The archer in the opening image is aiming his arrow a little bit above the bull’s eye of the target, rather than directly at it. Why doesn’t he aim at the bull’s eye instead? The answer is projectile motion.
When the archer releases the bowstring, the arrow will be flung forward toward the top of the target where she’s aiming. But another force will also act on the arrow in a different direction. The other force is gravity, and it will pull the arrow down toward Earth. The two forces combined will cause the arrow to move in the curved path shown in the Figure below. This type of motion is called projectile motion. It occurs whenever an object curves down toward the ground because it has both a horizontal force and the downward force of gravity acting on it.
Because of projectile motion, to hit the bull’s eye of a target with an arrow, you actually have to aim for a spot above the bull’s eye. You can see in theFigure below what happens if you aim at the bull’s eye instead of above it.
Another Example of Projectile Motion
You can probably think of other examples of projectile motion. One is shown in the Figure below. The cannon shoots a ball straight ahead, giving it horizontal motion. At the same time, gravity pulls the ball down toward the ground.
Q: How would you show the force of gravity on the cannon ball in the Figure above?
A: You would add a line pointing straight down from the cannon to the ground.
To get a better feel for projectile motion, try these interactive animations:
- http://jersey.uoregon.edu/vlab/ (Click on the applet “Cannon.”)
- Projectile motion is movement of an object in a curved path toward the ground because it has both a horizontal force and the downward force of gravity acting on it.
- Examples of objects that have projectile motion include arrows and cannon balls.
Play the game at the following URL by shooting the cannon at a stationary target. Experiment with three variables—cannon power, height of barrel, and angle of barrel—until you find at least three different combinations of variables that allow the cannon ball to hit the target. Record the values for the three combinations of variables. Then summarize what you learned by doing the activity.
- What is projectile motion? When does it occur?
- How might knowledge of projectile motion help you shoot baskets in basketball?