Why Coop doesn't control a pencil to communicate directly using gravity?

In the movie Interstellar,

Coop uses the watch he gave to his daughter as a communication mean. He can do that since he masters gravity. He can also drop books to send Morse messages, etc. Gravity is the only thing that can swallow time. If he's so skilled in gravity, why can't he use it to control forces applies to, let's say, a pencil, so he can write an understandable message on a sheet of paper?

Controlling gravity means to nullify or increase it: by applying correct forces on both sides of the pen, this should allow writing!

• I don't think the spoiler tags are necessary given the nature of the question. – Valorum Apr 12 '15 at 8:43
• Isn't picking up some thing and using it by holding difficult by sending gravitational wave like force, that Coop used? – VacuuM Apr 12 '15 at 8:47
• When was the last time anyone in the Nolan film did things the easy way?! – 22nd Century Fza Jun 3 '15 at 5:42

Because Coop needs to send a message that is;

• Sufficiently cryptic as to allow him to get to NASA

but

• Non-obvious who's sending the message

and

• Contains some sort of secondary message that Murph (and only Murph) can access years later

Oh yeah...

• and because the whole film is an enormous bootstrap paradox, he can't change the future because he didn't do it in the past.

This is covered in more detail in the film's official novelisation;

“Cooper,” Tars said, “They didn’t bring us here to change the past.” Of course they didn’t. Cooper paused, calming himself. No, he couldn’t change the past. But there was something else… something about what Tars was saying.
They didn’t bring us here to change the past.
They…
“We brought ourselves here,” he said, and he pushed off, found another angle, saw the room in a slightly different moment. It was full of dust from the storm, the storm that had come upon them at the baseball game. Murph had left her window open…
“Tars,” he said, studying the dust. “Feed me the coordinates of NASA in binary.” And with his fingers, he traced the pattern, the lines he had found after the dust storm—

then

“So what are we to do?” Tars asked.
Cooper looked down the time dimension. The books? No, and not the lander. But the watch, on the shelf, as far as he could see…
“The watch,” he realized. “That’s it. She’ll come back for it.”

“How do you know?” Tars asked.
And again he felt the certainty, a pull as strong as a black hole. Stronger—it was like the pull that had brought him here. That would bring Murph back, too. “Because I gave it to her,” he said, excitement building. He scrutinized the watch for a moment. It would have to be simple, binary, or…