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Cerrena [4.2K]
3 years ago
9

A hand lifts a block vertically upward at constant velocity. The work done by gravity on the block ____ if the system consists o

f the block only. The work done by gravity on the block ____ if the system consists of the block and the earth. AND WHY?
1. is zero, is zero2. is zero, is negative3. is zero, is positive4. is negative, is negative5. is positive, is positive
Physics
1 answer:
adelina 88 [10]3 years ago
6 0

Answer:

If the system consists of the block only, the work done by the gravity is negative.

If the system consists of the block and the earth the work done by the gravity is zero.

Explanation:

If the system consists of the block only, then the system experiences two external forces: one exerted by the hand that lifts the block vertically upward and other exerted by the earth (gravity), which is opposed to the movement of the system, so the work done by gravity is negative.

On the other hand, if the system consists of the block and the earth, then only exists a external force which is the exerted by the hand. So, the force exerted by gravity is zero.

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Answer:

b.) All these technologies use radio waves, including high-frequency microwaves

d.) Microwave ovens emit in the same frequency band as some wireless Internet devices.

e.) The radiation emitted by wireless Internet devices has the shortest wavelength of all the technologies listed above.

f.) All these technologies emit waves with a wavelength in the range of 0.10 to 10.0 m.

Explanation:

For option D. The frequency range of microwave ovens is 2450 MHz = 2.4 GHz, which intersects with wireless internet technology with range of 2.4 to 2.6 GHz.

For E, wavelenght and frequency are inversely proportional. Wireless internet service has the greatest frequency band and hence the shortest wavelenght band.

For F, in all these radiations, the highest Freq is 2.6 GHz and the lowest is 40 MHz. Wavelenght is speed of light (3x10^8 m/s) divided by the frequency.

2.6 GHz = 2.6x10^9 Hz

Wavelenght = 3x10^8 ÷ 2.6x10^9 = 0.1 m

40 MHz = 40x10^6

Wavelenght = 3x10^8 ÷ 40x10^6 = 7.5 m

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3 years ago
An escalator is 18.3 m long. If a person stands on the escalator, it takes 47.8 s to ride from the bottom to the top. If a perso
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Answer:

20.243 s

Explanation:

Because the escalator and person moving in the same direction, relative velocity can be calculated by summing the velocity of the escalator and velocity of the person.  

The speed of escalator can be calculated as,

v_{escalator}=\frac{x}{t}=\frac{18.3}{47.8}=0.383\ m/s

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Therefore total time required to take the person to get to the top

t=\frac{x}{v_{relative}}= \frac{18.3}{0.904}=20.243\ s

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The correct response is D.

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Answer:

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