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Physics 8.012 Fall 2008
Final Exam
S O L U T I O N S
NAME: _________________________________________________
Instructions:
- Do all SEVEN (7) problems. You have 2.5 hours.
- Show all work. Be sure to CIRCLE YOUR FINAL ANSWER.
- Read the questions carefully
- All work and solutions must be done in the answer booklets provided
- NO books, notes, calculators or computers are permitted. A sheet of useful equations is provided on the last page.
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Problem Maximum Score Grader 1 10 2 15 3 15 4 15 5 15 6 15 7 15
Total 100
[NO TEST MATERIAL ON THIS PAGE]
Page 2 of 25
Without an external force, the ice skater’s momentum doesn’t change; similarly, as there are no external torques, angular momentum is conserved. However, rotational energy scales as L^2 /2I, and the moment of inertia (I) is reduced for the skater as she pulls in her arms, so her total mechanical energy must increase.
(d) [2 pts] What are the dimensions of the gravitational constant G?
[M]-1[L]^3 [T]-
(e) [2 pts] A gyroscope whose spin angular velocity vector points toward the left is observed to precess such that its precession angular velocity vector points at an angle as shown. In which direction does the gravity vector point?
The precession direction points in the opposite direction as the spin vector initially moves toward as the gyroscope falls under gravity. In this case, the gravity vector must therefore be parallel to the precession vector.
(f) [BONUS 2 pts] A diver is the middle of a dive as shown below. Based on clues in the photo, indicate in your answer booklet the direction that his total spin vector points, and determine whether the diver is doing a front flip or a back flip.
One clue is the hair, which lies in a plane perpendicular to the spin vector (twirl a handful of string to convince yourself of this). The placement of the arms breaks the degeneracy, indicating an applied torque that causes a twist rotation whose direction points along the feet. So the total spin vector points in the direction shown, and the flip component indicates a front flip (head over feet).
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Problem 2: Atwood Machine [15 pts]
α
2M
M
M R
μ d
An Atwood machine consists of a fixed pulley wheel of radius R and uniform mass M (a disk), around which an effectively massless string passes connecting two blocks of mass M and 2M. The lighter block is initially positioned a distance d above the ground. The heavier block sits on an inclined plane with opening angle α. There is a coefficient of friction μ between the surfaces of this block and the inclined plane. Constant gravitational force acts downwards, and assume that the string never slips.
(a) [5 pts] Determine two conditions on the angle α which allow the lighter block to move up or move down.
(b) [10 pts] Assuming that the lighter block moves down, determine its acceleration.
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(b) Choosing our coordinate systems for each component as shown above so that all objects move in a positive direction, we can write down the following equations of motion:
leftmost block:
pulley wheel:
rightmost block:
The constraint equation tying all of these objects together (connected by an massless and hence inextensible string) is:
Using this and the first two equations of motion we can relate to two tension forces:
and using the first and third equations of motion we can solve for the individual tensions:
Note that the first conditions from part (a) is necessary for acceleration to be positive.
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2R MR +^ MF N^ particles/m^3
Problem 3: Rocket in an Interstellar Cloud [15 pts]
A cylindrical rocket of diameter 2R, mass MR and containing fuel of mass MF is coasting through empty space at velocity v 0. At some point the rocket enters a uniform cloud of interstellar particles with number density N (e.g., particles/m^3 ), with each particle having mass m (<< MR) and initially at rest. To compensate for the dissipative force of the particles colliding with the rocket, the rocket engines emit fuel at a rate dm/dt = γ at a constant velocity u with respect to the rocket. Ignore gravitational effects between the rocket and cloud particles.
(a) [5 pts] Assuming that the dissipative force from the cloud particles takes the form F = –Av^2 , where A is a constant, derive the equation of motion of the rocket (F = ma) through the cloud as it is firing its engines.
(b) [5 pts] What must the rocket’s thrust be to maintain a constant velocity v 0?
(c) [5 pts] If the rocket suddenly runs out of fuel, what is its velocity as a function of time after this point?
(d) [BONUS 5 pts] Assuming that each cloud particle bounces off the rocket elastically, and collisions happen very frequently (i.e., collisions are continuous), prove that the dissipative force is proportional to v^2 , and determine the constant A. Assume that the front nose-cone of the rocket has an opening angle of 90º.
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(d) As illustrated in the figure to the right, each particle that collides with the rocket is deflected through 90º (due to geometry), which means that each particle imparts an impulse on the rocket of Δp = mv in the horizontal direction opposite of motion (it also imparts an impulse of mv in the vertical direction, but that is balanced by particles striking the other side of the nosecone). The number of particles that strikes the rocket per unit time is simple the volume swept through by the rocket per unit time, AΔx/Δt = πR^2 v. The total momentum transfer onto the rocket is:
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mv
Problem 4: Sticky Disks [15 pts]
M
2M 2R
2R M
2M
A uniform disk of mass M and diameter 2R moves toward another uniform disk of mass 2M and diameter 2R on the surface of a frictionless table. The first disk has an initial velocity v 0 and spin rate ω 0 as indicated, while the second disk is initially stationary. When the first disk contacts the second (a “glancing” collision), they instantly stick to each other and move as a single object.
(a) [5 pts] What are the velocity and spin angular velocity of the combined disks after the collision? Indicate both magnitudes and directions.
(b) [5 pts] For what value of ω 0 would the combined disks not rotate?
(c) [5 pts] How much total mechanical energy is lost in this collision assuming that the combined disk system is not rotating?
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(c) To keep the math clean, it is assumed that the final system is not spinning, so that ω 0 and v 0 are related as in part (b). The initial energy is therefore:
The final energy is just the translation energy:
so the total energy loss is:
Page 13 of 25
Problem 5: Cylindrical Top [15 pts]
M
COM
r
μ = 0
Δp
L
R
A cylinder of mass M, length L and radius R is spinning about its long axis with angular velocity on a frictionless horizontal surface. The cylinder is given a sharp, horizontal strike with impulse Δp at a distance r from its center of mass (COM). Assume that constant gravitational acceleration acts downward. NOTE: you do not need to use Euler’s equations to solve this problem.
(a) [5 pts] What is the translational velocity of the cylinder after the impulse (magnitude and direction)?
(b) [5 pts] The strike imparts an angular momentum impulse to the cylinder which causes it to lift up at one end. At what angle α will the cylinder be tilted after the impulse and which end of the cylinder lifts up? Assume that the angular momentum impulse is much smaller than the spin angular momentum.
(c) [5 pts] After the cylinder tilts up, it effectively becomes a top. Determine its precessional rate and the direction of precession. Assume that nutational motion is negligible (i.e., α remains effectively constant) and that R << L (i.e., that the cylinder can be approximated as a thin rod for this part).
(d) [5 pts BONUS] For a strong enough impulse, the cylinder will tilt high enough to precess in the opposite direction. What is the minimum tilt angle for this to happen and what is the minimum impulse required? (Note that you cannot assume R << L here. This problem is similar to the “tipping battery” trick pointed out by one of the 8.012 students.)
Page 14 of 25
using here our expression for the time derivative of a vector (LS) in a rotating reference frame. The precession vector must point along the z-direction, a fact we can ascertain by considering that gravity would initially pull the spin angular momentum vector down, so to conserve total angular momentum the precession angular momentum vector must point upward. The precession vector rotates only the radial component of the spin angular momentum vector, hence:
Solving for the precession rate:
note that the I here is the moment inertia about the spinning axis, not about the
L/
R
α α
90º-α
precession axis.
from the solution to (a) this places a constraint on the impulse required:
Note that this is approximate, as we no longer satisfy the gyroscopic approximation that LS >> ΔL (indeed, they are of the same order of magnitude in this case).
(d) To precess in the other direction, the center of mass must be inside the pivot point of the disk, which happens at a critical angle (see right):
Page 16 of 25
Problem 6: Bead on a Spinning Rod [15 pts]
M
ω
r 0
A bead of mass M is placed on a frictionless, rigid rod that is spun about at one end at a rate ω. The bead is initially held at a distance r 0 from the end of the wire. For the questions below, treat the bead as a point mass. Ignore gravitational forces.
(a) [5 pts] What force is necessary to hold the bead in place at r 0? Indicate both magnitude and direction.
(b) [5 pts] After the bead is released, what is its position in the inertial frame (in polar coordinates) as a function of time?
(c) [5 pts] Now calculate the fictitious forces on the bead in a reference frame that is rotating with the wire. What real force must the rod exert on the bead in both the rotating and inertial frames?
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The angular equation of motion in the rotating frame is:
where N is again the angular normal force acting on the bead, and the net angular acceleration component is 0 since θ is constant in the rotating frame. Hence
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Problem 7: Central Potential [15 pts]
A particle of mass m moves within a region under the influence of a force of the form
The particle is initially at a distance r 0 from the origin of the force, and initially moves with velocity v 0 in a tangential direction.
(a) [5 pts] Derive and sketch the effective potential of this system as a function of radius from the origin. Indicate all important inflection points. Can the particle pass through the origin of this reference frame?
(b) [5 pts] Find the velocity v 0 required for the particle to move in a purely circular orbit at a radius r 0 with this force law.
(c) [5 pts] Compute the frequency of small oscillations about this equilibrium radius. How does the period of these oscillations compare to the orbital period?
Page 20 of 25
