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Homework Problems
1. A little old lady and a pro football player collide head-on. Compare
their forces on each other, and compare their accelerations. Explain.
2. The earth is attracted to an object with a force equal and opposite to the
force of the earth on the object. If this is true, why is it that when you
drop an object, the earth does not have an acceleration equal and opposite
to that of the object.
3. When you stand still, there are two forces acting on you, the force of
gravity (your weight) and the normal force of the floor pushing up on
your feet. Are these forces equal and opposite. Does Newton's third law
relate them to each other. Explain.
In problems 4-8, analyze the forces using a table in the format shown in
section 5.3. Analyze the forces in which the italicized object participates.
4. A magnet is stuck underneath a parked car.
5. Analyze two examples of objects at rest relative to the earth that are
being kept from falling by forces other than the normal force. Do not use
objects in outer space, and do not duplicate problem 4 or 8.
6. A person is rowing a boat, with her feet braced. She is doing the part of
the stroke that propels the boat, with the ends of the oars in the water (not
the part where the oars are out of the water).
7. A farmer is in a stall with a cow when the cow decides to press him
against the wall, pinning him with his feet off the ground. Analyze the
forces in which the farmer participates.
8. A propeller plane is cruising east at constant speed and altitude.
9 . Today’s tallest buildings are really not that much taller than the tallest
buildings of the 1940s. One big problem with making an even taller
skyscraper is that every elevator needs its own shaft running the whole
height of the building. So many elevators are needed to serve the building’s
thousands of occupants that the elevator shafts start taking up too much of
the space within the building. An alternative is to have elevators that can
move both horizontally and vertically: with such a design, many elevator
cars can share a few shafts, and they don’t get in each other’s way too much
because they can detour around each other. In this design, it becomes
impossible to hang the cars from cables, so they would instead have to ride
on rails which they grab onto with wheels. Friction would keep them from
slipping. The figure shows such a frictional elevator in its vertical travel
mode. (The wheels on the bottom are for when it needs to switch to
horizontal motion.) (a) If the coefficient of static friction between
rubber and steel is
µ
s
, and the maximum mass of the car plus its passengers
is M, how much force must there be pressing each wheel against the rail in
order to keep the car from slipping. (Assume the car is not accelerating.)
(b) Show that your result has physically reasonable behavior with respect
to
µ
s
. In other words, if there was less friction, would the wheels need to
be pressed more firmly or less firmly. Does your equation behave that
way.
SA solution is given in the back of the book.A difficult problem.
A computerized answer check is available.
.
A problem that requires calculus.
Problem 9.
steel rail
car
rubber wheel
Chapter 5Analysis of Forces
Problem 6.