Which of the lines shows the trajectory of a projectile?
In our diagram, we have three trajectories marked out: one indicated by the purple dashed line, one by the blue dotted line, and one by the yellow solid line. When an object travels through space under certain conditions, it’s known as a projectile. Creating a projectile is not difficult. If we have, say, a ball on our hand and we throw it through the air, we have a projectile. A projectile moves the way it does because of the forces acting on it. It experiences no horizontal force but only a constant vertically downward one. Thanks to this, a projectile follows a path with a certain shape. That shape is called a parabola.
In picking which of our three trajectories show the motion of a projectile then, we want to identify which of these paths is parabolic. A parabola is a smooth curve with no sharp corners. Notice that both the blue dotted line and the yellow solid line do have a sharp corner at their peak. This does not represent how projectiles, like a ball thrown in the air, will move. We can eliminate these two trajectories from consideration. That leaves us with the purple dashed line. This is the only one of our three lines that shows the trajectory of a projectile.