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.

An implanted pacemaker supplies the heart with 72 pulses per minute, each pulse providing 6.0 v for 0.65 ms. the resistance of the heart muscle between the pacemaker’s electrodes is 550 ω. find (a) the current that flows during a pulse, (b) the energy delivered in one pulse, and (c) the average power supplied by the pacemaker.

An uncharged nonconductive hollow sphere of radius 18.0 cm surrounds a 12.0 µc charge located at the origin of a cartesian coordinate system. a drill with a radius of 1.00 mm is aligned along the z axis, and a hole is drilled in the sphere. calculate the electric flux through the hole.

Which of the following harmful chemicals are found in tobacco smoke? a. carbon monoxide b. carbon dioxide c. nicotine b. carbon dioxide d. both a and c