A 25-kg block on a horizontal surface is attached to a light spring (force constant = 8.0 kN/m). The block is pulled 10 cm to the right from its equilibrium position and released from rest. When the block has moved 2.0 cm toward its equilibrium position, its kinetic energy is 12 J. What is the change in mechanical energy caused by the frictional force on the block as it moves the 2.0 cm?
a. ?4.0 J
b. ?3.5 J
c. ?2.4 J
d. ?2.9 J
e. ?15 J
c
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A filament of tungsten wire is 0.001 m in diameter, and the filament is heated to 2500 K. The work function of tungsten is 4.5 eV.
(a) Calculate the probability of electrons having sufficient energy to be emitted from the filament. (b) Calculate the emission current density from the tungsten wire. (c) What length of wire can produce a thermionic electron current of 1 A?
Thermodynamic Devices: A Carnot air conditioner has a coefficient of performance of 17.0 and removes 72.0 MJ of heat from the interior of a house every hour. How much power does it consume?
A. 1180 W B. 1320 W C. 520 kW D. 3.14 MW E. 1.25 MW
If you draw a spacetime diagram, how will the worldline of an object that is accelerating away from you appear?
A) vertical B) horizontal C) slanted D) curved E) a circle
Explain why the lines of force never cross one another. (Hint: Consider the direction of a magnetic line of force.)