final - Final Exam SOLUTIONS Physics 1301 Section 001 Print...

PHYS 1301 Section 001 Final Exam page 1 of 11
Fall 2009 W. Wayne Kinnison
Final Exam SOLUTIONS
Physics 1301 Section 001
Print your name (very legibly) on EACH PAGE of paper that you turn in including this test sheet. Staple all
pages including the test sheet together. Be sure to write legibly for maximum credit.
Concept Questions (Answer all six (6)—5 points each):
1. You have learned this semester that energy is always conserved in any process where no external
forces are acting on the system. However, the total kinetic energy of two cars measured before and
after a collision is not the same. How then is the concept of energy conservation reconciled with
what happens in an automobile collision?
There are two issues. One is that kinetic energy is not conserved but the sum of kinetic and potential
energy is what is conserved. The other is that the total mechanical energy (kinetic and potential) are
only conserved if there is no external work.
So on the one hand, some of the total kinetic energy is converted to heat, sound, potential energy in
the form of bent parts, etc. So kinetic energy would not be expected to be conserved.
Second, there are external forces involved in that some work goes into deforming the automobiles,
making noise, mass changing because parts fell off, etc. Due to the external energy or forces, the
total kinetic energy is not conserved.
2. Is friction a conservative or non-conservative force? Explain your answer.
It is a non-conservative force. It is always directed opposite to the direction of motion.
Consequently, movement around a closed path always requires non-zero work.
.

PHYS 1301 Section 001 Final Exam page 2 of 11
Fall 2009 W. Wayne Kinnison
3. The Archimedes Principle says that an object in a fluid has a buoyant force acting on it. So a boat
floating on a lake has an upward-directed buoyant force acting on it. But, wouldn’t Newton’s Second
Law then imply that the boat must be accelerating upward due to that force? What is wrong with this
logic?
Newton’s Second Law says that there is an acceleration proportional to the net force. In this case we
have not only the buoyant force but also the weight of the boat. Those two forces balance resulting
in a net force of zero so there is zero acceleration.
4. As most things get colder they get denser. If something is denser than the fluid it is in, it sinks. Then
why does ice float in water?
Unlike most substances, ice and water at temperatures close to 0  C expands instead of contracts as it
cools. Thus it gets less dense than warmer water so the ice floats.