phys. 121: thursday, 11 sep. ● reading: read ch. 7 for tuesday. ● written hw 3: due by 5:00 pm...
TRANSCRIPT
![Page 1: Phys. 121: Thursday, 11 Sep. ● Reading: Read ch. 7 for Tuesday. ● Written HW 3: Due by 5:00 pm today. ● Written HW 4: 6.17, 6.54, 7.8, 7.23, 7.26, and](https://reader030.vdocuments.mx/reader030/viewer/2022032706/56649dfe5503460f94ae635e/html5/thumbnails/1.jpg)
Phys. 121: Thursday, 11 Sep.
● Reading: Read ch. 7 for Tuesday.● Written HW 3: Due by 5:00 pm today.● Written HW 4: 6.17, 6.54, 7.8, 7.23, 7.26, and 7.29. (Use table 6.1 on p. 149 for friction coefficients.) Due in one week.● Mastering Phys.: Third assign. due Tuesday.● NM Tech Physics YouTube video on ch. 5 material; link on class web page. Please watch it by Tuesday!
![Page 2: Phys. 121: Thursday, 11 Sep. ● Reading: Read ch. 7 for Tuesday. ● Written HW 3: Due by 5:00 pm today. ● Written HW 4: 6.17, 6.54, 7.8, 7.23, 7.26, and](https://reader030.vdocuments.mx/reader030/viewer/2022032706/56649dfe5503460f94ae635e/html5/thumbnails/2.jpg)
How many of you still need an iClicker?
![Page 3: Phys. 121: Thursday, 11 Sep. ● Reading: Read ch. 7 for Tuesday. ● Written HW 3: Due by 5:00 pm today. ● Written HW 4: 6.17, 6.54, 7.8, 7.23, 7.26, and](https://reader030.vdocuments.mx/reader030/viewer/2022032706/56649dfe5503460f94ae635e/html5/thumbnails/3.jpg)
Hints for today's HW?
![Page 4: Phys. 121: Thursday, 11 Sep. ● Reading: Read ch. 7 for Tuesday. ● Written HW 3: Due by 5:00 pm today. ● Written HW 4: 6.17, 6.54, 7.8, 7.23, 7.26, and](https://reader030.vdocuments.mx/reader030/viewer/2022032706/56649dfe5503460f94ae635e/html5/thumbnails/4.jpg)
Clickers: When is Newton's SecondLaw not applicable in the formF = m a (even at low speeds)?
• a) When force is not constant• b) When momentum is not constant• c) When acceleration is not constant• d) When acceleration is not zero• e) When mass is not constant
![Page 5: Phys. 121: Thursday, 11 Sep. ● Reading: Read ch. 7 for Tuesday. ● Written HW 3: Due by 5:00 pm today. ● Written HW 4: 6.17, 6.54, 7.8, 7.23, 7.26, and](https://reader030.vdocuments.mx/reader030/viewer/2022032706/56649dfe5503460f94ae635e/html5/thumbnails/5.jpg)
As a vector formula, this gives 2 equationsin 1 (or 3-in-1 in 3 dimensions). We willuse it over and over again!
Strategy: draw a force diagram forthe relevant object(s). Construct acoordinate system which simplifiesthings as much as possible.
![Page 6: Phys. 121: Thursday, 11 Sep. ● Reading: Read ch. 7 for Tuesday. ● Written HW 3: Due by 5:00 pm today. ● Written HW 4: 6.17, 6.54, 7.8, 7.23, 7.26, and](https://reader030.vdocuments.mx/reader030/viewer/2022032706/56649dfe5503460f94ae635e/html5/thumbnails/6.jpg)
[Clickers: A given force F pulls straight down.What value for the angle minimizes the force on
each anchor point (R and L)?] a) 0 degrees b) Between 0 and 45
degrees c) Between 45 and 90
degrees d) Exactly 90 degrees e) Need more
information to tell.F
X X
RL
(This angle) (equal to this)
![Page 7: Phys. 121: Thursday, 11 Sep. ● Reading: Read ch. 7 for Tuesday. ● Written HW 3: Due by 5:00 pm today. ● Written HW 4: 6.17, 6.54, 7.8, 7.23, 7.26, and](https://reader030.vdocuments.mx/reader030/viewer/2022032706/56649dfe5503460f94ae635e/html5/thumbnails/7.jpg)
Example:
Find T in eachstring.
![Page 8: Phys. 121: Thursday, 11 Sep. ● Reading: Read ch. 7 for Tuesday. ● Written HW 3: Due by 5:00 pm today. ● Written HW 4: 6.17, 6.54, 7.8, 7.23, 7.26, and](https://reader030.vdocuments.mx/reader030/viewer/2022032706/56649dfe5503460f94ae635e/html5/thumbnails/8.jpg)
Example: Table isfrictionless; find the tension in thestring and the period of circular motion.
![Page 9: Phys. 121: Thursday, 11 Sep. ● Reading: Read ch. 7 for Tuesday. ● Written HW 3: Due by 5:00 pm today. ● Written HW 4: 6.17, 6.54, 7.8, 7.23, 7.26, and](https://reader030.vdocuments.mx/reader030/viewer/2022032706/56649dfe5503460f94ae635e/html5/thumbnails/9.jpg)
Example: Rock massis 940 g; string breaks after 120 N; L is1.3 meters. Find (a) max. speed; (b) θ.
![Page 10: Phys. 121: Thursday, 11 Sep. ● Reading: Read ch. 7 for Tuesday. ● Written HW 3: Due by 5:00 pm today. ● Written HW 4: 6.17, 6.54, 7.8, 7.23, 7.26, and](https://reader030.vdocuments.mx/reader030/viewer/2022032706/56649dfe5503460f94ae635e/html5/thumbnails/10.jpg)
Frictional Forces: Static and Kinetic
• All frictional forces due to surface contact point parallel to the surface (so, 90 degrees to the
normal force N).• Static friction force points opposite to the sum of
the other forces parallel to the surface.• Kinetic friction force points opposite to velocity.• Both have magnitude depending on N; however static frictional force can adjust its magnitude!
![Page 11: Phys. 121: Thursday, 11 Sep. ● Reading: Read ch. 7 for Tuesday. ● Written HW 3: Due by 5:00 pm today. ● Written HW 4: 6.17, 6.54, 7.8, 7.23, 7.26, and](https://reader030.vdocuments.mx/reader030/viewer/2022032706/56649dfe5503460f94ae635e/html5/thumbnails/11.jpg)
Static Frictional Forces
( f⃗ s) μ⩽ s ( n⃗ )
![Page 12: Phys. 121: Thursday, 11 Sep. ● Reading: Read ch. 7 for Tuesday. ● Written HW 3: Due by 5:00 pm today. ● Written HW 4: 6.17, 6.54, 7.8, 7.23, 7.26, and](https://reader030.vdocuments.mx/reader030/viewer/2022032706/56649dfe5503460f94ae635e/html5/thumbnails/12.jpg)
![Page 13: Phys. 121: Thursday, 11 Sep. ● Reading: Read ch. 7 for Tuesday. ● Written HW 3: Due by 5:00 pm today. ● Written HW 4: 6.17, 6.54, 7.8, 7.23, 7.26, and](https://reader030.vdocuments.mx/reader030/viewer/2022032706/56649dfe5503460f94ae635e/html5/thumbnails/13.jpg)
Static Frictional Forces
( f⃗ s) μ⩽ s ( n⃗ )
![Page 14: Phys. 121: Thursday, 11 Sep. ● Reading: Read ch. 7 for Tuesday. ● Written HW 3: Due by 5:00 pm today. ● Written HW 4: 6.17, 6.54, 7.8, 7.23, 7.26, and](https://reader030.vdocuments.mx/reader030/viewer/2022032706/56649dfe5503460f94ae635e/html5/thumbnails/14.jpg)
Kinetic Friction
Always pointsopposite to the (relative)velocity (with respect tothe surface it's sliding against).
∣⃗f k∣=μk∣⃗n∣
![Page 15: Phys. 121: Thursday, 11 Sep. ● Reading: Read ch. 7 for Tuesday. ● Written HW 3: Due by 5:00 pm today. ● Written HW 4: 6.17, 6.54, 7.8, 7.23, 7.26, and](https://reader030.vdocuments.mx/reader030/viewer/2022032706/56649dfe5503460f94ae635e/html5/thumbnails/15.jpg)
![Page 16: Phys. 121: Thursday, 11 Sep. ● Reading: Read ch. 7 for Tuesday. ● Written HW 3: Due by 5:00 pm today. ● Written HW 4: 6.17, 6.54, 7.8, 7.23, 7.26, and](https://reader030.vdocuments.mx/reader030/viewer/2022032706/56649dfe5503460f94ae635e/html5/thumbnails/16.jpg)
Simple (“cartoon”) picture of friction
![Page 17: Phys. 121: Thursday, 11 Sep. ● Reading: Read ch. 7 for Tuesday. ● Written HW 3: Due by 5:00 pm today. ● Written HW 4: 6.17, 6.54, 7.8, 7.23, 7.26, and](https://reader030.vdocuments.mx/reader030/viewer/2022032706/56649dfe5503460f94ae635e/html5/thumbnails/17.jpg)
Clickers: This system moves at constant 10 m/s to the right. Not shown is the frictional force, which
must be...
• a) static friction of 2 N to the left• b) static friction of 2 N to the right• c) kinetic friction of 2 N to the left• d) kinetic friction of 2 N to the right• e) More information is needed to decide.
![Page 18: Phys. 121: Thursday, 11 Sep. ● Reading: Read ch. 7 for Tuesday. ● Written HW 3: Due by 5:00 pm today. ● Written HW 4: 6.17, 6.54, 7.8, 7.23, 7.26, and](https://reader030.vdocuments.mx/reader030/viewer/2022032706/56649dfe5503460f94ae635e/html5/thumbnails/18.jpg)
This means thatstatic frictioncan be (but doesnot have to be)stronger thankinetic friction.
μk≤μs .
![Page 19: Phys. 121: Thursday, 11 Sep. ● Reading: Read ch. 7 for Tuesday. ● Written HW 3: Due by 5:00 pm today. ● Written HW 4: 6.17, 6.54, 7.8, 7.23, 7.26, and](https://reader030.vdocuments.mx/reader030/viewer/2022032706/56649dfe5503460f94ae635e/html5/thumbnails/19.jpg)
Clickers: Your advisor compels you to push a box up an incline. You push upward with a force greater than m g sin(θ), but it doesn't move,
because...a) Kinetic friction pushes up
the incline
b) Kinetic friction pushes down the incline
c) Static friction pushes up the incline
d) Static friction pushes down the incline
e) The box must have infinite weight
You push in thisdirection...
... harder thanthe componentof weight in this direction.