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

Always opposes the relative motion of two surfaces Friction Always opposes the relative motion of two surfaces

Friction

Static Friction

Static Friction

Static Friction

Without Friction

Friction

Friction

Friction

Friction

Friction

Friction

Friction

Main Points

Main Points

Pre-lecture thoughts

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 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: 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.

Clicker Question 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? M f T A) Mg B) mMg C) T D) 0 Since acceleration is zero.

Clicker Question 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? M f T A) Mg B) mMg C) 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 36

Checkpoint

38% got this right on first try

Checkpoint

CheckPoint m2 m2 Case 1 g Case 2 g m1 m1 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 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 40

m2 m2 Case 1 (No Friction) g 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 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. 41

Resume here

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 43

Tension in String 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? T1 m2 T2 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 > T2 B) T1 = T2 C) T1 < T2 m1 44

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 = a2 B) a1 < a2 C) a1 > a2 45

m2 1) FBD N T m2 g f T m1 m1 m2g m1g 46

m2 1) FBD 2) SF=ma N T m2 g f T m1 m1 m2g N = m2g m1g T – m m2g = m2a m1g – T = m1a add m1g – m m2g = m1a + m2a m1g – m m2g a = m1 + m2 47

T is smaller when a is bigger 1) FBD 2) SF=ma N T m2 g f T m1 m1 m2g m1g m1g – T = m1a m1g – m m2g T = m1g – m1a a = m1 + m2 T is smaller when a is bigger 48

Accelerating Blocks

Accelerating Blocks

Carnival Ride

Carnival Ride

Accelerating Truck

Accelerating Truck

Mass on Incline

Mass on Incline

Mass on Incline

Mass on Incline 2

Mass on Incline 2

Mass on Incline 2

Mass on Incline 2

Block

Pushing Blocks