Please show a step by step process for each problem, Thanks. The friction force

Question

Please show a step by step process for each problem, Thanks.

The friction force acts to retard the motion, requiring more power. For a descending elevator car, the friction force can actually reduce the power requirement. QUESTION In general, are the minimum power requirements of an elevator car ascending at constant velocity greater than, less than, or equal to the minimum power requirements of an elevator car descending at constant velocity? greater than less than equal to Use the worked example above to help you solve this problem. A 1.03 times 10^3-kg elevator car carries a maximum load of 8.80 times 10^2 kg. A constant friction force of 3.95 times 10^3 N retards its motion upward, as shown in the figure. What minimum power, in kilowatts and horsepower, must the motor deliver to lift the fully loaded elevator at a constant speed of 3.00 m/s? Use the values from PRACTICE IT to help you work this exercise. Suppose the same elevator car with the same load descends at 3.00 m/s. What minimum power is required? (Here, the motor removes energy from the elevator by not allowing it to fall freely.)

Explanation / Answer

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"greater than" (the weight of the elevator is added to the motor power, if a constant velocity is required/desired).

PRACTICE IT

P=W/t=(F*d)/t=F*v (1)

P=(W+Ff)*v=[melev+mload)*g+Ff]*v=68.004*103 W~91.2 hp

EXERCISE

P=(W-Ff)*v=[melev+mload)*g-Ff]*v=44.304*103 W~59.41 hp

OBS: About horsepower:

A hp is a unit in the foot-pound-second (fps, English system).  It was originally defined as 550 foot-pounds per second (ft-lb/s).

1 hp~746 W (Wikipedia gives 1hp=745.7 W)

From this equality you may find how many hp are included in 1W.

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