A 3.0-kg block slides on a rough horizontal surface. A force of 8.0 N acting parallel to the surface is applied to the block. The coefficient of kinetic friction between the block and the surface is 0.15 . What is the magnitude of the block's acceleration?
a. 1.9 m/s2
b. 1.2 m/s2
c. 2.3 m/s2
d. 1.5 m/s2
e. 2.9 m/s2
b
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A hot-wire anemometer consists of a 5-mm-long, 5-?m-diameter platinum wire. The probe is operated at constant current of 0.03 amp. The electrical resistivity of platinum is 17 ??-cm at 20°C and increases by 0.385% per °C. (a)If the voltage across the wire is 1.75 volts, determine the velocity of the air flowing across it and the wire temperature if the free-stream air temperature is 20°C. (b)What are the wire temperature and voltage if the air velocity is 10 m/s? Neglect radiation and conduction heat transfer from the wire.
GIVEN • A hot wire in air • Wire diameter (D) = 5 m = 5 × 10–6 m • Wire length (L) 5 mm = 0.005 m • Current (I) = 0.03 A (constant) • Electrical resistivity (rel) = 17 W cm = 17 × 10–8 W m at 20°C and increases 0.385% per °C. • Air temperature (T?) = 20°C FIND (a) The air velocity (U?) and the wire temperature (Tw) if the voltage across the wire (Vel) =1.75V (b) The wire temperature (Tw) and voltage (Vel) if the air velocity (U?) = 10 m/s ASSUMPTIONS • Radiative heat transfer is negligible • Variation of Prandtl number with temperature id negligible PROPERTIES AND CONSTANTS Thermal conductivity (k) = 0.0251 W/(m K) Kinematic viscosity (?) = 15.7 × 10–6 m2/s Prandtl number (Pr) = 0.71 At 90°C: Pr = 0.71
You and your friends sit at opposite ends in the back of a pickup traveling north at 60 mph. Your friend, at the front of the pickup bed, throws a ball to you with a speed of 4 mph. An observer on the ground perceives the ball traveling with a velocity
A. 4 mph north. B. 4 mph south. C. 64 mph north. D. 56 mph north. E. 56 mph south.
A neutron passes by Earth at relativistic speed. Earth scientists observe the process of the neutron decaying into a proton, an electron, and a neutrino. Compared to the same type of process on Earth, earthbound observers would say that the process in the moving neutron
A. No process in another reference frame can be observed. B. appears to happen faster. C. appears to happen at the same rate. D. appears to happen more slowly.
A gasoline engine absorbs 2 500 J of heat at 250°C and expels 2 000 J at a temperature of 50°C. The magnitude of the change in entropy (in J/K) for the system is
a. 6.2 b. 4.7 c. 1.4 d. 10.9 e. 3.2