Because all though he knows his sister is dead, he can still hear knocking and scratching sounds coming from her coffin. She's dead, but comes back to life to haunt him causing his madness to grow stronger.
Irony is a situation in which the statement someway has an outcome that would not have normally been perceived.
"So foul and fair a day" is ironic because if a day has been negative is most likely isn't also seen to be positive.
300N - 50N = 250N
250N to the right
