18 $$

    19 \begin{tikzpicture}

    20 \fill (0,0) circle (.1);

    21 \draw (0,0) -- (2,0);

    22 \draw (1,0.1) -- (1,-0.1);

    24 \draw [->,red] (1,0.25) -- (1,0.75);

    25 

    26 \fill (0,1) circle (.1);

    27 \draw (0,1) -- (2,1);

    28 \end{tikzpicture}

    29 \qquad

    30 \begin{tikzpicture}

    31 \fill (0,0) circle (.1);

    32 \draw (0,0) -- (2,0);

    33 \draw (1,0.1) -- (1,-0.1);

    34 

    35 \draw [->,red] (1,0.25) -- (1,0.75);

    36 

    37 \fill (0,1) circle (.1);

    38 \draw (0,1) -- (1,1);

    39 \end{tikzpicture}

    40 \qquad

    41 \begin{tikzpicture}

    42 \fill (0,0) circle (.1);

    43 \draw (0,0) -- (3,0);

    44 \foreach \x in {0.5, 1.0, 1.25, 1.5, 2.0, 2.5} {

    45 	\draw (\x,0.1) -- (\x,-0.1);

    46 }

    47 

    48 \draw [->,red] (1,0.25) -- (1,0.75);

    49 

    50 \fill (0,1) circle (.1);

    51 \draw (0,1) -- (2,1);

    52 \foreach \x in {1.0, 1.5} {

    53 	\draw (\x,1.1) -- (\x,0.9);

    54 }

    55 

    56 \end{tikzpicture}

    57 $$