Zubrzycki: Power
Power Up!
REFERENCES:
 PTPA Section 6.9 (posted online on my website)
 Wilson/Buffa Section 5.6 (Problems pg 172 MC #'s 21-22, CQ #'s 19-20, EX #'s 63, 64,
66, 67, 69, 70, 71)
8.1 Hans, a weightlifter, can bench press 100 kg (220 lbs). Hans can lift the 100 kg, from a
height of 0.8 m above the ground to a height of 1.3 m in 0.2 seconds. Hans wants to
determine the rate at which work is done on the barbell and weights. What would you tell
Hans to do, to determine the rate at which he does work on the barbell and weights?
8.2 Caroline is doing what is called a dead lift. She lifts a 30-pound barbell (13.6 kg) from the
floor to the level of her waist (a vertical distance of 1.0 m) in 0.80 s. Determine the power
during the lift.
8.3 Caroline performs an overhead press—lifting the same barbell from her shoulders to above
her head. Determine the power involved in this process. The length of her arm is
approximately 40 cm. It takes her 1.0 s to lift the bar.
8.4 Mr. Zubrzycki (mass 60 kg) moves on rollerblades on a smooth linoleum floor a distance of
4.0 m in 5.0 s. Determine the power of this process.
8.5 A crane lifts an I-beam up the side of a building. The crane’s power output is 1750W for 20
seconds. After 20 seconds the I-beam was moving at 2 m/s and the mass has 200 kg. Use the
work-energy process to determine the change in height of the I-beam.
8.6 A 1400-kg car is traveling on a level road at a constant speed of 27 m/s (60 mph). The drag
force exerted by the air on the car, and the rolling friction force exerted by the road on the car
add to a net force of 680 N pointing opposite the direction of motion of the car. (a)
Determine the power due to the work done by this opposing force during this process. (b)
The car instead drives up a 4.0 degree incline at the same speed. Determine the power due to
the work of the opposing friction-like forces and the force exerted on the car by Earth during
this process. Express both results in watts and in horsepower (1 hp = 746 W).
8.7 The quickest times for the Sears Tower stair climb (103 flights, or 2232 steps) is around 20
minutes. (a) Estimate the mechanical power in watts for a top climber. Indicate any
assumptions you made. (b) If the body is 20 percent efficient at converting chemical energy
into mechanical energy, approximately how many joules and kilocalories of chemical energy
does the body expend during the stair climb? Note: 1 food calorie = 1 kilocalorie = 4186 J.
Efficiency is a very complicated subject—especially relative to mechanical work done by the
human body. For example, it takes about 10 J of food energy for the food we eat to produce
the equivalent of 1 J of chemical energy in the body. If the body can convert only 20 percent
of this chemical energy to mechanical work, then we get 0.2 J of work for each 10 J invested
in producing food.