Physics 1a, Physics 1a, Section 2Section 2

October 25, 2010October 25, 2010

Second QuizSecond QuizWas due 3 hours ago.Was due 3 hours ago.

Problem Solving Problem Solving Hints:Hints:

Normal Force and Normal Force and FrictionFriction suggested order:suggested order:

1) draw gravity and other fixed external forces 1) draw gravity and other fixed external forces (direction and magnitude are unambiguous)(direction and magnitude are unambiguous)

2) draw normal force (prevents interpenetration of 2) draw normal force (prevents interpenetration of objects)objects)

3) draw friction force (perpendicular to normal force)3) draw friction force (perpendicular to normal force)

kinetic friction:kinetic friction:

F = F = kkNNdirection opposite to relative velocitydirection opposite to relative velocity

static friction:static friction:

F ≤ F ≤ ssNNdirection and magnitude to prevent relative direction and magnitude to prevent relative

accelerationacceleration

Quiz Quiz ProbleProblem 11: m 11: static static

& & kinetikineti

c c frictiofrictio

nn

• Answer:

a) a = g(1scos - sin)

b) F = (m1+m2)g(2k + 1s)cos

Quiz Quiz ProbleProblem 11m 11

Tension in a Rope Tension in a Rope Draped over a TableDraped over a Table

L

mass per unit length

relevant for HW QP20

Tension in a Rope Tension in a Rope Draped over a TableDraped over a Table

L

mass per unit length

T=0

T’ = gL

T=0

T=T’ T=T’T=T’

Fictitious (Inertial) Fictitious (Inertial) ForcesForces

Accelerating (non-inertial) frames:Accelerating (non-inertial) frames:Linear acceleration:Linear acceleration:

Fictitous force ma opposite direction of Fictitous force ma opposite direction of accelerationacceleration

Rotating frame:Rotating frame:Fictitious force mFictitious force m22r outwardr outward(If an object is moving with respect to the (If an object is moving with respect to the rotating frame, there is also a fictitious rotating frame, there is also a fictitious Coriolis force on it.)Coriolis force on it.)

Why introduce fictitious forces and work in Why introduce fictitious forces and work in non-inertial frame?non-inertial frame?

Often simplifies solution of the problem.Often simplifies solution of the problem.

Quiz Quiz ProbleProblem 29m 29

• Answer:

a) mA to the left

b) ma = mAcos – mgsin – N, N = m(Asin + gcos)

c) a = A(cos – sin) – g(cos + sin); < 37°

Quiz Quiz ProbleProblem 29m 29

Homework due Homework due WednesdayWednesday

I think the intention is for you to work I think the intention is for you to work these problems in these problems in nonnon-inertial frames -inertial frames with fictitious forces:with fictitious forces:

QP21QP21

QP28QP28

but they can be solved without that.but they can be solved without that.

Good practice to try in non-inertial Good practice to try in non-inertial frame.frame.

Thursday, Thursday, October 28:October 28:

Quiz Problem 49 (springs)Quiz Problem 49 (springs)

Quiz Problem 50 (energy)Quiz Problem 50 (energy)

Optional, but helpful, to try these in advance.Optional, but helpful, to try these in advance.