Physics 1A, Section 7

October 29, 2007

Quiz Problem

29

Quiz Problem 29

• Answer:a) fictitious force mA to the left

b) mA to the left, normal to upper right, mg down, friction to lower right;

up the incline: mAcos - N - mgsin = ma

perp. incline: -mAsin + N - mgcos = 0

c) a = A(-sin+ cos) - g(cos + sin)

tan < 0.75 < 37º

OM, Chapter 7

OM, Chapter 7

OM, Chapter 7

Energy Conservation• Energy is conserved: K + U + heat + … = constant• Sometimes, mechanical energy is conserved:

K + U = mechanical energy = constant• Example: 1/2 mv2 + mgh = constant

This often allows a quick solution of a difficult problem.

Energy Conservation• Energy is conserved: K + U + heat + … = constant• Sometimes, mechanical energy is conserved:

K + U = mechanical energy = constant• Example: 1/2 mv2 + mgh = constant

This often allows a quick solution of a difficult problem.

• However, in other cases, mechanical energy is not conserved, so K + U constant: friction: Energy is lost to heat. inelastic collision: Energy is lost to heat. This is the same thing as saying the force can’t be described

by a potential energy; the force is a function of some variable other than position.

• In many of those cases, one can resort to using the force to calculate the energy added to the system:energy input = W = ∫F•ds

OM, Problem 7-79

OM, Problem 7-79

• Answer: cos = 2/3 = 48.2º

Thursday, November 1:

• Collision Problems (OM Chapter 11)– Quiz Problem 30– Quiz Problem 31– Quiz Problem 38

• Optional to work on these in advance.