Physics 1A, Section 7 October 29, 2007

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

Energy Conservation Energy is conserved: K + U + heat + … = constant Sometimes, mechanical energy is conserved:  K + U = mechanical energy = constant Example: 1/2 mv 2 + 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 mv 2 + 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 = ∫Fds

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.