A chemistry student named Hector had a cup of hydrogen peroxide, H2O2, that weighed 40.27 g. He walked away and forgot about it so when he returned he reweighed it just to be sure. Shockingly, he found it only weighed 33.16 g!?! Hector knows his chemistry and knows that matter can’t be created or destroyed. Look closely at the equation - what could explain the reduction in mass?

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

o conteúdo de calor de um sistema é igual à entalpia apenas para um sistema que é constante