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PHY 113, Summer 2007
Langenbrunner
HW 5 – due Wed, June 6 at 5:00 (I'll have "office hours" between class and 4:00 and I'll be in
email contact until 11:00 PM.)
1. Some people believe that the positions of the planets at the time of birth influence the
newborn. Others deride this belief and claim that the gravitational force exerted on a body by the
obstetrician is greater than that exerted by the planets. To check this claim, calculate and
compare the magnitude of the gravitational force exerted on a 3 kg baby by
a) a 70 kg obstetrician who is 1 m away and roughly estimated as a point particle
b) the massive planet Jupiter (m=2 x 1027 kg) at its closest approach to Earth (=6 x 1011m)
c) What do you think about this claim?
2. Certain neutron stars (extremely dense stars) are believed to be rotating at about 1 rev/s. If
such a star has a radius of 20 km, what must be its minimum mass so that the material on its
surface remains in place during the rapid rotation?
3. a) What linear speed must an Earth satellite have to be in a circular orbit at an altitude of 160
km?
b) What is the period of revolution?
4. Calculate the kinetic energies of the following objects moving at the given speeds:
a) a 110 kg football linebacker running at 8.1 m/s (that seems really fast to me, but that's what
the problem I found says)
b) a 4.2 g bullet at 950 m/s
c) the aircraft carrier Nimitz, 91,400 tons at 32 knots (1 ton = 907 kg)
5. To pull a 50 kg crate across a horizontal frictionless floor, a worker applies a force of 210 N,
directed 20 above the horizontal. As the crate moves 3.0 m, what work is done on the crate by
a) the worker's force,
b) the gravitational force on the crate, and
c) the normal force on the crate from the floor?
d) What is the total work done on the crate?
6. A spring with a spring constant of 15 N/cm has a cage attached to one end (see below).
a) How much work does the spring force do on the cage when the spring is stretched from its
relaxed length by 7.6 mm? How much additional work is done by the spring force when the
spring is stretched by an additional 7.6 mm?