Physics for Scientist and Engineers third edition Newton s Laws. Example Problem. Variables. Drawing. Solution. Answer: O, 10 N, 20 N, 38N, 39N, 29N

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1 Our 10.0 kg mystery bx rest on a horizontal floor. The coefficent of static friction is µs= 0.40 and the coefficent of kinetic friction is µk= Determine the force of friction Ffr acting on the box if a horizontal external applied Force Fa is exerted on it of magnitude A) 0, B) 10N, C) 20 N D) 38 N E) 40 N Answer: O, 10 N, 20 N, 38N, 39N, 29N RoessBoss 1

2 A 10.0 kg box is pulled along a horizontal surface by a force Fp of 40.0 N which is applied at a 30 angle. There is now friction. The coefficent of friction µk= Determine the acceleration Answer: 78.0 N, 23.4 N, 34.6N, 1.1 m/s^2 RoessBoss 2

3 Two boxes are connected by a cord running over a pulley. Box one (5.0kg) is on the table and Box two (2.0kg) is suspended off of the table. The coefficent of kinetic friction between box 1 and the table is We ignore the mass of the cord and pulley and any friction in the pulley, which means we can assume the force applied to one end of the cord will have the same magnitude at the other end. We wish to find the acceleration a of the sytem, which will have the same magnitude for both boxes assuming the cord doesn t stretch. As box II moves down, Box i moves to the right Answer: 1.4 m/s^2, 17N RoessBoss 3

4 The skier has begun descending on the 30 slope. Assuming the coefficent of kinetic friction is 0.10 calculate a) her acceleration and b) the speed she will reach after 4.0 s Answer: 0.41g, 16 m/s RoessBoss 4

5 Suppose that in the example previously that the snow is slushy and the skier moves down the 30 slope at constant speed. Wht can you say about the coefficent of friction µk. Answer: 0.58 RoessBoss 5

6 A box of mass m1= 10.0 kg rests on a surface inclined at ø= 37 to the horizontal. It is connected by a lightweight cord, which passes over a massless and frictionless pulley to a second mass m2, which hangs freely. A) If the coefficent of static friction is 0.40, determine what range of values for mass m2 will keep the system at rest B) If the coefficent of kinetic firction is 0.30 and m2= 10.0 kg, determine the acceeleration of the system Answer: 0.78 m/s^2 RoessBoss 6

7 Estimate the force a person must exert on a string attached to a kg ball to make the ball revlove in a horizontal circle of radius m. The ball makes 2.00 revolutions per second (T=0.500 s) Answer: 14 N RoessBoss 7

8 A small ball of mass m suspended by a cord of length L revloves in a circle of radius r=l sin ø, where ø is the angle the string makes with the vertical. A) in what direction is the acceleration of the ball, and what causes the acceleration B) Calculate the speed and period of the ball in terms of L, ø, and m Answer: RoessBoss 8

9 A kg ball on the end of a 1.10 m long cord is swung in a vertical circle. Determine the minimum speed the ball must have at the top of its arc so that it continues moving in a circle B) Calcualte the tension in the cord at the bottom of the arc if the ball is moving at twice the speed of part a Answer: 7.34 N RoessBoss 9

10 A 1000 kg car rounds a curve on a flat road of radius 50 m at a speed of 50 km/h (14 m/s). Will the car make the turn, or will it skid if A) the pavement is dry and the coefficent of static friction is µs= 0.60 B) the pavement is icy and µs = 0.25 Answer: 5900 N, 2500 N RoessBoss 10

11 For a car traveling with speed v around a curve of radius r, determine a formula for the angle at which a road should be banked so that no friction is required. B) What is this angle for an expressway off ramp curve radius 50 m at a design speed of 50 km/h Answer: 22 RoessBoss 11

12 A racing car starts from rest in the pit area and accelerates at a uniform rate to a speed of 35 m/s in 11 s, moving on a circular track of radius 500 m. Assuming constant tangential acceleration, find A) the tangential acceleration B) radial acceleration at the instant when the speed is v= 15 m/s and again when v= 30 m/s Answer: 0.45 m/s^2, 1.8 m/s ^2 RoessBoss 12