def plot_force(ax, coord, load_matrix, min_size, size):
    """ Add load vector to the plot of deformed mesh.
        
    Args:    
        ax (matplotlib.axes.Axes): Matplotlib axes object.
        coord (:obj:`numpy.array`): mesh coordinates.
        load_matrix (:obj:`numpy.array`): The columns are respectively node, x direction, y direction, force value. 
        min_size (:obj:`float`): Value to define the width of load arrow.
        size (:obj:`float`): Load arrow length.
    
    Returns:
        Matplotlib axes object.
    """
    factor = coord[:, min_size].max() - coord[:, min_size].min()
    ind = (load_matrix[:, 0] - 1).astype('int')
    for i in range(load_matrix.shape[0]): 
        if load_matrix[i, 1] == 1:
            ax.arrow(coord[ind[i], 1], coord[ind[i], 2], size, 0, shape='full', length_includes_head=True, width = factor * 0.01)
        if load_matrix[i, 1] == -1:
            ax.arrow(coord[ind[i], 1], coord[ind[i], 2], -size, 0, shape='full', length_includes_head=True, width = factor * 0.01)
        
        if load_matrix[i, 2] == -1:
            ax.arrow(coord[ind[i], 1], coord[ind[i], 2], 0, -size, shape='full', length_includes_head=True, width = factor * 0.01)
        
        if load_matrix[i, 2] == 1:
            ax.arrow(coord[ind[i], 1], coord[ind[i], 2], 0, size, shape='full', length_includes_head=True, width = factor * 0.01)
    return ax