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Classical Mechanics Lecture 6. Today’s Concept: Friction. Midterm Redux. Seems like time is a big issue!!! Practice makes faster…. Average=18.6/20. Friction. Friction. Friction. Friction. Always opposes the relative motion of two surfaces. Friction. Static Friction. Static Friction.
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Classical Mechanics Lecture 6 Today’s Concept: Friction
Midterm Redux Seems like time is a big issue!!! Practice makes faster… Average=18.6/20
Friction Always opposes the relative motion of two surfaces
Microscopic explanation of Friction • What is Friction? • Why can’t we walk • through walls? • Basically the same • answer to both questions… • Electron clouds of atoms repel (or bond to) each other • http://www.virneth.co.uk/topFriction/friction0.php • http://astro1.panet.utoledo.edu/~vkarpov/Static_Friction_nature.pdf
Checkpoint • 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. • If the truck moves with constant acceleration to the left as shown, which of the following diagrams best describes the static frictional force acting on the box: S a A B C
CheckPoint S a 56% correct 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. 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:
Clicker Question T f M A) MgB) mMgC) T D) 0 Since acceleration is zero. 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?
Clicker Question T f M • A box of mass M sits on a horizontal table. A horizontal string having tensionTapplies 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) MgB) mMgC) T D) 0 Since the box is not moving the forces must be equal, otherwise there would be an acceleration. 38% correct
CheckPoint 47% got this right on first try
CheckPoint m2 m2 g Case 1 (No Friction) Case 2 (With Friction) g m1 m1 In which case is the tension in the string biggest? A) Case 1 B) Case 2 C) Same 65% got this right on first try 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.
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 mk. What is the tension in the string? m2 g m1
Tension in String T1 T2 m2 g 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 > T2B)T1 = T2 C) T1 < T2 m1 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 mk. What is the tension in the string?
Acceleration of coupled blocks 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 mk. What is the tension in the string? m2 g m1 What is the relationship between the magnitudes of the acceleration of the two blocks? A) a1 = a2B) a1 < a2 C) a1 > a2
m2 1) FBD N T m2 g f T m1 m1 m2g m1g
m2 1) FBD 2) SF=ma N T m2 g f T m1 m1 m2g N = m2g m1g m1g – T = m1a T – mm2g = m2a add m1g – mm2g = m1a + m2a m1g – mm2g a = m1 + m2
m2 1) FBD 2) SF=ma N T m2 g f T m1 m1 m2g m1g m1g – T = m1a m1g – mm2g T = m1g – m1a a = m1 + m2 T is smaller when a is bigger
