Answer:
16
Explanation:
The atomic mass of an element is defined as the average of the isotopic masses, weighted according to the naturally occurring abundances of the isotopes of the element. Although oxygen has nine isotopes with mass numbers ranging from 13 to 21, oxygen-13, oxygen-14 and oxygen-15 are not naturally occurring isotopes. So, we do not need to include them when we calculate the relative atomic mass of oxygen.
Solar energy is a form of renewable energy that is harnessed using the technology of the photovoltaic cells. These cells are composed of semiconductors. When the light from the sun hits the semiconductor, the energy from the light particles, or photons, cause the electrons in the semiconductor to be 'excited'. When the electron moves across the semiconductor platforms, the electron flows, thus creating electricity.
Therefore, as much as possible, you want to capture all of the sun's light. This can be achieved when the Sun is high up the sky. But if there is low sun angle, then the concentration of the sun's energy that strike the PV cells would be lesser.
A. Minerals would dissolve faster at the bottom of a waterfall than at a still pond because the water is more turbulent at the base of the waterfall. The turbulent water has more energy and would facilitate faster dissolution of minerals.
B. Minerals would dissolve faster in warmer water downstream because the warm water would speed up the rate of dissolution.
C. Minerals in the sand would dissolve faster because the sand is in small grains and so more surface area is available for dissolution.
Answer is D.
Speed:
Use relative speed to simplify the situation. Since the trains are moving in opposite directions, you can add the speeds and pretend the first train is stationary (moving at 0m/s) and the second train is moving at 50m/s.
Distance:
The front of the second train needs to travel 120m to get from the front to the back of the first train. When the front of the second train is at the back of the first train, the back of the second train is still 10m in front of the first train. The back therefore has to travel 130m to clear the first train. The total distance over which the trains are overlapping in this scenario is therefore 120 + 130 = 250m.
You have speed and you have distance so now just calculate time:
v = d / t
50 = 250 / t
t = 5s
