applicationsCollege Physics A04 Prof. KinoshitawwwCollege Physics A04 Prof. Kinoshitawww6.3 collisionsTypes (p. 164)• an elastic collision is one in which both momentum and kineticenergy are conserved.• an inelastic collision is one in which momentum is conserved butkinetic energy is not.• a perfectly inelastic collision is an inelastic collision in which thetwo objects stick together after the collision, so that the final velocitiesare the same and the momentum of the system is conserved.perfectly inelastic (animation)6.3Exercise26. A 75.0-kg ice skater, moving at 10.0 m/s, crashes into astationary skater of equal mass. After the collision, the two skatersmove as a unit at 5.00 m/s. Suppose the average force a skater canexperience without breaking a bone is 4 500 N. If the impact time is0.100 s, does a bone break?Write equations:College Physics A04 Prof. KinoshitawwwCollege Physics A04 Prof. Kinoshitawww