Astudent throws a 5.0-newton ball straight up.
what is the net force on the ball at its maximum
height?
(1) 0.0 n (3) 5.0 n, down
(2) 5.0 n, up (4) 9.8 n, down

If the ball weighs 5 newtons, then that's the force of gravity that
attracts it to the center of the Earth.  The force only depends on
the mass of the ball and its distance from the center of the Earth. 
The force of gravity doesn't care whether the ball is moving or at
rest. It doesn't change significantly until the ball moves to some
significant distance off the Earth's surface, which isn't happening
if it's tossed by a human being. 

So we have a downward force of 5 newtons on the ball at all times.
In order to find the net force on it, we have to identify any other
forces besides gravity that are also acting on it.

If the ball were moving through the air, then there would be a force
of air resistance on it, acting opposite to its motion. But when it's
at its maximum height, in the process of turning around from rising
to falling, there's an instant when it's not moving through the air at all,
so gravity is the only force acting on it. At that moment, the net force
on the ball is 5.0 newtons downward.

Acetic acid  /əˈsiːtɪk/, systematically named  ethanoic acid  /ˌɛθəˈnoʊɪk/, is a colourless liquid  organic compound  with the  chemical formula  ch3cooh (also written as ch3co2h or c2h4o2). when undiluted, it is sometimes called  glacial acetic acid.  vinegar  is no less than 4% acetic acid by volume, making acetic acid the main component of vinegar apart from water. acetic acid has a distinctive sour taste and pungent smell. in addition to household vinegar, it is mainly produced as a precursor to  polyvinyl acetate  and  cellulose acetate. it is classified as a  weak acid  since it only partially  dissociates  in solution, but concentrated acetic acid is corrosive and can attack the skin.acetic acidnamespreferred iupac nameacetic acidsystematic iupac nameethanoic acidother namesvinegar (when dilute); hydrogen acetate; methanecarboxylic acid[1][2]identifierscas number64-19-7  3d model (jsmol)interactive image3dmetb00009 reference506007chebichebi: 15366  chemblchembl539  chemspider171  drugbankdb03166  echa infocard100.000.528ec number200-580-7e numbere260  (preservatives)gmelin reference1380iuphar/bps1058keggd00010  meshacetic+acidpubchem  cid176rtecs numberaf1225000uniiq40q9n063p  un number2789inchiinchi=1s/c2h4o2/c1-2(3)4/h1h3,(h,3,4)  key:   qtbsbxvteameqo-uhfffaoysa-n  smilescc(o)=o