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

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Unformatted text preview: fm Physics 211 Lecture 6 Today's Concept: Friction Physics 211 Lecture 6, Slide 1 Physics Stuff you asked about: Pudding How could the Bears lose? go over boxes being pulled on a table by a string connected to a box hanging off the table The wheel rolling, not sliding. If you tape a piece of toast butter side up on the back of the cat and drop the entire system, which law of the universe is violated? But more to the point, can you go over in more detail the derivations of the equations in the prelecture? In the case of the car going around the track, how are we able to assume that the friction is counteracting the acceleration? It makes sense but I don't fully understand it from a physics point of view. I thought centripetal acceleration pointed towards the of center of the circle, thus to counter act it why wouldn't the static friction force point outwards? Physics 211 Lecture 6, Slide Physics 2 Preflight 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. μS 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 3 Physics μS 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) The friction is causing the box to accelerate in the same The direction as the truck. B) Frictional force is always in the opposite direction of the motion Frictional Physics 211 Lecture 6, Slide 4 Physics μS 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 5 Physics Act Act A box of mass M sits on a horizontal table. A horizontal sits string having tension T applies a force on the box, but static applies 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 Mg B) μMg B) C) T C) D) 0 f Physics 211 Lecture 6, Slide 6 Physics Preflight Preflight A box of mass M sits on a horizontal table. A horizontal sits string having tension T applies a force on the box, but static applies 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 Mg B) μMg B) C) T C) D) 0 f Since there is no acceleration, the forces must be equal. Physics 211 Lecture 6, Slide 7 Physics Physics 211 Lecture 6, Slide 8 Physics 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 g (no friction) (no m1 Case 2 g (with friction) (with m1 In which case is the tension in the string biggest? A) Case 1 B) Case 2 C) Same Physics 211 Lecture 6, Slide 9 Physics Act A 3kg block (m2) slides on a table pulled by a string attached slides to a 5kg mass (m1) hanging over the side. The coefficient of hanging kinetic friction between the sliding block and the table is 0.3. What is the tension T2 in the string? What in m2 T2 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? T1 g m1 A) T1 > T2 B) T1 = T2 C) T1 < T2 B) C) Physics 211 Lecture 6, Slide 10 Physics m2 m2 Case 1 g (no friction) (no m1 Case 2 g (with friction) (with m1 In which case is the tension in the string biggest? A) Case 1 B) Case 2 C) Same B) In case 2, the acceleration of m1 is less than in case 1, In which means there is a larger tension force holding it back in case 2. C) They both have the same pull from m1 They Physics 211 Lecture 6, Slide 11 Physics Lets work it out A 3kg block (m2) slides on a table pulled by a string attached slides to a 5kg mass (m1) hanging over the side. The coefficient of hanging kinetic friction between the sliding block and the table is 0.3. What is the tension in the string? What m 2 g m1 Physics 211 Lecture 6, Slide 12 Physics Act A 3kg block (m2) slides on a table pulled by a string attached slides to a 5kg mass (m1) hanging over the side. The coefficient of hanging kinetic friction between the sliding block and the table is 0.3. What is the tension in the string? What m 2 g m1 What is the relationship between the magnitudes of the acceleration of the two blocks? A) a1 = a2 B) a1 < a2 C) a1 > a2 B) C) Physics 211 Lecture 6, Slide 13 Physics m2 1) FBD FBD 2) ΣF=ma f N m2 m2g T T m1 g m1 m1g Physics 211 Lecture 6, Slide 14 Physics m2 1) FBD FBD 2) ΣF=ma f N T m2 g T m1 m2g m1 m1g N = m2 g m1 g − T = m1a T − μ m2 g = m2 a add m1 g − μ m2 g = m1a + m2 a m1 g − μ m2 g a= m1 + m2 Physics 211 Lecture 6, Slide 15 Physics m2 1) FBD FBD 2) ΣF=ma f N m2 m2g m1 g − μ m2 g a= m1 + m2 T g T m1 m1 m1g m1 g − T = m1a T = m1 g − m1a T is smaller when a is bigger Physics 211 Lecture 6, Slide 16 Physics ...
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## This note was uploaded on 09/22/2011 for the course PHYSICS 211 taught by Professor Selig during the Fall '10 term at University of Illinois, Urbana Champaign.

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