The materials which allow heat to pass through them easily are conductors of heat. For examples,aluminum, iron, and copper.
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The best explanation, though it's controversial, is that a great flood had once covered the earth for a brief period of time.
Atomic mass deals with the number of protons and neutrons added together, atomic number deals with protons only, though isotopes can be explained like this, say you had a hydrogen atom it's one proton and one electron if you add a neutron to it, the hydrogen becomes Deuterium which is a isotope of hydrogen it's the same atom just a neutron added to it for a different atomic mass if only slightly changed
Answer:
The force that keeps the planets in orbit is gravity.
