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Ymorist [56]
3 years ago
10

Which of the following has the lowest U value?

Physics
1 answer:
jarptica [38.1K]3 years ago
4 0

Answer:

c. expanded polyurethane

Explanation:

Thermal performance of a building fabric is measured in terms of heat loss and is expressed as U-value or R-value. U-value is the rate of heat transferred through a structure divided by the difference in temperature across the structure with a unit of measurement of W/m²K.You can calculate the U-value of a by getting the reciprocal of the sum of thermal resistances , R, making the building material.

If you have the value of R, then U=1/R

Material                         size            R                      U

plywood                          1"              1.25                0.8

Poured concrete            2"              0.99               1.010

Expanded polyurethane  1"            6.5                   0.1538

Asbestos shingles            1"             0.03                33.33

The material with lowest U-value is expanded polyurethane

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Finger [1]

Forward thrust has positive values and reverse thrust has negative values.

Thrust is a sudden push or pull in a certain direction.

a)

Flight speed u = 150 km/h

1 km/h = \frac{1}{3.6} km/s

therefore, 150 km/h =  41.67 km / s

The thrust force represents the horizontal or x-component of momentum equation:

T = m_{exhaust} * U_{exhaust} - U_{flight}

T = 50 * (150 - 41.67)

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b)

Now the exhaust velocity is now vertical due to reverse thrust application, then it has a zero horizontal component,

thus thrust equation is:

T = m_{exhaust} * U_{exhaust} - U_{flight}

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T = -2083.5 N

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c)

Now the exhaust velocity and flight velocity is zero, then it has a zero horizontal component, thus thrust is also zero becauseU_{exhaust} = U_{flight} = 0\\

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The given question is incomplete, the complete question is,

"The idling engines of a landing turbojet produce forward thrust when operating in a normal manner, but they can produce reverse thrust if the jet is properly deflected. Suppose that while the aircraft rolls down the runway at 150 km/h the idling engine consumes air at 50 kg/s and produces an exhaust velocity of 150 m/s.

a. What is the forward thrust of this engine?

b. What are the magnitude and direction (i.e., forward or reverse) if the exhaust is deflected 90 degree without affecting the mass flow?

c. What are the magnitude and direction of the thrust (forward or reverse) after the plane has come to a stop, with 90 degree exhaust deflection and an airflow of 40 kg/s?"

To know more about thrust,

brainly.com/question/14552836

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3 0
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DiKsa [7]

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A crate of 45.2-kg tools rests on a horizontal floor. You exert a gradually increasing horizontal push on it and observe that th
iragen [17]

Answer:

 μ = 0.725

Explanation:

This problem refers to Newton's second law.

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Let's write the equations on each axis

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8 0
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