Download WORK POWER MACHINES assignment

WORK, POWER, MACHINES 1
INDEPENDENT PRACTICE
1. In the following problems, assume that the fulcrum is located at the NBP of the meter stick.
a. Where would a 200 N weight need to be located to balance a 100 N weight located 30 cm from
the fulcrum?
b. Where would a 280 N weight need to be located to balance a 2800 N weight located 5 cm from
the fulcrum?
c. A 475 N weight is placed 25 cm from the fulcrum. How much weight is needed to balance the
meter stick if the weight is located 50 cm away on the other side of the fulcrum?
d. A 820 N weight is placed 50 cm from the fulcrum. How much weight is needed to balance the
meter stick if the weight is located 35 cm away on the other side of the fulcrum?
e. A 750 N weight is placed is placed 40 cm from the fulcrum of a balance. A weight will
balance the meter stick if it is placed 30 cm on the opposite side of the fulcrum. What is the
mass in kilograms of this weight?
f. A 1680 N weight is placed 32 cm from the fulcrum of a balance. A weight will balance the
meter stick if it is placed 20 cm on the opposite side of the fulcrum. What is the mass in
kilograms of this weight?
2. A boy wants to sit on a see-saw by himself. Explain how the see-saw would have to be set up. Also,
explain why your setup would enable the boy to make the see-saw work by himself.
3. Why is it dangerous to roll open the top drawers of a fully-loaded file cabinet that is not secured to the
floor?
4. What part of an object follows a smooth path when the objects made to spin through the air or across a
flat smooth surface?
5. Describe the motion of a projectile before and after it explodes in midair.
6. Cite an example of an object that has a CG where no physical material exists.
7. How far can an object be tipped before it topples over?
8. Why do some high-jumpers arch their bodies into a U-shape when passing over the high bar?
9. The bars shown in the figures below are of the same thickness over their entire length. How much
does each weigh?
3m
40 cm
7 kg
5m
1.5 m
12 kg
2m
80 cm
1.5 kg
1m
10 cm
750 kg
10. The bars shown in the figures below are of the same thickness over their entire length. How much
does each weigh?
3m
10 cm
1m
750 g
2.5 m
10 cm
75 cm
1200 g
WORK, POWER, MACHINES 2
GUIDED PRACTICE
1. A force of 825 N is needed to push a car across a lot. Two students push the car 35 m.
a. How much work is done?
b. After a rainstorm, the force needed to push the car doubled because the ground became soft.
By what amount does the work done by the students change?
2. What work is done by a forklift raising a 583 kg box 1.2 m?
3. How much work does the force of gravity do when a 25 N object falls a distance of 3.5 m?
4. An airplane passenger carries a 215 N suitcase up stairs, a displacement of 4.20 m vertically and
4.60 m horizontally.
a. How much work does the passenger do?
b. The same passenger carries the same suitcase back down the same stairs. How much work
does the passenger do now?
5. A rope is used to pull a metal box 15.0 m across the floor. The rope is held at an angle of 46.0 with
the floor and a force of 628 N is used. How much work does the force on the rope do?
INDEPENDENT PRACTICE
1. In what units is work measured?
2. A satellite orbits Earth in a circular orbit. Does Earth’s gravity do any work on the satellite?
3. An object slides at constant speed on a frictionless surface.
a. What forces act on the object?
b. What work is done?
4. Which requires more work, carrying a 420 N knapsack up a 200 m high hill or carrying a 210
knapsack up a 400 m high hill? Explain.
5. Guy has to get a piano onto a 2.0 m high platform. He can use a 3.0 m long, frictionless ramp or a 4.0
m long, frictionless ramp. Which ramp will Guy use if he wants to do the least amount of work?
6. Grace has an after-school job carrying cartons of new copy paper up a flight of stairs and then
carrying used paper back down the stairs. The mass of the paper does not change. Grace’s physics
teacher suggests that Grace does no work all day, so she should not get paid.
a. In what sense is the physics teacher correct?
b. What arrangement of payments might Grace make to ensure compensation?
7. Grace now carries the copy paper boxes down a 15.0 m long hall. Is Grace working now? Explain.
8. Lee pushes horizontally with a 80 N force on a 20 kg mass 10 meters across a floor. Calculate the
amount of work Lee did.
9. Chris carried a carton of milk, weight 10.0 N, along a level hall to the kitchen, a distance of 3.50 m.
How much work did Chris do?
10. The third floor of a house is 8.0 m above street level. How much work is needed to move a 150 kg
refrigerator to the third floor?
11. A student librarian picks up a 22 N book from the floor to a height of 1.25 m. He carries the book 8.0
m to the stacks and places the book on a shelf that is 0.35 m high. How much work does he do on the
book?
12. Stan does 176 J of work lifting himself 0.300 m. What is Stan’s mass?
13. Sally applies a horizontal force of 462 N with a rope to drag a wooden crate across a floor with a
constant speed. The rope tied to the crate is pulled at an angle of 56.0.
a. How much force is exerted by the rope on the crate?
b. What work is done by Sally if the crate is moved 24.5 m?
c. What work is done by the floor through force of friction between the floor and the crate?
14. Mike pulls a sled across level snow with a force of 225 N along a rope that is 35.0 above the
horizontal. If the sled moved a distance of 65.3 m, how much work did Mike do?
15. An 845 N sled is pulled a distance of 185 m. The task requires 1.20 x 104 J of work and is done by
pulling on a rope with a force of 125 N. At what angle is the rope held?
16. Karen has a mass of 57.0 kg and she rides the up escalator at Woodley Park Station of the Washington
D.C. Metro. Karen rode a distance of 65 m, the longest escalator in the free world. How much work
did the escalator do on Karen if it has an inclination of 30?
17. A crane lifts a 2.25 x 103 N bucket containing a 1.15 m3 of soil (density = 2.00 x 103 kg/m3 to a height
of 7.50 m. Calculate the work the crane performs.
18. John pushes a crate across the floor of a factory with a horizontal force. The roughness of the floor
changes and John must exert a force of 20 N for 5 m, then 35 N for 12 m, then 10 N for 8 m.
a. Draw a graph of force as a function of distance.
b. Find the work John does pushing the crate.
19. A 60.0 kg crate is slid up an inclined ramp 2.0 m long onto a platform 1.0 m above floor level. A
400.0 N force, parallel to the ramp is needed to slide the crate up the ramp at a constant speed.
a. How much work is done is sliding the crate up the ramp?
b. How much work would be done if the crate were simply lifted straight up from the floor to the
platform?
20. A 4200 N piano is to be slid up a 3.5 m frictionless plank that makes an angle of 30.0 with the
horizontal. Calculate the work done in sliding the piano up the plank.
21. Pete slides a crate up a ramp at an angle of 30.0 by exerting a 225 N force parallel to the ramp. The
crate moves at constant speed. The coefficient of friction is 0.28. How much work has been done
when the crate is raised a vertical distance of 1.15 m?
WORK, POWER, MACHINES 3
GUIDED PRACTICE
1. A box that weighs 575 N is lifted a distance of 20.0 m straight up by a rope. The job is done in 10.0 s.
What power is developed in watts and kilowatts?
2. A rock climber wears a 7.50 kg knapsack while scaling a cliff. After 30.0 min., the cliber is 8.2 m
above the starting point.
a. How much work does the climber do on the knapsack?
b. If the climber weighs 645 N, how much work does she do lifting herself and the knapsack?
c. What is the average power developed by the climber?
3. An electric motor develops 65 kW of power as it lifts a loaded elevator 17.5 m in 35.0 s. How much
force does the motor exert?
4. A motor exerts a force of 12 N and moves a load at 5m/s. What power does the motor develop?
INDEPENDENT PRACTICE
1. Define work and power.
2. What is a watt equivalent to in terms of kg, m, and s?
3. Two people of the same mass climb the same flight of stairs. The first person climbs the stairs in 25 s;
the second person takes 35 s.
a. Which person does more work? Explain.
b. Which person produces more power? Explain.
4. A force of 300 N is used to push a 145 kg mass 30.0 m horizontally in 3.00 s.
a. Calculate the work done on the mass.
b. Calculate the power.
5. Robin pushes a wheelbarrow by exerting a 145 N force horizontally. Robin moves it 60.0 m at a
constant speed for 25.0 s.
a. What power does Robin develop?
b. If Robin moves the wheelbarrow twice as fast, how much power is developed?
6. Brutus, a champion weight lifter, raises 240 kg a distance of 2.35 m.
a. How much work is done by Brutus lifting the weights?
b. How much work is done holding the weights above his head?
c. How much work is done lowering them back to the ground?
d. Does Brutus do work if the weights are let go and fall back to the ground?
e. If Brutus completes the lift in 2.5 s, how much power is developed?
7. A 188 W motor will lift a load at the rate (speed) of 6.50 cm/s. How great a load can the motor lift at
this speed?
8. A car is driven at a constant speed of 21 m/s (76 km/h) down a road. The car’s engine delivers 48 kW
of power. Calculate the average force of friction that is resisting the motion of the car.
9. A horizontal force of 805 N is needed to drag a crate across a horizontal floor with a constant speed.
Pete drags the crate using a rope held at an angle of 32.0.
a. What force does Pete exert on the rope?
b. How much work does Pete do on the crate when moving it 22.0 m?
c. If Pete completes the job in 8.0 s, what power is developed?
10. Wayne pulls a 305 N sled along a snowy path using a rope that makes a 45.0 angle with the ground.
Wayne pulls with a force of 42.3 N. The sled moves 16 m in 3.0 s. What is Wayne’s power?
11. A lawn roller is rolled across a lawn by a force of 115 N along the direction of the handle, which is
22.5 above the horizontal. If George develops 64.6 W of power for 90.0 s, what distance is the roller
pushed?
12. Use the graph in Figure 10-17 on page 214 of your textbook.
a. Calculate the work done to pull the object 7.0 m.
b. Calculate the power if the work were done in 2.0 s.
13. In 35.0 s, a pump delivers 550 dm3 of oil into barrels on a platform 25.0 m above the pump intake
pipe. The density of the oil is 0.820 g/cm3.
a. Calculate the work done by the pump.
b. Calculate the power produced by the pump.
14. A 12.0 m long conveyer belt, inclined at 30.0, is used to transport bundles of newspapers from the
mail room up to the cargo bay to be loaded on delivery trucks. Each newspaper has a mass of 1.00 kg
and there are 25 newspapers per bundle. Determine the useful power of the conveyor if it delivers 15
bundles per minute.
15. An engine moves a boat through the water at a constant speed of 15 m/s. The engine must exert a
force of 6.0 x 103 N to balance the force that water exerts against the hull. What power does the
engine develop?
Physics: Work and Pulley Lab (All Work and No Play)
WAHS D. Vernon
Purpose: To determine the amount of work done when a force acts through a distance and to compare this work
to that done by a pulley.
Procedure:
A cart will be pulled up an incline with a force sensor. First the cart will be pulled parallel to the plane of
the incline. Second, the cart will be pulled at an angle higher than the plane. Third, the cart will be lifted straight
up. Use the rear of the cart to determine the height raised. Finally, the cart will be lifted straight up with a pulley
system.
Part 1. Create a table of values (will include data and calculated results)
Table of Values
Height of the plane (y-axis) = ________ m
Trial
1
Angle of Pull
(relative to
plane of
movement)
Mass
of
Cart
(kg)
Weight
of Cart
(N)
Distance
moved
along
plane (m)
0
Force
applied
(avg)
(N)
Force
applied
on the
plane
(avg) (N)
Calculated
Work (N m)
or (J)
Calculated
Mechanical
Advantage
MA= F
resistance/F
effort
Calculate
Ideal MA
IMA= d
effort/d
resistance
Calculate
Efficiency
= MA/IMA
N/A
2
3
4
90 (straight
up)
N/A
Part 2. Create a table of values (will include data and calculated results) for the pulley system.
Trial
Pulley System
5
1 pulley
6
2 pulleys
Load
(N)
Distance
load
moves
(m)
Pulling
Force
(N)
Distance
Force
Moves (m)
Calculate
Work “IN”
(you did it)
(N m) or (J)
Calculate
Work “OUT”
(what happened
as a result)
(N m) or (J)
Calculate
Efficiency
(%) =
W out/W in
Calculate
Mech.
Advantage
Calculations:
-Show at least one sample calculation from each of the eight calculate columns above.
- Show ALL calculations for the components of force if an angle is involved (there should be
two).
Questions:
1.Determine the theoretical amount of work done by lifting the cart vertically. Compare this to the calculated
work done in trial 4 (using the force sensor). Are they the same ?, should they be?, Explain.
2. How does the amount of work done in trials 1, 2, and 3 compare? Explain your results.
3. How does the amount of work done in trial 4 compare to the other trials? Explain your results.
4. How are work and the change in potential energy related? Is energy always conserved in mechanical
operations?
5. If the mass of the object were greater in trials 1, 2, 3, 4, how would that affect work done to move the object
the same distances?
6. Compare trial 1 and 4. Which has the greater mechanical advantage? Explain your answer.
7. Compare trials 1, 2, 3. Is there a mechanical advantage to pulling the object at an angle other than the angle
that is parallel to the plane of the ramp?
8. Compare trials 1, 2, 3, 4. Which has the highest efficiency?
9. What is the advantage of using trial 1 vs. trial 4?
10. What have you learned about the amount of force applied in trials 2 and 3 when compared to trial 1 and the
resulting work done?
11. Construct a small graph of the following data and tell what information is provided in the area beneath the
curve:
Y axis – Force (N)
2
4
6
X axis- displacement (m)
0.08
0.16
0.24
12. Using the TOV in part 2, does the calculated value of “work out” always equal less than “work in”? If yes,
where does the missing work go?
13. What is an advantage of using only 1 pulley?
14. What is the advantage of using two pulleys?
15. How do the calculated efficiency and mechanical advantage change as you use more pulleys?
Conclusion/New Terms:
Work
Mechanical Advantage
Efficiency
Physics: Human Power Lab (Play and Work)
WAHS D. Vernon
Purpose: To determine the amount of power that can be produced by various muscles in the human body.
Procedure:
Perform these activities and record the appropriate data in the TOV.
1. Walk up the stairs outside of the gym.
2. Walk up the stairs on the bleachers inside the gym.
3. Move a bowling ball from about waist high to a point near eye level.
4. Do a pull up.
5. Pull up a bowling ball using a rope and pulley-like system.
6. Push Mr. Vernon’s car in the parking lot.
Create a table of values (will include data and calculated results)
Table of Values
Activity
Weight of Object
being moved
(N)
Distance
moved
vertically
(m)
1
Time object is moved
(s)
1
2
3
===
Calculated
Work (N m) or
(J)
Calculated
Power (W)
Calculated
Horsepower (HP)
1 HP= 746W
AVG.
===
===
===
===
===
2
3
4
5
(linearly)
6
Calculations:
-Show at least one sample calculation from each of the four “calculate” columns above.
For activity #6, the force used to move the car must be calculated.
vavg = x/t
vfinal = 2vavg
a= vfinal – v initial/ t
F= ma (use this force as a replacement for weight)
Questions:
1. In what activity done was the most work done?
2. In what activity done was the most power done?
3. Excluding activity 6, which muscle groups in the human body produced the most power and the least power?
4. Did the activity that used the largest force result in the largest power produced?
5. Explain how a large force could result in a relatively small power.
6. What was the advantage of using the rope and “one-pulley” system on the bowling ball in activity #5?
7. Can a two-pulley system, winch, or lever increase the rate that work can be done? Explain.
Conclusion
New Terms:
Power
Horsepower
Watt