Monday
Monday: Why a Throw Makes an Arc
Rocket tosses a foam ball gently across the lawn. It rises, slows at the top, then drops onto the grass.
"I threw it straight," he says. "So why does it bend?"
Raven stretches the tape measure from his feet to the landing spot. "What do you notice about the top of the path?"
"It stopped going up for a second," Rocket says. "Then it fell. But it kept going forward the whole time."
Nova hovers and traces a glowing curve in the air that matches the ball's path exactly.
"Two things happened at once," she says. "One of them you have measured since week 4."
Rocket grins. "Gravity. It was pulling the whole time." Raven writes "arc" in the Field Log and underlines it.
A ball in flight is a projectile
A physics text defines projectile motion as the motion of an object thrown or projected into the air, subject only to gravity.
The object is called a projectile. Its path is called its trajectory.
The most important fact about a projectile is that its forward motion and its up and down motion are independent.
Forward, nothing pushes the ball once it leaves your hand, so it keeps a steady pace. Scientists at NASA call this uniform motion.
Up and down, gravity pulls the whole time. The ball rises more slowly each moment, stops rising at the apex, then falls faster and faster.
| Direction | Force acting (ignoring air) | What the motion does |
|---|---|---|
| Forward | none | stays at a steady pace |
| Up and down | gravity, pulling down | rises, slows, stops at the apex, then falls |
- Read the question.
- Tap your answer.
Why the path curves
Put the two motions together and you get the arc. Steady forward motion plus a pull downward makes a curve.
NASA explains that in ballistic flight the horizontal motion is uniform, because there is no external force in that direction.
In the vertical direction, weight is the only external force acting on the object. Weight is the pull of gravity on the ball.
On Earth a dropped object speeds up by 9.8 metres per second every second. NASA uses this number for every falling ball.
| Statement | True or false? |
|---|---|
| A thrown ball's forward motion and up and down motion are independent. | ? |
| Gravity stops pulling on the ball at the apex. | ? |
| A projectile's path is called its trajectory. | ? |
| Something must keep pushing the ball forward or it would fall straight down. | ? |
Toss again and watch only the height. Notice the pause at the top. Write both in your Field Log.
Please pick a spot away from windows, roads, pets and younger children. Nothing is thrown at a person, and nothing is thrown indoors.
The flight facts come from NASA Glenn Research Center's Beginner's Guide to Aeronautics and a college physics text at libretexts.org.
Good eye. Tomorrow you learn what launch speed and launch angle do to a throw.