Relationship between Applied Force and Friction: A Graphical Analysis

[SpY]]

So my lecturer asked a physics audience (final year and postgrad) this question and no one got it right (at first):

Consider a block pushed back and forth on a flat table. Draw a graph of the applied force vs. friction. Take a minute to figure this one out yourself, then click this link for his solution.
http://i55.tinypic.com/2zpl53s.jpg
Trouble I'm having is interpreting the two lines, because it implies on the x-intercept that you will have a positive acceleration when there is no applied force, so the resultant force must be friction - which cannot exist without an applied force!

 

[CompuChip]

the resultant force must be friction - which cannot exist without an applied force!

It can if the block was already moving.

 

[nasu]

So my lecturer asked a physics audience (final year and postgrad) this question and no one got it right (at first):

Consider a block pushed back and forth on a flat table. Draw a graph of the applied force vs. friction. Take a minute to figure this one out yourself, then click this link for his solution.

When you say "a graph of applied force vs. friction" I would expect that the friction is on the x axis. Here it seems that you have acceleration. Is this correct?

Edited later.
Now I see that the title is actually "friction vs. applied force".
This makes more sense.
As you increase the applied force from zero, the friction will be equal with the applied force until the body starts to move. Then it will remain constant. So the first portion of the graph F vs Friction will be a line at 45 degrees from either axis. Then it will be a horizontal line.