Document 6573937

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Phvsics 120 Exam #2
(Fall 2014)
Show All Work.
Use Energy Methods when appropriate!!
TURN OFF YOUR CELL PHONES!!!
1.
lt is well known that bullets and other missiles fired at Superman
simply bounce off his chest as in the figure below. Suppose a
gangster sprays Superman's chest with 3 gram bullets at the rate
of 100 bullets/min, the speed of each bullet being 500 m/s.
Suppose too that the bullets rebound straight back with no loss in
speed. Calculate the average force exerted by the stream of
bullets on Superman's chest.
F= 5r.f
Dors tT suRrRtsn Yct, T}+AT Tl+E ForcE t5' 5p sl4 HLLT'
NoTF 5N e I' Lh r:R 18 OZ
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I
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2.
William Tell shoots an apple from his son's head. The speed of
the 125-g arrow just before it strikes the apple is 25.0 m/s, and at
the time of impact it is traveling horizontally. lf the arrow sticks in
the apple and the arrow/apple combination strikes the ground
8.50 m behind the son's feet, how massive was the apple?
Assume the son is 1.85 m
_b
tall.
Lnr ffi= fYl+ss ol= AnnoW = lr5g= la5 xto ru
M = fnAss 0F
BY
CCl.tsERVfti?un/
cl-
{+?fLE
LINFAR Mcl"lEn{Turt,
" = (rYl+
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(Y\V"tO
( penpp.rLY rN*.osrl(
\
cot-tls,sr',
(ta5 xrC)Gs/ = f,ragxrD3) + Hltr
oNcE ThE A-(not^., Flrts Tr+F l+?Pl_E
iT
ts
%=
A PRc;FcTt LE
(1o',0)x-#*
2_
o
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-/
-\
3
e=O +
k=X -9t
/eY
$= axaf
= (8,a)
(q, y)
l<-x--l
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fA= 13,83 nls
& M-W-0"'t{
oR.
vg = ourotry
fl= lel $wns
f,= 8'5 t*
,0= - l.85rt
)
K- srFlN
3. A uniform spherical shell (1.,' = ;MR')
rotates about a vertical axis
on frictionless bearings. A light cord passes around the equator
of the shell, over a pulley of radius r and rotational inertia l, and is
attached to a small object of mass m that is otheruvise free to fall
under the influence of gravity. What is the speed of the object
after it has fallen a distance h from rest?
BY Con tsERvATlD,{ oF ENen6Y)
M,R
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t*rnvzt
No\d (/)r = P/il
Yvrgh
1
cda
= *rntf- ilvz+
YY\gh:
=
G*l,h)
(Yr)
+
gr (ut )'
o%,]u'
+%flv
=
H[+#
B+g
o
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4.
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STEIN
A 68 kg skier approaches the foot of a hill with a speed of 15 m/s.
The surface of this hill slopes upward at 40o above the horizontal.
The coefficients of static and kinetic friction are 0.75 and 0.25
respectively, with the skis. (a) Use energy conservation to find
the maximum height above the foot of the hill that the skier will
reach. (b) Will the skier remain at rest once she stops, or will
she begin to slide down the hill? Prove your answer.
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'--->
(o - *rnvl) t (i'n6h -o)= - dA
-*rnv;
+
ry'g
[ = - /* b1*ru)( n/r,nr)
t Fg + r.rng cD+{:
h=
-6?E
i rnvi +
v:
a t0* )^nc*+E
h= 8.8+
?FKD
Srn
(rs)'
a$,r/[*(o,u)(r,r{
D: VA
+
m
(b) Wr$rl T$p sKtFR Comns rc REst or{ Tl+E SACIP F/
TIIF
DotnNruerD RBcE TFNliNq T0 n/LL l+E( $f+c*, gownJ
F--([lt
ods,me)
= (68)U1.8)srn +d =
tl-av
u
)
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50
IS
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l1+F $rLL.
q
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5. The cable of an 1800 kg elevator
snaps when the elevator is at
rest on the first floor so that the bottom is a distance d = 4 m
above a cushioning spring whose spring constant is
k = 1 .46 x 105 Nlm. A safety device clamps the guide rails so
that a constant friction force of 4.45 x 103 N opposes the motion
of the elevator. (a) Find the speed of the elevator just before it
hits the spring. (b) Find the distance s that the spring is
compressed. (c) Find the distance that the elevator will "bounce"
back up the shaft.
(a) AKtAUz=q
*(rroo)
+ *rqvl ytgA= -$ul-
t
f = E,,ooXt,v/ - (u*.tr (+) + l@
(b)-Mtr (.1+d
+ **x'=
- f(/+x)
(+tx) + *(t'q" ,'3 )f = - $+ro)(++x)
(z,or, xiu) - (t;zr,r,J)x + (7,3DXp) xt= - (r ,n x#) - (++ro)x
-(
r
tooX4,v/
{= 0, q+5 rn
(c)
**- (a"1+Ef + flZU= 51
-* ( I r'tbx r6/(c "q45)t + ( tsoo)(c 'ilV = (++ra) \
e (o,laxl# = K- lJt xlo+) -(++r
';lu +
yl
= e,qs M