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.

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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 (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 Second Law not applicable in the form F = 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 equations in 1 (or 3-in-1 in 3 dimensions). We will use it over and over again! Strategy: draw a force diagram for the relevant object(s). Construct a coordinate system which simplifies things 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 R L (This angle) (equal to this)

Example: Find T in each string.

Example: Table is frictionless; find the tension in the string and the period of circular motion.

Example: Rock mass is 940 g; string breaks after 120 N; L is 1.3 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

Kinetic Friction Always points opposite to the (relative) velocity (with respect to the surface it's sliding against).

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 that static friction can be (but does not have to be) stronger than kinetic friction.

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 this direction harder than the component of weight in this direction.