A soccer ball flies over a field after a kick. A model rocket climbs over the same field with its engine firing. Both are in the air. What acts on each one?
This lesson sorts flights by the forces acting on the flying object. Air resistance is ignored throughout. The sorting question is always the same: once the object is moving, is gravity the only force acting on it?
For example, take the soccer ball mid-kick, while the boot still touches it. Is gravity the only force acting on it? No: the boot's force acts on the ball too.
But a moment later the same ball is in flight, past the boot. Is gravity the only force acting on it? Yes: nothing touches the ball now, so gravity is the only force left acting on it.
And take a rock just released from a climber's hand, falling. Is gravity the only force acting on it? Yes: the hand has let go, so gravity is the only force acting on the rock.
And take a model rocket whose engine has burned out, still climbing. Is gravity the only force acting on it? Yes: the engine's force has ended, so gravity is the only force acting on the rocket, even while it moves upward.
But rewind that rocket a few seconds, to when its engine still fired. Is gravity the only force acting on it? No: the engine's force acts on the rocket too.
So one question sorts every flight: once the object is moving, is gravity the only force acting on it? A flight passes while nothing else acts on the object: no contact force, no engine's force, no cable's force.
A T-shirt cannon fires a shirt over a crowd. Once the shirt has left the cannon, is gravity the only force acting on it? Air resistance is ignored.
The shirt is in flight, and nothing touches it.
No engine's force and no contact force acts on it.
So gravity is the only force acting on the shirt.
Yes: once the shirt leaves the cannon, gravity is the only force acting on it.
Now sort four fresh flights. The question is the same every time.
A basketball is in the air during a free throw, past the shooter's fingertips. Ignoring air resistance, is gravity the only force acting on it?
A drone flies level over a park with its rotors spinning. Is gravity the only force acting on it?
A pop fly is still rising, just off the bat. Ignoring air resistance, is gravity the only force acting on it?
An empty skateboard leaves the top of a ramp and is still rising. Ignoring air resistance, is gravity the only force acting on it?
Air resistance is ignored. Gravity is the only force acting on one of these objects. Which one?
A tennis ball is still rising just after a lob, past the racket. Ignoring air resistance, which forces act on it?
A bungee jumper has fallen until the cord pulls tight. Gravity is not the only force acting on the jumper. Which other force acts?