Boat 1 goes directly across a stream a distance L and back taking a time t1 . Boat 2 goes down stream a distance L and back taking a time t2 . If both boats had the same speed relative to the water, which of the following statements is true?
a. t2 > t1
b. t2 < t1
c. t2 = t1
d. t2 = 2t1
e. t2 = 0.5t1
a
You might also like to view...
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
A farsighted girl has a near point at 2.0 m but has forgotten her glasses at home. The girl borrows eyeglasses that have a power of +2.75 diopters
With these eyeglasses, what is the near point of the girl, assuming that she wears them extremely close to her eyes? A) 28 cm B) 31 cm C) 33 cm D) 37 cm E) 40 cm
Which of the following devices makes use of an electromagnet?
a. relay c. both A and B b. motor d. None of the above.
Why couldn't life have originated on the Moon or Mercury?
What will be an ideal response?