Copyright © 2010 Pearson Education, Inc. Lecture Outline Chapter 6 Physics, 4 th Edition James S. Walker

Copyright © 2010 Pearson Education, Inc. Chapter 6 Applications of Newton’s Laws

Copyright © 2010 Pearson Education, Inc. Units of Chapter 6 Frictional Forces Strings and Springs Translational Equilibrium Connected Objects Circular Motion

Copyright © 2010 Pearson Education, Inc. 6-1 Frictional Forces Friction has its basis in surfaces that are not completely smooth: Even “smooth” surfaces have irregularities when viewed at the microscopic level. This type of roughness contributes to friction. Friction Is caused by the random, microscopic irregularities of a surface and since it is greatly affected by by other factors such as the presence of lubricants, there is no simple “law of nature” for friction. Two types of friction most commonly used- kinetic friction and static friction.

Copyright © 2010 Pearson Education, Inc. 6-1 Frictional Forces Kinetic friction is the friction encountered when surfaces slide against one another with a finite relative speed. The force generated by this friction, which will be designated with the symbol f k, acts to oppose the sliding motion at the point of contact between the surfaces The static frictional force depends on the normal force: (6-1) The constant is called the coefficient of kinetic friction.

Copyright © 2010 Pearson Education, Inc. 6-1 Frictional Forces

Copyright © 2010 Pearson Education, Inc. 6-1 Frictional Forces The kinetic frictional force is also independent of the relative speed of the surfaces, and of their area of contact. In the top of the figure, a force F is required to pull the brick with constant speed v. Thus the force kinetic friction is f k = F. In the bottom part of the figure, the normal force has been doubled, and so has the force of kinetic friction, to f k = 2F. Adding a second brick doubles the normal force -- which doubles the force of kinetic friction.

Copyright © 2010 Pearson Education, Inc. 6-1 Frictional Forces Someone at the other end of the table asks you to pass the salt. Feeling quite dashing, you slide the 50.0-g shaker in their direction, giving it an initial speed of 1.15 m/s. (a) If the shaker comes to rest with constant acceleration in 0.84 m, what is the coefficient of kinetic friction between the shaker and the table? (b) How much time is required for the shaker to come to rest if you slide it with an initial speed of 1.32 m/s? V x 2 = V ox 2 + 2a x ∆x => a x = (V x 2 – V ox 2 ) / 2∆x = 0 – (1.15 m/s) 2 / 2 (0.840 m) a x = m/s 2 ∑F y = ma x = 0 => ∑F y = N + (-W) = ma y = 0 or N = W = mg ∑F x = -f k = ma x => f k = -ma x = µ k N => µ k = f k / N = -ma x / mg = -a x /g = -( m/s 2 ) / 9.81 m/s 2 = = µ k (b). Vx = Vox + axt = > t = V x – V ox ) / a x = 0 – (1.32 m/s) / m/s 2 t = 1.68 s

Copyright © 2010 Pearson Education, Inc.

The static frictional force keeps an object from starting to move when a force is applied. The static frictional force has a maximum value, but may take on any value from zero to the maximum, 6-1 Frictional Forces depending on what is needed to keep the sum of forces zero.

Copyright © 2010 Pearson Education, Inc. 6-1 Frictional Forces (6-2) where (6-3) The constant of proportionality is called µ s the coefficient of static friction. Note that µ s, like µ k, is dimensionless. In most cases, µ s is greater than µ k, indicating that the force of static friction is greater than the force of kinetic friction. In fact, it is not uncommon for µ s to be greater than 1. The static frictional force is also independent of the area of contact and the relative speed of the surfaces. Rules of Thumb for Static Friction The force of static friction between two surfaces has the following properties: 1.It takes on any value between zero and the maximum possible force of static friction, f s,max = µ s N 2.It is independent of the area of contact between the surfaces. 3.It is parallel to the surface of contact, and in the direction that opposes relative motion.

Copyright © 2010 Pearson Education, Inc. 6-1 Slightly Tilted A flatbed truck slowly tilts its bed upward to dispose of a 95.0 –kg crate. For small angles of tilt the crate stays put, but when the tilt angle exceeds 23.2 o, the crate begins to slide. What is the coefficient of static friction between the bed of the truck and the crate? F s,x = -ma x = - f s,max = - µ s N W s = mg sin θ W y = -mg cos θ ∑F y = N + W y = N – mg cos θ = ma y = 0 N = mg cos θ ∑F x = - f s,x + W x = µ s N + mg sin θ = ma x = 0 ∑F x = - µ s mg cos θ + mg sin θ => µ s mg cos θ = mg sin θ µ s = mg sin θ / mg cos θ = tan θ = tan 23.2 o = µ s = 0.429

Copyright © 2010 Pearson Education, Inc. 6-1 Slightly Tilted

Copyright © 2010 Pearson Education, Inc. 6-2 Strings and Springs A common way to exert a force on an object is to pull on it with a string, a rope, a cable, or a wire. Similarly, you can push or pull on an object if you attach it to a spring. Imagine picking up a light string and holding it with one end in each hand. If you pull to the right with your right hand with a force T and to the left with your left hand with a force T, the string become taut. We say that there is tension T in the string. If your friend were to cut the string at some point, the tension T is the force pulling the ends apart.

Copyright © 2010 Pearson Education, Inc. 6-2 Strings and Springs The tension in a real rope will vary along its length, due to the weight of the rope. Here, we will assume that all ropes, strings, wires, etc. are massless unless otherwise stated. Tension in a heavy rope Because of the weight of the rope, the tension is noticeably different at points 1,2 and 3. As the rope becomes lighter, however, the difference in tension decrease. In the limit of a rope of zero mass, the tension is the same throughout the rope.

Copyright © 2010 Pearson Education, Inc. 6-2 Strings and Springs An ideal pulley is one that simply changes the direction of the tension in a string, without changing its magnitude.

Copyright © 2010 Pearson Education, Inc. 6-2 Strings and Springs A traction device employing three pulleys is applied to a broken leg, as shown in the sketch. The middle pulley is attached to the sole of the foot, and a mass m supplies the tension in the ropes. Find the value of the mass m if the force exerted on the sole of the foot by the middle pulley is to be 165 N.

Copyright © 2010 Pearson Education, Inc. 6-2 Strings and Springs T 1,x = T cos 40.0 O T 1,y = T sin 40.0 O T 2,x = T cos 40.0 O T 2,y = -T sin 40.0 O ∑F x = T cos 40.0 O + T cos 40.0 O = 2 T cos 40.0 O ∑F y = T sin 40.0 O - T sin 40.0 O = N = 2 T cos 40.0 O T = 165 N / 2 cos 40.0 O T = 108 N T = mg => m = T/g m = 108 N / 9.81 m/s 2 = 11.0 kg m = 11 kg

Copyright © 2010 Pearson Education, Inc. 6-2 Strings and Springs Hooke’s law for springs states that a spring exerts a force that is proportional to the amount, x by which it is stretched or compressed. Thus, if F is the magnitude of the spring force, we can say that In this expression, k is a constant of proportionality, referred to as the force constant, or equivalent ly, as the spring constant. Since F has units of Newtons and x has units of meters, it follows that k has units of Newtons per meter, or N/m. The larger the value of k, the stiffer the spring.

Copyright © 2010 Pearson Education, Inc. 6-2 Strings and Springs The force exerted by a spring When dealing with a spring, it is convenient to choose the origin at the equilibrium (zero force) position. In the cases shown here, the force is strictly in the x direction, and is given by F x = -kx. Note that the minus sign means that the force is opposite to the displacement, that is, the force is restoring.

Copyright © 2010 Pearson Education, Inc. 6-2 Strings and Springs Rules of Thumb for Springs (Hooke’s Law) A spring stretched or compressed by the amount x from its equilibrium length exerts a force whose x component is given by Fx = -kx (gives magnitude and direction) If we are interested only in the magnitude of the force associated with a given stretch or compression, we use the somewhat simpler form of Hooke’s law: F = kx ( gives magnitude only). In this text, we consider only ideal springs – that is, springs that are massless, and that are assumed to obey Hooke’s law

Copyright © 2010 Pearson Education, Inc. 6-3 Translational Equilibrium When an object is in translational equilibrium, the net force on it is zero: (6-5) This allows the calculation of unknown forces.

Copyright © 2010 Pearson Education, Inc. 6-3 Translational Equilibrium Raising a bucket A person lifts a bucket of water from the bottom of a well with a constant speed, v. Because the speed is constant, the net force acting on the bucket must be zero.

Copyright © 2010 Pearson Education, Inc. 6-4 Connected Objects A 1.84 kg bag of clothespins hangs in the middle of a clothesline, causing it to sang by an angle θ = 3.5 o. Find the tension, T, in the clothesline. Ty = T sin θ W y = -mg ∑Fy = o T sin θ + T sin θ – mg = 0 T = mg / (2 sin θ) T = (1.84 kg) * (9.81 m/s 2 ) / (2 sin 3.5 O ) T = 148 N

Copyright © 2010 Pearson Education, Inc. 6-4 Connected Objects When forces are exerted on connected objects, their accelerations are the same. If there are two objects connected by a string, and we know the force and the masses, we can find the acceleration and the tension:

Copyright © 2010 Pearson Education, Inc. 6-4 Connected Objects We treat each box as a separate system: It is straight forward to solve for the acceleration in terms of the applied force F: a = F / (m 1 + m 2 )

Copyright © 2010 Pearson Education, Inc. 6-4 Connected Objects If there is a pulley, it is easiest to have the coordinate system follow the string: T = m 1 a = (m 1 m 2 / m 1 + m 2 ) g

Copyright © 2010 Pearson Education, Inc. 6-4 Connected Objects Atwood’s machine consists of two masses connected by a string that passes over a pulley, as shown below. Find the acceleration of the masses for general m 1 and m 2, and evaluate for the specific case m 1 = 3.1 kg, m 2 = 4.4 kg ∑F 1,x = T – m 1 g = m 1 a ∑F 2,x = T – m 2 g = m 2 a Sum the two relations obtained above to eliminate T T – m 1 g = m 1 a T – m 2 g = m 2 a => (m 2 – m 1 )g = (m 1 + m 2 )a a = ((m 2 – m 1 ) / (m 1 + m 2 ))g a = (4.4 kg -3.1 kg) * 9.81 (3.1 kg kg) a = 1.7 m/s 2

Copyright © 2010 Pearson Education, Inc. 6-5 Circular Motion An object moving in a circle must have a force acting on it; otherwise it would move in a straight line. The direction of the force is towards the center of the circle.

Copyright © 2010 Pearson Education, Inc. 6-5 Circular Motion Some algebra gives us the magnitude of the acceleration, and therefore the force, required to keep an object of mass m moving in a circle of radius r. The magnitude of the force is given by: (6-15)

Copyright © 2010 Pearson Education, Inc. 6-5 Circular Motion This force may be provided by the tension in a string, the normal force, or friction, among others.

Copyright © 2010 Pearson Education, Inc. 6-5 Circular Motion If a roadway is banked at the proper angle, a car can round a corner without any assistance from friction between the tires and the road. Find the appropriate banking angle for a 900-kg car traveling at 20.5 m/s in a turn of radius 85.0 m. Since the weight W has no x component, it follows that the normal force N must supply the needed centripetal force. ∑Fy = N cos θ - W = 0 N = W / Cos θ = mg /cos θ ∑Fx = N sin θ ma x = ma cp = mv 2 / r N sin θ = mg sin θ / cos θ = mv 2 / r tan θ = v 2 / gr or θ = tan -1 (v 2 /gr) θ = tan -1 ((20.5 m/s) 2 / (9.81 m/s 2 )(85.0 m)) = 26.7 o

Copyright © 2010 Pearson Education, Inc. 6-5 Circular Motion While driving along a country lane with a constant speed of 17.0 m/s, you encounter a dip in the road. The dip can be approximated as a circular arc, with a radius of 65 m. What is the normal force exerted by a car seat on an 80.0 kg passenger when the car is at the bottom of the dip? ∑F y = ma y N – mg = ma y Replace a y with the centripetal acceleration a y = V 2 /r N = mg + mv 2 / r N = 1140 N

Copyright © 2010 Pearson Education, Inc. 6-5 Circular Motion An object may be changing its speed as it moves in a circle; in that case, there is a tangential acceleration as well:

Copyright © 2010 Pearson Education, Inc. Summary of Chapter 6 Friction is due to microscopic roughness. Kinetic friction: Static friction: Tension: the force transmitted through a string. Force exerted by an ideal spring:

Copyright © 2010 Pearson Education, Inc. Summary of Chapter 6 An object is in translational equilibrium if the net force acting on it is zero. Connected objects have the same acceleration. The force required to move an object of mass m in a circle of radius r is: