PHYS 250 MT2 Review

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PHYS 250 - MT2 Review

• MT 2 Mon 4/6 - 630-745 pm
• Thomas 100
• (Room opens at 625)
• Bring pencils, calculator nothing else!
• Website Sample exams, eqn sheet
• Use the sample exams as exams (take under exam
  conditions)!

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Physics is Articulating Relationships
conservation laws
W?E
dynamics
W
E
F12-F21
W F?x
Kp2/2m
kinematics
UG mgh
x
v
a
F
p
F?t?p
a ?v/?t
v ?x/?t
Fnetma
h
FPA
P ?gh 1/2?v2 const
aTr?
vTr?
?r?F
P
?
?
?
?
FB wdisp
?net I?
?2?f
FB
f1/T
k2?/?
fluids
T
f
?
k
waves
v?f
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Key Ideas
1) x - vx - ax relationships (slope area) 2)
Vector representations (v,?) ? (vx,vy) 3)
Independence of horizontal vertical motion 4)
Newtons Second Law a Fnet/m (Fnet ma) 5)
Newtons Third Law F12 -F21 6) Forces w mg,
n, f (fs lt ?sn, fk ?kn) , D, T 7) Applying
forces to systems of connected objects 8)
Centripetal acceleration, circular motion
gravity 9) Angular kinematics (?-?-?
relationships) 10) Torque r?F I?
11) Center of mass 12) Moment of Inertia (I) 13)
Impulse J F?t ?p m?v 14) Momentum
conservation (pmv)
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Quiz 1
1) Which box below has the greatest net force
acting on it?
Fnet,x max All axs the same
2) Rank the three identical mass objects below
(two hoops and one solid disc) from greatest to
least ?. (There is a fixed axis at the center of
each.)
(C)
(A)
C gt A B ? ?/I
?C ?A IC IA/2
?B 4?A IB 4IA
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Quiz 2
3) You are going around a curve of radius R while
moving at speed V. There is a force F (what
direction?) that keeps you from skidding (what is
the type of force?). If you went around twice as
fast, what would be the force required to keep
you moving in the circle? (a) F (b) F/2
(c) 2F (d) 4F (e) F/4 4) Two
children are sliding together on a slippery sheet
of ice with velocity v. One of the children
shoves the other child (who happens to weigh
twice as much) and stops. What is the velocity of
the other child? (a) v (b) v/2 (c)
2v (d) 3/2 v (e) 2/3 v
F ma mv2/R if v doubles, F quadruples
pi pf 3m(v) m(0) (2m)vf
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Quiz 3
5) You hold your arm straight out horizontal and
then bend your arm at the elbow (so that your
fingers touch your shoulder). If you can treat
your arm as a rod of uniform density (a crude
approximation), what happens to the torque around
the shoulder joint exerted by your
muscles? (a) doesnt change (b) doubles (c) halve
s (d) quadruples (e) decreases by factor of 4 6)
A force that varies with time is applied to a 0.5
kg object that had an initial velocity of 10 m/s
(see figure above). What is the objects velocity
at t 0.14 s?
100 N
Fx
t
0.10 s
0.14 s
Area m(v - 10 m/s) ? v 24 m/s
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Quiz 4

• 7) A 2 kg bomb is sliding along a surface with
  a velocity of (5 m/s, -2 m/s) when it explodes
  into three pieces
• 0.7 kg moving at (50 m/s, 0 m/s)
• 0.5 kg moving at (-40 m/s, -10 m/s)
• ??
• What is the velocity of the third piece?

pi (2 kg) (5 m/s, -2 m/s) (10, -4) kgm/s pf
(0.7 kg) (50 m/s, 0 m/s) (0.5 kg) (-40 m/s, -10
m/s) (0.8 kg)(vx,vy) (15, -5) kgm/s
(0.8 kg)(vx,vy) pi pf, so (10, -4) kgm/s
(15, -5) kgm/s (0.8 kg)(vx,vy) (vx,vy)
(-6.25 m/s, 1.25 m/s)
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Quiz 5
M2g(0.5 m) - M3g(2 m) 0
5 m
1 m
8) What are M2 and M3?
2 m
0.5 m
10 kg
M2 40 kg, M3 10 kg
M2
M3
(10 kg)g(5 m) - (M2M3)g(1 m) 0
9) A MGR is spinning with angular velocity ?. A
child (not on the MGR) dumps a big bucket of sand
from the sandbox onto the MGR. What happens to ?
and why?
Adding sand (mass) increases I. Since L I?
conserved, ? must decrease.
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Quiz 6
10) Where is the COM of this system
(x-coordinate)?
4 kg
2 m
6 kg
4 m
xcm (6 kg)(2 m) (4 kg)(3 m) / (10 kg) 2.4
m
11) The sculpture above is supported from below
at two points at the midpoint of the bottom beam
and at the right edge of the beam. What is the
force exerted by each support?
Use the left support as the pivot Fnet FL FR
(10 kg)g 0 tnet - (0.4 m)(10 kg)g (2
m)(FR) 0 gt FR 19.6 N, FL 78.4 N
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Quiz 7
12) A 10 kg box sits still on a horizontal
surface that is not frictionless. A spring (k
300 N/m) is connected to the box and is stretched
by 0.1 m (horizontally). If the coefficient of
static friction is 0.5, what is the force of
friction on the box?
Fs k?x (300 N/m)(0.1 m) 30 N lt (0.5)(10
kg)(g)
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Quiz 8
13a) An object falls over when tipped at an angle
of 16 degrees. If you stretched it so that you
doubled its height and halved its width, at
approximately what angle would it fall over?
a) 16 degrees b) 8 degrees c) 4 degrees d) 32
degrees e) 12 degrees
?
h
tan? (w/2) / (h/2) w/h tan? decreased by
factor of 4, so ? decreased by approximately a
factor of 4.
w
13b) What is the effect on ? if the object is
denser at the top than at the bottom?
Raises center of mass so decreases ?.