Homework Ch. 6 Newton’s Laws II - Due W 3/14 in class.
PHY 161 General Physics I: Mechanics and Thermodynamics
Physics Department --- Mercer University --- Spring 2007
PLEASE NOTE: The instructor’s strong preference is that your homework solutions be
handwritten on printed copies of these pages. If necessary, write on the back of these
sheets or attach work on standard printer paper sheets (8 ½” x 11”). Only if it is
absolutely necessary should you attach notebook pages to your work; in that case,
please remove the paper fringes or ‘chads.’
1. A 3-kg block is held at rest against a vertical wall by a horizontal force of 120 N.
a) What is the frictional force exerted by the wall on the block?
b) What is the minimum horizontal force needed (instead of the 120 N) to prevent the block
from falling if the coefficient of static friction between the wall and the block is μs = 0.4?
2. (NOTE UNITS) The coefficient of static friction between the tires of a car and the horizontal
road is μs = 0.5. If the net force on the car is the force of static friction exerted by the road,
a) What is the maximum acceleration of the car, in miles per hour per second (mi/h·s)?
b) What is the least distance, in feet (ft), in which the car can stop if it is initially traveling at
65 mi/h?
3. (NOTE UNITS) A curve of radius 30 m is banked so that a car traveling at 20 mi/h can
round it even if the road is so icy that the coefficient of static friction is effectively zero.
a) What is the angle of the bank?
b) Find the range of speeds (Vmin, Vmax), in miles per hour (mi/h), at which a car can travel
around this curve without skidding on a non-icy day, if the coefficient of static friction
between the road and the tires is μs = 0.25.
4. In certain amusement park rides, people stand on a floor, with their backs against the inside
vertical wall of a hollow cylinder. The cylinder begins to spin, and eventually the floor
supporting the riders is removed. But the riders do not fall; rather they are pushed flat against
the cylinder walls in complete safety as they spin…
a) Show with a force diagram how this is possible.
b) The radius of the cylinder is R = 10 m and the coefficient of static friction between the
riders and the wall is μs = 0.5. What is the minimum speed that must be achieved before the
floor drops out? At this speed, what is the period of revolution?
5. In the figure below, the mass m2 = 10 kg slides on a frictionless table. The coefficients of
friction between m2 and m1 = 5 kg are μs = 0.6 and μk = 0.4.
a) What is the maximum acceleration (a1) of m1?
b) What is the maximum value of m3 if m1 moves with m2 without slipping?
c) If m3 = 30 kg, find the acceleration of each body (a1, a2 = a3 = a) and the tension (T) in the
string.