Phy 121 - Assignment 9
A. 1. a. One from your hand, one from the nail.
b. The force from the nail is larger. (Force is just how hard you pull. Try
pulling a nail from a board by hand if you think you can exert a larger force
than it does.)
c. The torque from your hand is larger. (The hammer starts turning the way you
want, so your torque is winning. Torque is how hard you pull times the
leverage, and your hand is farther from the pivot than the nail. Your small
force times a lot of leverage overcomes a big force times not much leverage.)
2.
B. Στ =
+(.25 m)(90 N) – (.10 m)(120 N)
= 22.5 – 12 = 10.5 N·m
The 30° is irrelevant. θ in r sinθ F
is the angle between r and F,
which is 90°, not the angle from
the horizontal.
at = rα
α=
τ = Iα
I=
=
=
= 20.4 rad/s2
= .515 kg·m2
C. 1. No. With linear motion, no force is needed for an object to keep moving
along a straight line. (Newton's first law.) In the same way, with circular
motion, no torque is needed for something to keep spinning about the same
axis. (Like the Earth does, for example.) Torque causes angular acceleration,
not angular motion.
2. Since you have to have radians for τ = Iα, might as well convert right at the
start. 90° = π/2 radians.
D. a.
b. Centripetal acceleration = v2/r. v = 0 when released, so this component is
zero.
Tangential acceleration = r  = (.55 m)(13.4 rad/s2) = 7.37 m/s2
E.