A skydiver has jumped and is falling through the air. Her parachute is still closed.
Nothing touches her but the air. What is her free-body diagram?
Her weight acts on her: one arrow, straight down.
No surface touches her, so there is no normal force. No rope holds her, so there is no tension.
But she moves through air, and air pushes back on anything moving through it. That force is drag.
She is falling, so her motion is downward, and the drag on her points upward: one arrow, up.
Her diagram carries two arrows: weight down, drag up.
An acorn falls from a branch through the air. Which forces are on its free-body diagram?
A hailstone falls toward the ground. Which way does the drag force on it point?
Which object exerts the drag force on the falling hailstone?
The same two arrows fit any object falling through air: a raindrop, an acorn, a dropped glove.
Where there is no air, there is no drag. A wrench falling near the Moon's surface gets one arrow only: its weight.
A tennis ball drops from a balcony and is falling through the air. Build its free-body diagram.
The ball becomes a box.
Its weight: one arrow, straight down.
The ball moves down through the air, and the air pushes back on it: drag, one arrow, up.
Nothing else touches the ball.
Two arrows: weight down, drag up.
Clara tosses her keys straight up, and they are still rising. Build the keys' free-body diagram.
The keys become a box.
Their weight: one arrow, straight down.
The keys move UP through the air, and the air pushes back against that motion: drag, one arrow, down.
Nothing else touches the keys.
Two arrows, both pointing down: the weight and the drag.
A water balloon is dropped from a balcony and is falling through the air. Which set of arrows is its correct free-body diagram?
A volleyball has just been hit straight up and is still rising. Which set of arrows is its correct free-body diagram?
An astronaut on the Moon drops a hammer. The Moon has no air. Which diagram is correct while the hammer falls?