free body diagrams

a student is sitting in his chair

normal up = gravity down

a rightward moving car skids to a stop with locked wheels

normal up = gravity down = friction left

a leftward moving car skids to a stop with locked wheels

normal up = friction right = gravity down

a bundle of hay, attached to a truck by a rope, is moved across the dirt to the left at a constant speed

normal up = friction right = gravity down = tension left

a skydiver with an unopened parachute is falling downward through the air at a constant speed

air resistance up = gravity down

a baseball is moving through the air, upward and leftward towards the peak of its trajectory

gravity down

a leftward moving sledder is slowing down as she glides across a patch of loosely packed snow

normal up > friction right < gravity down = applied left

a book is suspended motionless from the end of a string

tension up = gravity down

a student pushes a box of books to the right across the floor at a constant speed

normal up = applied right = gravity down = friction left

a rightward moving baseball player slows to a stop while sliding across the infield dirt

normal up = gravity down = friction left

a leftward moving baseball player slows to a stop while sliding across the infield dirt

normal up = friction right = gravity down

what is the upward force of an object that is hanging

tension

what is the upward force of an object that is sitting

normal

what is the upward force of an object that is falling

air resistance

what is the net force of a constant speed

equal to zero