Classical Mechanics Lecture 6. Today s Concept: Fric3on. Mechanics Lecture 6, Slide 1

Transcription

1 Classical Mechanics Lecture 6 Today s Concept: Fric3on Mechanics Lecture 6, Slide 1

2 can we watch a movie in class please? More daffy duck vids...?

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4 Stuff you asked about: I know this is more of a eometrical issue, but I couldn't see how the anle used to break down the weiht components of a block si9n on a ramp into its x and y components, is the same as the anle of the ramp. If you could briefly explain that in lecture it would be very nice. y mcos(θ) 90 θ 90 θ 90 θ x msin(θ) m Mechanics Lecture 6, Slide 3

5 Clicker Question A block weihin 4 N is hun from a rope a@ached to a scale. The scale is then a@ached to a wall and reads 4 N. What will the scale read when it is instead a@ached to another block weihin 4 N?? m m m A) 0 N B) 4 N C) 8 N Mechanics Lecture 6, Slide 4

6 Friction Always opposes the rela3ve mo3on of two surfaces Mechanics Lecture 6, Slide 5

7 CheckPoint A box sits on the horizontal bed of a movin truck. StaBc fricbon between the box and the truck keeps the box from slidin around as the truck drives. a µ S If the truck moves with constant accelerabon to the lec as shown, which of the followin diarams best describes the stabc fricbonal force acbn on the box: A B C Mechanics Lecture 6, Slide 6

8 CheckPoint a If the truck moves with constant accelera3n to the lej as shown, which of the followin diarams best describes the sta3c fric3onal force ac3n on the box: µ S A B C A) When you're drivin and you have somethin loose si9n in the car, that object slides backwards if you accelerate too fast so the fricbonal force must be acbn in the direcbon of the car's accelerabon. B) as the truck accelerates, the box wants to slide backwards, the forces of fricbon opposes mobon, and is thus opposite the mobon of the box Mechanics Lecture 6, Slide 7

9 CheckPoint a If the truck moves with constant accelera3n to the lej as shown, which of the followin diarams best describes the sta3c fric3onal force ac3n on the box: µ S A B C the force has to o in the same direc<on as the accelera<on. A B C Mechanics Lecture 6, Slide 8

10 Clicker Question A box of mass M sits on a horizontal table. A horizontal strin havin tension T applies a force on the box, but sta3c fric3on between the box and the table keeps the box from movin. What is the manitude of the total force ac3n on the box? A) M B) µm C) T D) 0 f M T Since accelera3on is zero. Mechanics Lecture 6, Slide 9

11 CheckPoint A box of mass M sits on a horizontal table. A horizontal strin havin tension T applies a force on the box, but stabc fricbon between the box and the table keeps the box from movin. What is the manitude of the sta3c fric3onal force ac3n on the box? A) M B) µm C) T D) 0 f M T Since the box is not movin the forces must be equal, otherwise there would be an accelera3on. Mechanics Lecture 6, Slide 10

12 Mechanics Lecture 6, Slide 11

13 CheckPoint A block slides on a table pulled by a strin aiached to a hanin weiht. In Case 1 the block slides without fricbon and in Case 2 there is kinebc fricbon between the slidin block and the table. Case 1 (No Fric<on) Case 2 (With Fric<on) In which case is the tension in the strin biest? A) Case 1 B) Case 2 C) Same ~80% ot this riht Mechanics Lecture 6, Slide 12

14 Clicker Question A block slides on a table pulled by a strin aiached to a hanin weiht. In Case 1 the block slides without fricbon and in Case 2 there is kinebc fricbon between the slidin block and the table. Case 1 (No Fric<on) Case 2 (With Fric<on) What is the tension in the strin in Case 1? A) T = 0 B) T = m C) T is between 0 and m Mechanics Lecture 6, Slide 13

15 Case 1 (No Fric<on) Case 2 (With Fric<on) In which case is the tension in the strin biest? A) Case 1 B) Case 2 C) Same B) M1 will not be accelera<n as fast in case two than in case one because there is the extra force of fric<on ac<n aainst mass 1. Since the accelera<on is smaller in case two, there has to be more of a force ac<n aainst ravity, the only other possible force is Tension. C) The tension on the strin is m1* reardless of what is happenin on top of the table; the force is equal. Mechanics Lecture 6, Slide 14

16 Case 1 (No Fric<on) Case 2 (With Fric<on) In which case is the tension in the strin biest? A) Case 1 B) Case 2 C) Same Some wron explanabons for pickin B) : In case one the tension in the strin is just equal to m, but in case 2 the tension is equal to m + fricbon. In case 1, is like free fall, so there is no tension in the strin. In case 2, the tension of the strin is equal to the kinebc fricbon. Mechanics Lecture 6, Slide 15

17 Lets work it out A block ( ) slides on a table pulled by a strin a@ached to a mass ( ) hanin over the side. The coefficient of kine3c fric3on between the slidin block and the table is µ k. What is the tension in the strin? (pulley is massless) Mechanics Lecture 6, Slide 16

18 Clicker Question A block ( ) slides on a table pulled by a strin a@ached to a mass ( ) hanin over the side. The coefficient of kine3c fric3on between the slidin block and the table is µ k. What is the tension in the strin? T 1 T2 What is the rela3onship between the manitude of the tension of the strin at block 2 and the manitude of the tension in the strin at block 1? A) T 1 > T 2 B) T 1 = T 2 C) T 1 < T 2 Mechanics Lecture 6, Slide 17

19 Clicker Question A block ( ) slides on a table pulled by a strin a@ached to a mass ( ) hanin over the side. The coefficient of kine3c fric3on between the slidin block and the table is µ k. What is the tension in the strin? What is the rela3onship between the manitudes of the accelera3on of the two blocks? A) a 1 = a 2 B) a 1 < a 2 C) a 1 > a 2 Mechanics Lecture 6, Slide 18

20 1) FBD N f T T Mechanics Lecture 6, Slide 19

21 1) FBD 2) ΣF=ma f N T T N = T µ = a T = a add µ = a + a a = µ + Mechanics Lecture 6, Slide 20

22 1) FBD 2) ΣF=ma f N T T a = µ + T = a T = a T is smaller when a is bier Mechanics Lecture 6, Slide 21