Phys 1021 Ch 7, P 3 - George Washington University

This week’s homework . 2 partslast one due tonightToday’s class: Newton’s third law Pulleys TensionCircular motionQuiz on Friday, Ch 5 and 6First exam Weds, Feb 22, Chs 1-8, 6 PM to 8 PM, Funger 108 Email me if there is a class conflict for you at the exam timePHYS 2: Chap. 19, Pg 21

When two or more objects are connected by strings, pulleys, or are rigidlyconnected, then they no longer move independently. The constraintsbetween their positions, velocities and accelerations can be used to easethe solving of their motion.For example in the picture below, all three types of constraints are in play: Ropes: Masses 1 and 2 move andaccelerate at the same ratePulleys: The velocity of the masses is equal tothe radius times the angular velocity of thepulley (as are the linear and angularaccelerationsRigid body: All points at a given distance from the pulley center have thesame acceleration.Phys 1021 Ch 7, p 3For each picture, write the acceleration constraint in terms of components. Forexample, write (a1)x (a2)y, if that is the appropriate answer, rather than a1 a2Phys 1021 Ch 7, p 42

Often in physics and engineering problems themass of the string or rope is much less than themasses of the objects that it connects. In suchcases, we can adopt the following massless stringapproximation:Third law: TsonA TsonBPhys 1021 Ch 7, p 5Phys 1021 Ch 7, p 63

A. Equal toB. Greater thanC. Less thanPhys 1021 Ch 7, p 7A. Equal toB. Greater thanC. Less thanEach block feels an upward force of 490 N,which is provided by the tension.Therefore both ropes have the sametension.Phys 1021 Ch 7, p 84

A. Equal toB. Greater thanC. Less thanPhys 1021 Ch 7, p 9A. Equal toB. Greater thanC. Less thanThe blocks are accelerating, since there isno force to hinder block A. This meansthat the net force on B {mg – T} is not zeroand is downward. Thus mg TPhys 1021 Ch 7, p 105

A. Less than 500gB. equal to 500 gC. more than 500 gPhys 1021 Ch 7, p 11A. Less than 500gB. equal to 500 gC. more than 500 gNeither the rubber band not the weight are accelerating.Therefore the net force on each is zero. Gravity pulls down ofthe weight and the rubber band pulls up with equal andopposite force. The weight pulls down on the rubber bandwith the same force, and in turn, the scale pulls up on therubber band with the identical force, mg, (m 0.5 kgPhys 1021 Ch 7, p 126

An Atwood’s machine is shown inthe figure. The 100 kg block on theright side takes 6.0 s to reach thefloor after it is released from rest.What is the mass of the block onthe left side?Phys 1021 Ch 7, p 13 A 1.0 kg block is tied to thewall with a rope. It sits on topof a 2.0 kg block. The lowerblock is pulled to the rightwith a tension force of 20 N.The coefficient of kineticfriction at both the lower andupper surfaces of the 2.0 kgblock is µk 0.40What is the tension in the rope attached to the 1.0 kg block?What is the acceleration of the 2.0 kg block?Phys 1021 Ch 7, p 147

1. Take one rubber band and hang a heavy weight from it. Hang the other endfrom the spring scale. How far does it stretch? Call this length l0. What isthe reading of the scale?2. Combine two rubber bands in a chain (end to end) and hang the sameweight from the combination. How far does it stretch? Why?For the questions below, answer only in integer numbers of rubber bands3. About how many identical rubber bands must be combined ( and how ) tostretch by l0 under the load of two heavy weights? Try this. Calculate theforce on each rubber band? Draw the fbd for the mass. Draw the fbd foreach rubber band.4. About how many identical rubber bands must be combined ( and how ) tostretch by l0 under the load of a mass smaller than the original one? Trythis. Calculate the force on each rubber band? Draw the fbd for the mass.Phys 1021 Ch 7, p 158

the figure. The 100 kg block on the right side takes 6.0 s to reach the floor after it is released from rest. What is the mass of the block on the left side? Phys 1021 Ch 7, p 14 A 1.0 kg block is tied to the wall with a rope. It sits on top of a 2.0 kg block. The lower block