On the earth, when an astronaut throws a 0.250-kg stone vertically upward, it returns to his hand a time T later. On planet X he finds that, under the same circumstances, the stone returns to his hand in 2T
In both cases, he throws the stone with the same initial velocity and it feels negligible air resistance. The acceleration due to gravity on planet X (in terms of g) is
A) g/4.
B) g/2.
C) g/ .
D) g .
E) 2g.
Answer: B
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A child is trying to throw a ball over a fence. She gives the ball an initial speed of 8.0 m/s at an angle of 40° above the horizontal. The ball leaves her hand 1.0 m above the ground and the fence is 2.0 m high
The ball just clears the fence while still traveling upwards and experiences no significant air resistance. How far is the child from the fence? A) 0.73 m B) 1.6 m C) 2.7 m D) 3.8 m E) 7.5 m
When is the angular momentum of a system constant?
A) Only when its total kinetic energy is constant. B) Only when the linear momentum and the energy are constant. C) Only when no net external torque is exerted on the system. D) Only when the moment of inertia is constant.
A small sphere (2.5 cm-diam) is placed in a heating oven. The oven cavity is a 30 cm cube filled with air at 101 kPa(abs); it contains 3 % water vapor at 810 K, and its walls are at 1370 K. The emissivity of the sphere is equal to 0.44 – 0.00018 T, where T is the surface temperature in K. When the surface temperature of the sphere is 810 K determine (a) the total irradiation received by the walls of the oven from the sphere, (b) the net heat transfer by radiation between the sphere and the walls of the oven, and (c) the radiant heat transfer coefficient.
GIVEN
A small sphere in a 30 cm cubic heat oven filled with air
Sphere diameter (D) = 30 cm – 2.5 cm= 27.5 cm
Air pressure = 101 kPa(abs) = 1.01*105 Pa
Air contains 3% water vapor
Air temperature (Tm) = 810 K
Oven wall temperature (T2) = 1370 K
Sphere emissivity (?1) = 0.44 – 0.00018 T (T in K)
Sphere surface temperature (T1) = 810 K
FIND
(a) Total irradiation received by the walls from the sphere (q2)
(b) The net heat transfer by radiation between the sphere and the walls (q12)
(c) The radiant heat transfer coefficient (hr)
ASSUMPTIONS
The gas is a gray body
The oven walls are black (?2 = 1)
The sphere is near the center of the oven
SKETCH
PROPERTIES AND CONSTANTS
the Stephan-Boltzmann constant
The oldest parts of Earth's crust are located along the mid-ocean rifts
a. True b. False Indicate whether the statement is true or false