(b) 105 NA setup similar to the one shown at right is often used in hospitals to support and apply a traction force to an injured leg. (a) Determine the force of tension in the rope supporting the leg. (b) What is the traction force exerted on the leg? Assume the traction force is horizontal.
Example Problem #4.3013 NAn object of mass 2.0 kg starts from rest and slides down an inclined plane 80 cm long in 0.50 s. What net forceis acting on the object along the incline? Geometry: ° Solution (details given in class):
Example Problem #4.36Find the acceleration of each block and the tension in the cable for the following frictionless system:m1m2+y+xCoupled system: mass m2moves same distance in same time as mass m1v1= v2a1= a2Free–body diagrams:m1m2Nm1gm2gTTApply Newton’s 2ndLaw to block m1Fx= m1axT= m1ax= m1aFy= 0 m1g– N= 0 m1g= N (a= 0 in y–direction) 5.00 kg10.0 kg = a : (1)
Example Problem (continued) Apply Newton’s 2 nd Law to block m 2 : F y = m 2 a y = m 2 a m 2 g – T = m 2 a (2) Combining equations (1) and (2): m 2 g – m 1 a = m 2 a a = [ m 2 / ( m 1 + m 2 )] g = 6.53 m/s 2 Using equation (1) and plugging in for a : T = m 1 a = 32.7 N (Note that this analysis will be useful for Experiment 5 in lab!)
CQ5: Interactive Example Problem: Rocket Blasts Off What is the maximum height reached by the rocket? ActivPhysics Problem #2.4, Pearson/Addison Wesley (1995–A) 0 m B) 240 m C) 960 m D) 2880 mE) 3840 m 2007)
Friction•Frictionis a contact force between two surfaces that always opposes motion•The kind of friction that acts when a body slides over a surface is called the kineticfriction force (“kinetic” for motion)–The magnitude of is proportional to the magnitude of the normal force N– Constant k= coefficient of kinetic friction(depends on the two surfaces in contact)WNFfis always tofNkfkfNfkk :
Friction • When pushing a car, have you ever noticed that it’s harder to start car moving than to keep it moving? • The magnitude of the frictional force varies! • There is a static frictional force , with variable magnitude, that is almost always larger (at it’s maximum value) than the kinetic frictional force • For the case of pushing a piano across the floor: s f W N (1) (no pushing) W N (pushing but no sliding) (2) F f s
Friction • f s has a variable magnitude: – Constant s is the coefficient of static friction ( s > k ) • In situation (1) above: • In situation (2) above: • In situation (3) above: • In situation (4) above: N f s s N f f s s N f f s s N f f k k 0 f W N (just about to slide) (3) F f s, max f s ,max = f s max. value W N (piano sliding) (4) F f k
Friction • Variable magnitude of the friction force as a function of pushing force F summarized in the graph at right • Friction responsible for motion of wheeled vehicles (Note that f is in the same direction as the motion of the car, but opposite to the motion of the tires !)
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