Which of the following is seen on the surface of Europa suggests the existence of a briny ocean below?
A) eruptions of water onto the surface from below
B) spectroscopic signatures of salts on the surface
C) pools of water seen on the surface
D) all of these
B
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A 2.5 × 2.5 m steel sheet 1.5-mm-thick is removed from an annealing oven at a uniform temperature of 425°C and placed in a large room at 20°C in a horizontal position. (a) Calculate the rate of heat transfer from the steel sheet immediately after its removal from the furnace, considering both radiation and convection. (b) Determine the time required for the steel sheet to cool to a temperature of 60°C. Hint: This will require numerical integration.
GIVEN
• Horizontal steel sheet in air
• Sheet dimensions = 2.5 m × 2.5 m × 0.0015 m
• Sheet initial temperature (Tsi) = 425°C = 698 K
• Air temperature (T?) = 20°C = 293 K FIND
(a) The initial rate of heat transfer (q) (b) The time required for the sheet to cool to 60°C (333 K)
ASSUMPTIONS
• The room behaves as a black body at T?
• The steel sheet behaves as a black body (? = 1.0)
• Heat transfer takes place from both top and bottom of the sheet
• The steel is 1% carbon steel
• Heat transfer from the edges of the plate is negligible
SKETCH
PROPERTIES AND CONSTANTS
The Stephan-Boltzmann constant (?) = 5.67 × 10–8 W/(m2 K4)
for 1% carbon steel
Specific heat (cs) = 473 J/(kg K)
Density (?s) = 7801 kg/m3
for dry air at the mean temperature of 496 K (223°C)
Thermal expansion coefficient (?) = 0.00203 1/K
Thermal conductivity (k) = 0.0384 W/(m K)
Kinematic viscosity (?) = 38.7 × 10–6 m2/s
Prandtl number (Pr) = 0.71
You push a heavy crate down a ramp at a constant velocity. Only four forces act on the crate. Which force does the greatest magnitude of work on the crate?
(a) The force of friction. (b) The force of gravity. (c) The normal force. (d) The force of you pushing. (e) The net force.
Answer the following statement(s) true (T) or false (F)
1. Two blinking warning lights are 3000 m apart along a railroad track. Suppose that in the ground frame the west light blinks (event W ) 5 ?s before the east light blinks (event E). Now imagine that both lights are observed by a passenger on the train who passes the west light just as it blinks. It is possible for the train to be moving fast enough that the two blinks are observed (not necessarily seen) by the passenger to be simultaneous
2. Suppose an explosion occurs at x = 0 in the Home Frame (event A). Light from the explosion is detected by a detector at position x = –100 ns (event B) and by a detector at position x = +50 ns (event C). Events B and C are causally connected
3. If the space time interval between two events is time like, then the temporal order of the two events is the same in every inertial reference frame.
4. If the space time interval between two events A and B is spacelike and event A occurs before event B in some Home Frame, then it is always possible to find an Other Frame where the events occur in the other order
5. An unidentified spaceship cruises past Outpost 11, a space station floating in deep space, at a speed of 
, without acknowledging requests for identification. Ten seconds after the ship passes (according to Outpost 11 clocks), Outpost 11 sends a message to the departing ship, warning that it will commence firing if the ship does not respond. Fifteen seconds after that, Outpost 11 sensors indicate that the ship has raised its shields. This is clearly a response to the warning
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Answer the following statement true (T) or false (F)