1、Announcements:Midterm 1 in less than a week, Sept. 19 (Chapter 1-6)Can use a 3x5 inch cheat card with 10 formulasI will post practice test on the Web HW 10 is due on Friday, Sept. 23, 10 pmExtended office hours this Friday 1-3,In this chapter we will learn about static frictionkinetic frictioncentri
2、petal forcekinetic drag force,Chapter 6: Force and Motion - II,Reading assignment: Chapter 7.1-7.5 Homework : Due Friday Sept 23, 2005 (an extra week to do it! Chapter 7: Q13, 4, 9, 18, 22, 29,Black board example 5.6,Two objects of mass m1 and m2 are attached by a string over a pulley as shown in th
3、e Figure. m2 lies on an incline with angle q. Determine the magnitude of the acceleration of the two objects and the tension in the cord. m1 = 10.0 kg, m2 = 5.00 kg, q = 45,Forces of Friction,Static friction, fsKinetic friction, fk,The following laws hold true about friction: Friction force, f, is _
4、 to normal force, n.ms and mk: coefficients of _ and _ friction, respectively Direction of frictional force is _ to direction of relative motionValues of ms and mk depend on nature of surface.ms and mk dont depend on the _ of contactms and mk dont depend on _. Friction is due to the surfaces interac
5、ting with each other on the microscopic level (sliding over bumps, chemical bonds),Black board example 6.1,Measuring the coefficient of static friction A brick is placed on an inclined board as shown in the figure. The angle of incline is increased until the block starts to move.,Determine the stati
6、c friction coefficient from the critical angle, qc, at which the block starts to move. What is ms if the block starts sliding at qC = 31,Black board example 6.2,Measuring the coefficient of kinetic frictionA hockey puck is given an initial speed of 20.0 m/s. It slides 115 m before coming to rest.,De
7、termine the coefficient of kinetic friction between the puck and the ice. Could the coefficient of friction be larger than 1?,Thus far we have applied Newtons law, F = m*a to linear motion. Now well apply it to rotational motion,Particle moving with _ speed v in a circular path with radius r has an
8、acceleration ar:,(Derivation: see Chapter 4.7),The acceleration points towards the _ of the circle!Centripetal acceleration,Newtons law along the radial direction (along r):,Uniform Circular motion:The velocity of the particle is along the _The centripetal acceleration is towards the _The centripeta
9、l force acting on the particle is towards the _,The magnitude of the centripetal acceleration is: a =_,Newtons law: The force on the particle is (centripetal force) F= ma = _,Centripetal force causes a change in the _ but no change in _.,A particle is moving in a circular path. If the force on the p
10、article would suddenly vanish (string cut) in which direction would the ball fly off?,Black board example 6.3,Franz rotates a stone (m = 0.50 kg) that is attached to the end of a 1.5 m cord above his head in a horizontal circle. If the cord can hold 50 N of tension, at what maximum speed will it rup
11、ture. Which force “provides” the centripetal force?,Black board example 6.4,Jeff Gordon leads his race and must drive into a curve at top speed to win it all. The radius of the curve is 1000.0 m and the coefficient of static friction between his tires and the dry pavement is 0.500. Find the maximum
12、speed he can have and still make the turn. Which force “provides” the centipetal force?,Motion in the presence of drag (or _) forces Motion in _ media.,Objects interact with the medium through which they are moving. AirWater, oil, liquids.,The drag forces depend on the _ of the object.,F v: Objects
13、falling in _, tiny objects falling in air. F v2: _ objects moving in air.,Terminal velocity. When the _ force of the falling object is equal to the _ force on the object.,Examples: - skydiverssoap bubbles falling in airsmall spheres dropping in liquid.,Air drag at high speeds.,For objects moving at
14、high speeds through air: Drag force:,Where: _ Drag coefficient of object (depends on shape) _density of air A cross-sectional area of object v velocity of object,A careless person drives a pick-up truck with a mattress tied to the roof. The mattress is 50 cm high and 150 cm wide. Calculate the resis
15、tive force acting on the mattress if the truck drives with a speed of 30 m/s (67 mi/hr). Should the driver worry about his load if it is tied down with a rope capable of holding 250 N?,Drag coefficient = 0.5 density of air = 1.29 kg/m3,Black board example 6.5,Air drag at high speeds.,For objects mov
16、ing at high speeds through air: Drag force:,Where: C Drag coefficient of object (depends on shape) rdensity of air A cross-sectional area of object v velocity of object,A careless person drives a pick-up truck with a mattress tied to the roof. The mattress is 50 cm high and 150 cm wide. Calculate the resistive force acting on the mattress if the truck drives with a speed of 30 m/s (67 mi/hr). Should the driver worry about his load if it is tied down with a rope capable of holding 250 N?,Drag coefficient = 0.5 density of air = 1.29 kg/m3,Black board example 6.5,
