Why does a balloon filled with helium rise while a balloon filled with an equal volume of average atmospheric air does not?
a. helium has no mass, but air does.
b. helium increases the balloon's mass more than air.
c. helium has a higher density than air.
d. helium has a lower density than air.

You have been called to testify as an expert witness in a trial involving a head-on collision. car a weighs 1515 lb and was traveling eastward. car b weighs 1125 lb and was traveling westward at 44.0 mph. the cars locked bumpers and slid eastward with their wheels locked for 22.5 ft before stopping. you have measured the coefficient of kinetic friction between the tires and the pavement to be 0.750 . how fast (in miles per hour) was car a traveling just before the collision? (this problem uses english units because they would be used in a u.s. legal proceeding.)

In the image, the arrow is pointing to a celestial object. which attribute disqualifies the object from being a planet?

Consider a hydrogen atom in the ground state. what is the energy of its electron? =e= jj now consider an excited‑state hydrogen atom. what is the energy of the electron in the =5n=5 level? =e5= j