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Phys 150/140
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Physics 150/140
Solutions to Final Examination December 16, 2013
Problem 1 (20 Points) A 30.0 kg child stands on the edge of a solid uniform disk of mass 100. kg and radius 2.00 m. The disk rotates horizontally about a fixed vertical axis located at the opposite edge of the disk from the child. Initially, the disk and child are rotating counterclockwise and make 1.00 revolution every 8.00 seconds. Then the child catches a ball of mass 5.00 kg thrown by a friend. As shown in the figure, just before the ball is caught, it has a horizontal velocity of magnitude 12.0 m/s at an angle φ=37.0° to a line tangent to the disk.
(a) (8 Points) Before the ball is caught, what is the moment of inertia of the disk and child about the axis? Use the parallel axis theorem:
€
Ii = Idisk + Ichild =mdiskR
2
2+mdiskR
2 +mchild 2R( )2
Ii =3mdiskR
2
2+mchild 2R( )2
Ii =3 100kg( ) 2.00m( )2
2+ 30.0kg( ) 4.00m( )2 =1080 kg m2
(b) (12 Points) After the ball is caught, what is the angular speed of the disk+child+ball system? Li = Lf
Iiωi −mbvcosφ = I fω f
ωi =2π8s
=π4s−1
I f = Ii +mb 2R( )2 =1080 kg m2 + 5kg( ) 4m( )2 =1160 kg m2
ω f =1080 kg m2( ) π4 s
−1"
#$
%
&'− 5kg( ) 12m / s( ) cos37( )
1160 kg m2 = 0.566 rad / s
Problem 2 (20 Points) A wire is 30.0 cm long. It has density 0.230 grams/meter, and is under tension FT=5.70 N. It is attached to rigid posts at each end.
(a) (5 Points) Calculate the velocity of a transverse travelling pulse on this wire. (Be careful with units).
(b) (5 Points) Calculate the fundamental vibrational frequency.
(c) (5 Points) Calculate the period of the motion if the wire is in the n=3 standing wave mode.
(d) (5 Points) At time t=0 the wire is not moving. The top graph shows the displacement of the wire at time t=0. If T is the period of the motion that you calculated in part (c), sketch the wire’s configuration at times t=T/4, t=T/2, and t=T.
t = 0 milliseconds
t = 0.318 milliseconds
t = 0.635 milliseconds
t = 1.27 milliseconds
t=0
t=T/4
t=T/2
t=T
Problem 3 (20 Points) A newly discovered comet (Balley’s Comet) is in an elliptical orbit around the Sun (which has mass 1.99 x 1030 kg) with orbital period 162 years. Its distance of closest approach to the Sun is 8.98 x 1010 m where its speed is observed to be 54.1 km/s. The mass of the comet is 2.53 x 1014 kg. (a) (5 points) Calculate the semimajor axis of Balley’s orbit.
(b) (5 points) What is the farthest distance Balley will get from the Sun?
(c) (5 points) Balley is observed to fly by the Earth at a distance of 1.50 x 1011 m from the Sun. What is its speed there? (You can ignore the gravitational force exerted by the Earth. Note that you can solve this even if you could not do parts a or b.)
(d) (5 points) How much work needs to be done on Balley to change its orbit to a circular orbit at the distance of closest approach to the Sun? (Note that you can solve this even if you could not do parts a, b, or c.)
Problem 6 (10 Points) A uniform disk of mass m and radius r rests on the back of a truck. The truck and the disk are initially at rest, but then the truck accelerates to the right with acceleration atruck . Assume that the disk does not fall off the truck and that the static coefficient of friction is large enough that the disk rolls without slipping over the truck. What is the acceleration of the center of mass of the disk, as observed in the reference frame of a person standing on the ground? Specify both the direction and the magnitude of the acceleration of the disk in terms of some or all of m, r, and atruck but no other variables.
Note that afterwards the disk is rotating counterclockwise and accelerating to the left. The no-slip condition ensures that the bottom surface of the disk has the same acceleration as the truck.
Ff =maFf r = Iα
atan = rα =Ff r
2
I=
Ff r2
mr2 / 2=2Ffm
= 2a
atruck = a+ atan = 3a
a = atruck3
m,r
atruck