To test the quality of a tennis ball, you drop it onto a floor from a height of 4.00m . it rebounds to a height of 2.00 m. if the ball is in contact with the floor for 12.0ms,
(a) what is the magnitude of its average acceleration during the contact?
(b) is the acceleration up or down?

an ion is a charged atom or molecule. it is charged because the number of electrons do not equal the number of protons in the atom or molecule. an atom can acquire a positive charge or a negative charge depending on whether the number of electrons in an atom is greater or less then the number of protons in the atom.

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the skeletons of most mammals reach a certain size and then stop growing. however, many animals, including some mammals, keep growing throughout their lives. kangaroos, for example, just keep growing and growing until they die. also, the longer an animal lives the greater the chance it might catch some deadly disease.