PHYS 2048
Review Problems Unit 3
1. During a solar eclipse, the Moon, Earth, and Sun all on the same line, with the moon between the Earth
and the Sun. (a)What force is exerted by the Sun on the Moon? (b) What force is exerted by the Earth on
the Moon? (c) What force is exerted by the Sun on the Earth? (d) Why doesn’t the Sun capture the
Moon away from the Earth?
2. After our Sun exhausts its nuclear fuel, its ultimate fate may be to collapse to a white dwarf state. In this
state, it would have approximately the same mass as it has now but a radius equal to the radius of the
Earth. Calculate (a) the average density of the white dwarf, (b) the surface free-fall acceleration, and (c)
the gravitational potential energy associated with a 1.00-kg abject at its surface.
3. Four railroad cars, each of mass 2.50 x 104kg, are couple together and coasting along horizontal tracks
at speed v1 toward the south. A very strong movie actor, riding on the second car, uncouples the first car
and gives it a big push. Increasing its speed to 4.00 m/s southward. The remaining three cars continue
moving south, now at 2.00 m/s. (a) Find the initial speed of the cars. (b) How much work did the actor
do?
4. A 2.00-kg particle has a velocity (2.00i – 3.00j) m/s, and a 3.00-kg particle has a velocity (1.00i + 6.00j)
m/s. Find (a) the velocity of the center of mass and (b) the total momentum of the system.
5. A model airplane with mass 0.750-kg is tethered by a wire so that it flies in a circle 30.0 m in radius.
The airplane engine provides a net thrust of 0.800N perpendicular to the tethering wire. (a) Find the
torque the net thrust produces about the center of the circle. (b) Find the angular acceleration of the
airplane when it is in level flight. (c) Find the translational acceleration of the airplane tangent to its
flight path.
6. A bicycle is turned upside down while its owner repairs that tire. A friend spins the other wheel, of
radius 0.381 m and observes that drops of water fly off tangentially. She measures the height reached by
the drops moving vertically. A drop that breaks loose on the next turn rises 51.0 cm above the tangent
point. The height to which the drops rise decrease because the angular speed of the wheel decreases.
From this information, determine the magnitude of the average angular acceleration of the wheel.
7. A small flat puck with a mass m = 2.40 kg slides on a frictionless, horizontal surface. It is held in a
circular orbit about a fixed rod with a negligible mass and length R = 1.50 m, pivoted at one end.
Initially the puck has a speed of v = 5.00 m/s. A 1.30 kg ball of putty is dropped vertically onto the puck
from a small distance above it and immediately stick to the puck. (a) What is the new period of rotation?
(b) Is angular momentum of the puck-putty system about the axis of rotation conserved in this process?
(c) Is momentum of the system conserved in the process of the putty sticking to the puck? (d) Is
mechanical energy of the system conserved in the process?
8. Rigid rods of negligible mass lying along the y axis connect three particles. The system rotates about the
x axis with an angular speed of 2,00 rad/s. Find (a) the moment of inertia about the x axis and the total
rotational kinetic energy evaluated from ½ Iω2 and (b) the tangential speed of each particle and the total
kinetic energy evaluated from Ʃ½ m1v12. (c) Compare the answers for kinetic energy in parts (a) and
(b).
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