# Lect06 - Physics 211 Lecture 6 Friction Today's Concept...

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Unformatted text preview: Physics 211 Lecture 6 Friction Today's Concept: Physics 211 Lecture 6, Slide 1 Stuff you asked about: Blocks falling down ramps. I always confuse the forces going in the x and the y because I am used to cosine referring to the x direction and sine referring to the y direction but it is opposite when we consider boxes sliding down ramps. I don't know; I understand friction pretty well. The only reason by SmartScore sucks so much is because I didn't read the directions properly. The concept I felt was not hard to understand but hard to predict the answers to. Though I understood the material, I needed to do the calculations in order to get the right answer. I feel like it is hard to determine when to use which principles when dealing with these problems? If we could go over more examples, it would be really helpful! . I just want some more problems discussed in class regarding the difference between static and kinetic friction so I can become more comfortable with it. Why did the chicken cross the road? Chickens at rest tend to stay at rest. Chickens in motion tend to cross the road. Or: It was pushed on the road by another chicken, which went away from the road. Or: It was attracted to a chicken on the other side of the road nothing, it's just too easy Physics 211 Lecture 6, Slide 2 I know this is more of a geometrical issue, but I couldn't see how the angle used to break down the weight components of a block sitting on a ramp into its x and y components, is the same as the angle of the ramp. If you could briefly explain that in lecture it would be very nice. 90 q 90-q 90 q mg y x Act Revisited A block weighing 4 lbs is hung from a rope attached to a scale. The scale is then attached to a wall and reads 4 lbs. What will the scale read when it is instead attached to another block weighing 4 lbs? ? m m m A) 0 lbs. B) 4 lbs. C) 8 lbs. Physics 211 Lecture 6, Slide 4 Friction Always opposes the relative motion of two surfaces Physics 211 Lecture 6, Slide 5 Preflight A box sits on the horizontal bed of a moving truck. Static friction between the box and the truck keeps the box from sliding around as the truck drives. S a If the truck moves with constant accelerating to the left as shown, which of the following diagrams best describes the static frictional force acting on the box: A B C Physics 211 Lecture 6, Slide 6 S a If the truck moves with constant accelerating to the left as shown, which of the following diagrams best describes the static frictional force acting on the box: A B C A) In order to keep the box from sliding to the back of the truck as it accelerates, the frictional force needs to pull/push the box forward. B) Friction always opposes motion/acceleration. Physics 211 Lecture 6, Slide 7 S a If the truck moves with constant accelerating to the left as shown, which of the following diagrams best describes the static frictional force acting on the box: A B C the force has to go in the same direction as the acceleration. Physics 211 Lecture 6, Slide 8 Act A box of mass M sits on a horizontal table. A horizontal string having tension T applies a force on the box, but static friction between the box and the table keeps the box from moving. What is the magnitude of the total force acting on the box? A) Mg B) Mg C) T D) 0 f Since acceleration is zero Physics 211 Lecture 6, Slide 9 Preflight A box of mass M sits on a horizontal table. A horizontal string having tension T applies a force on the box, but static friction between the box and the table keeps the box from moving. What is the magnitude of the static frictional force acting on the box? A) Mg B) Mg C) T D) 0 f Since the box is not moving the forces must be equal, otherwise there would be an acceleration. Physics 211 Lecture 6, Slide 10 Physics 211 Lecture 6, Slide 11 Preflight A block slides on a table pulled by a string attached to a hanging weight. In Case 1 the block slides without friction and in Case 2 there is kinetic friction between the sliding block and the table. m2 m2 Case 1 (no friction) g m1 Case 2 g (with friction) m1 In which case is the tension in the string biggest? A) Case 1 B) Case 2 C) Same 52% got this right Physics 211 Lecture 6, Slide 12 Act A block slides on a table pulled by a string attached to a hanging weight. In Case 1 the block slides without friction and in Case 2 there is kinetic friction between the sliding block and the table. m2 m2 Case 1 (no friction) g m1 Case 2 g (with friction) m1 What is the tension in the string in Case 1? A) T = 0 B) T = mg C) T is between 0 and mg Physics 211 Lecture 6, Slide 13 m2 m2 Case 1 (no friction) m1 Case 2 (with friction) m1 In which case is the tension in the string biggest? A) Case 1 B) Case 2 C) Same B) M1 will not be accelerating as fast in case two than in case one because there is the extra force of friction acting against mass 1. Since the acceleration is smaller in case two, there has to be more of a force acting against gravity, the only other possible force is Tension C) The tension on the string is m1*g regardless of what is happening on top of the table; the force is equal. Physics 211 Lecture 6, Slide 14 m2 m2 Case 1 (no friction) g m1 Case 2 g (with friction) m1 In which case is the tension in the string biggest? A) Case 1 B) Case 2 C) Same Some popular wrong explanations for picking B) : Without friction there is no tension. in case 1, m1 is like free fall, so there is no tension in the string. In case 2, the tension of the string is equal to the kinetic friction Physics 211 Lecture 6, Slide 15 Lets work it out A block (m2) slides on a table pulled by a string attached to a mass (m1) hanging over the side. The coefficient of kinetic friction between the sliding block and the table is k. What is the tension in the string? m 2 g m1 Physics 211 Lecture 6, Slide 16 Act A block (m2) slides on a table pulled by a string attached to a mass (m1) hanging over the side. The coefficient of kinetic friction between the sliding block and the table is k. What is the tension in the string? m2 T2 T1 What is the relationship between the magnitude of the tension of the string at block 2 and the magnitude of the tension in the string at block 1? A) T1 > T2 B) T1 = T2 C) T1 < T2 g m1 Physics 211 Lecture 6, Slide 17 Act A block (m2) slides on a table pulled by a string attached to a mass (m1) hanging over the side. The coefficient of kinetic friction between the sliding block and the table is k. What is the tension in the string? m 2 g What is the relationship between the magnitudes of the acceleration of the two blocks? A) a1 = a2 B) a1 < a2 C) a1 > a2 m1 Physics 211 Lecture 6, Slide 18 1) FBD m2 N f m2 m 2g T T m1 m 1g m1 Physics 211 Lecture 6, Slide 19 1) FBD 2) SF=ma m2 N f m2 m 2g T T m1 m 1g m1 N m2 g T - m2 g m2 a add m1 g - T m1a m1 g - m2 g m1a m2 a m1 g - m2 g a m1 m2 Physics 211 Lecture 6, Slide 20 1) FBD 2) SF=ma m2 N f m2 m 2g T T m1 m 1g m1 m1 g - m2 g a m1 m2 m1 g - T m1a T m1 g - m1a T is smaller when a is bigger Physics 211 Lecture 6, Slide 21 ...
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## This note was uploaded on 01/27/2011 for the course PHYS 211 taught by Professor Liss during the Fall '08 term at University of Illinois at Urbana–Champaign.

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