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!

How many of you still need an iClicker?

Hints for today's HW?

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

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.

[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)

Example:

Find T in eachstring.

Example: Table isfrictionless; find the tension in thestring and the period of circular motion.

Example: Rock massis 940 g; string breaks after 120 N; L is1.3 meters. Find (a) max. speed; (b) θ.

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!

Static Frictional Forces

( f⃗ s) μ⩽ s ( n⃗ )

Static Frictional Forces

( f⃗ s) μ⩽ s ( n⃗ )

Kinetic Friction

Always pointsopposite to the (relative)velocity (with respect tothe surface it's sliding against).

∣⃗f k∣=μk∣⃗n∣

Simple (“cartoon”) picture of friction

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.

This means thatstatic frictioncan be (but doesnot have to be)stronger thankinetic friction.

μk≤μs .

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.