Which of these statements is not true?
a. atoms of the same elements can have different masses.
b. the nucleus of an atom has a positive charge.
c. atoms of isotopes of an element have different numbers of protons.
d. atoms are mostly empty space.

Heena wants to measure the volume of a ball that is 24 cm across. how should she set up her equation?

Consider an ordinary, helium-filled party balloon with a volume of 2.2 ft3. the lifting force on the balloon due to the outside air is the net resultant of the pressure distribution exerted on the exterior surface of the balloon. using this fact, we can derive archimedes’ principle, namely that the upward force on the balloon is equal to the weight of the air displaced by the balloon. assuming that the balloon is at sea level, where the air density is 0.002377 slug/ft3, calculate the maximum weight that can be lifted by the balloon. note: the molecular weight of air is 28.8 and that of helium is 4.

The boy of mass 30 kg is at r1= 2 meters from the fulcrum. of the girl is 45 kg, at what r2 must she sit so that they are balanced ?