《大學(xué)物理》英文課件6 Force and MotionⅡ

1、? ?大學(xué)物理大學(xué)物理? ?英文課件英文課件6 6 Force and Motion-Force and Motion-Kinetic frictional force, which acts when there is motion.Kinetic frictional force fk is less than maximum static frictional force fs,max .Nfkkk:coefficient of kinetic frictionN: normal force Sample ProblemN 147 033MgTMgT0321TTT047cos28coso

2、2o1TT047sin28sin3o2o1TTTN 134 N 10421TTSample Problem0gFNTN 7 .41 0sinTmgTN 1 .72 0cosNmgN2m/s 9 . 4 sinamamgProblem: let the mass of the block be 8.5kg and the angle = 30o. Find the tension in the cord, and the normal force acting on the block. Sample ProblemmgaNmamgN)( N 708)(mgmgaNN 939)( m/s 2 .

3、 32mgaNaupN 477)( m/s 2 . 32mgaNadownN 231netgupFNFNewtons second law does not hold relative to the frame of the cab! Problem:2.Stationary:Up:Down:Upward net force:Sample Problem2m/s 2 )(BABAmmFaammFN 12amFBBAProblem: a constant horizontal force Fapp of magnitude 20N is applied to block A of mass mA

4、 = 4.0kg, which pushes against block B of mass mB = 6.0kg. Find the acceleration and force of block A on block B.Sample Problem)(20202xxavvm 290 00 xxvmafkmgNfkkkm/s 58)(200 xxgvkProblem: If a cars wheels are “l(fā)ocked (kept from rolling) during emergency braking, the car slides along the road. Ripped

5、-offbits of tire and small melted sections of road form the “skid marks that reveal that cold-welding occurred during the slide. The record for the longest skid marks on a public road was reportedly set in 1960 by a Jaguar on the M1 highway in England the marks were 290 m long! Assuming that k = 0.6

6、0 and the cars acceleration was constant during the braking, how fast was the car going when the wheels became locked?Sample Problem0kgfFNT0cosNTk0sinmgNT)sin(TmgNfkkkN 91TProblem: A girl pulls the sled with a tub on it. Sample ProblemNfss0sgfFN0sinmgfs0cosmgNtancossinmgmgs6-4 Uniform Circular Motio

7、n Rva2Centripetal acceleration and centripetal forceRvmF2The magnitudes of the acceleration and velocity are constant, but their directions continuously varyThe circular coordinateSample Problem)( amFNggRv Problem: In a 1901 circus performance, Allo “Dare Devil Diavolo introduced the stunt of riding

8、 a bicycle in a loop-the-loop. Assuming that the loop is a circle with radius R = 2.7 m, what is the least speed v that Diavolo and his bicycle could have at the top of the loop to remain in contact with it there?Solution:RvamgFNg2, 0Sample ProblemmgNRvmNs )(221. 02gRvsProblem: a car of mass m moves at a constant sp