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CH4 Section Problems:
- What is the measure of an object’s ability to do work?
Energy
- What is the equation for work?
W= F X D
- How do you increase the potential energy of a rubber band?
by stretching it
- Explain why sliding a block does not increase its energy?
energy loss to friction, some increase in heat temperature though
- Give an example where applying force produces no work.
pushing down on a block, no movement no work
- When does a force do the most work on an object?
To do the greatest amount
of work, you must apply force in the
direction the object will move (force
C page 87).
- What is the equation for work when lifting an object?
W = mgh
- Alexander has a mass of 75.6 kilograms. His apartment is on the second floor, 5.70 meters up from ground level. How much work
does he do against gravity each time he climbs the stairs to his apartment?
W = mgh
W = 75.6*9.81*5.70
W = 4230 joules
- How much additional work does Alexander have to do if he is carrying 12.7 kilograms of groceries?
W = mgh
W = 12.7*9.81*5.70
W = 710. joules
- A car engine does 62,000 J of work to accelerate at 12.2 m/sec2 for 5.00 meters. What is the mass of the car?
W = mgh
62,000J = m * 12.2 * 5.00
m = 1.0e3 kg
- What is Power and what are the units for power?
Th rate at which work is done, watts
- What is one Watt equal to? What is one horsepower equal to?
1 watt = 1 joule/1 second; 1hp = 746watts
- What is the equation for power?
Watts = joules/second or power = Work/time
- Explain how the weightlifters Big Mike and Little Jim do the same amount of work, but they have different amounts of power
W = F * D
200J = 100N * 2m (for both)
To find Michael’s power
divide his work (200 joules) by his time (1 second) = 200 watts
To find Jim’s power
divide his work (200 joules) by his time (10 seconds) = 20 watts
- What is the maximum human power?
Highly trained athletes can keep up a power of 350 watts for about an hour.
An average person running or biking for a full hour produces an average
power of around 200 watts.
- Calculate the amount of work you do in each situation.
a. You push a refrigerator with a force of 50 N and it moves 3
meters across the floor.
W = F * d
150J = 50N * 3m
b. You lift a box weighing 25 N to a height of 2 meters.
W = F * d
50J = 25 * 2
c. You apply a 500 N force downward on a chair as you sit on it
while eating dinner.
W = F * d
0J = 500N * 0m
d. You lift a baby with a mass of 4 kg up 1 meter out of her crib.
PE = mgh
PE = 4kg * 9.81 * 1m
PE = 39.2J
e. You climb a mountain that is 1000 meters tall. Your mass is 60
kg.
PE = mgh
PE = 60kg * 9.81 * 1000m
PE = 590,000J
- Sal has a weight of 500 N. How many joules of work has Sal done
against gravity when he reaches 4 meters high on a rock climbing
wall?
W = F * d
2000J = 500N * 4m
- You do 200 joules of work against gravity when lifting your backpack
up a flight of stairs that is 4 meters tall. What is the weight of your
backpack in newtons?
W = F * d
200J = ?N * 4m
50N = backpack
- You lift a 200 N package to a height of 2 meters in 10 seconds.
a. How much work did you do?
W = F * d
400J = 200N * 2m
b. What was your power?
Watts = joules/time
40w = 400j/10sec
- One machine can perform 500 joules of work in 20 seconds. Another
machine can produce 200 joules of work in 5 seconds. Which
machine is more powerful?
Watts = joules/time
25w = 500j/20sec
Watts = joules/time
40w = 200j/5sec
- Two cranes use rope and pulley systems to lift a load from a truck to
the top of a building. Crane A has twice as much power as crane B.
a. If it takes crane A 10 seconds to lift a certain load, how much
time does crane B take to lift the same load?
2x as long so 20 seconds
b. If crane B can do 10,000 joules of work in a minute, how many
joules of work can crane A do in a minute?
2x as many joules so 20,000 joules
- An elevator lifts a 500 kg load up a distance of 10 meters in 8
seconds.
a. Calculate the work done by the elevator.
PE = mgh
49,050J = 500kg * 9.81 * 10m
b. Calculate the elevator’s power.
Watts = joules / time
Watts = 49,050 / 8sec
6000 watts
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