def calculate_inst_vel(x_disp, y_disp, time_steps):
    """
    Calculate instantaneous velocities of (x,y) coordinates.

    Keyword arguments:
    x_disp -- list of step values of x-coordinates
    y_disp -- list of step values of y-coordinates
    time_steps -- list of time step values for frames of a tiff stack
    """
    
    x_inst_vel = []
    y_inst_vel = []
    
    #  Calculate instantaneous velocities
    for (x_disp_value, time_step) in zip(x_disp, time_steps):
        x_inst_vel_value = x_disp_value/time_step
        x_inst_vel.append(x_inst_vel_value)
        
    for (y_disp_value, time_step) in zip(y_disp, time_steps):
        y_inst_vel_value = y_disp_value/time_step
        y_inst_vel.append(y_inst_vel_value)
        
    return x_inst_vel, y_inst_vel