every object in the universe with mass experiences an attractive gravitational force due to every other object with mass. why don't you feel a pull from objects close to you?

that tool is called an altimeter which measures your altitude which can be used to tell which atmosphere you are currently in, including the stratosphere.

hope this ! : )

t = 1/400 h   ⇔ 0.0025 h ⇔ 9s ⇔ 0.15 m

any of the answers are correct.