The human brain consumes about 21.0 W of power under normal conditions, though m

Question

The human brain consumes about 21.0 W of power under normal conditions, though more power may be required during exams. How long can one candy bar (203 Calories per bar) power the normally functioning brain? (Note: The nutritional Calorie is equivalent to 1000 calories (1 kcal) as defined in physics; 1 kcal = 4186 J).

At what rate must you lift a 0.90-kg container of milk (1/4 gallon) if the power output of your arm is to be 21.0 W?

How long does it take to lift the milk container through a distance of 1.06 m at this rate?

Explanation / Answer

1. Power = Energy/time
time = Energy/Power
= 203*4.186/21 = 40.465 secs
2. Power = Force*velocity
Velocity = Power/Force
=21/(0.9*9.8) = 2.38m/sec
3. Time = distance/velocity = 1.06/2.38 = 0.45secs

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