19. H2O, CO2, CaCl2, and Na2CO3 (the numbers are subscripts) are all examples of that tell which atoms and how much of each atom is needed to make a molecule of a substance. A. chemical formulas
B. types of atoms
C. elements

A. Chemical formulas

Explanation:

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In this case, since chemical formulas are written or drawn representations of what atoms and how much of them are present in a molecule, which is composed by two or more different atoms, we can infer that molecules such as H2O (water), CO2 (carbon dioxide), CaCl2 (calcium chloride) and Na2CO3 (sodium carbonate) are given as a. Chemical formulas, because we can see there are two hydrogens and one oxygen in water, one carbon and two oxygens in carbon dioxide, one calcium and two chlorines in calcium chloride and two sodiums, one carbon and three oxygens in sodium carbonate.

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first let us lay out the formula for potential

and kinetic energy:

pe = mgh

ke = (mv^2)/2

where: m=mass, v=velocity, g=gravitational

acceleration, h=height

calculating for mass using the known values of

pe:

340j = m (9.81 m/s^2) (50ft) (1m/3.28ft)

m = 2.27 kg

the law of conservation of energy states that

energy is neither created nor destroyed. therefore, the change in kinetic

energy from 50 ft to 35 ft would be equal to the change in potential energy

from 50 ft to 35 ft. however it would be opposite in signs since one is losing

while other is gaining.

ke(35ft) – ke(50ft) = - [ pe(35ft) – pe(50ft]

ke(50ft) = 0 since the brick is initially at rest

ke(35ft) = 340j - 2.27kg (9.81 m/s^2) (35ft)

(1m/3.28 ft)

ke(35ft)

= 102.38 j