1. Aim: How do we explain the motion of projectiles launched horizontally?

2. Turn and Talk:Why do both hit the ground at the same time?

3. Why do a bowling ball and a bullet fired horizontally hit the ground at the same time?
They both have the same initial vertical velocity which is 0 m/s
They both accelerate downwards at 9.8 m/s2
Thus according the equation d = vit + 1/2at2, both projectiles will take the same time to fall a distance d

4. Vocabulary:What is a projectile?
A projectile is an object that falls under the influence of gravity. Projectiles often move vertically and horizontally.

5. Projectiles Launched Horizontally
When a projectile is launched horizontally, it moves vertically and horizontally.

6. Independence of Vertical and Horizontal Motion
The horizontal motion of the projectile has NO effect on the vertical motion of the projectile.

7. Comparing the horizontal and vertical motion
Type of Motion
(Acceleration or Constant Velocity?)
Direction of Motion
Horizontal Motion (x-direction)
Constant Velocity
Left or Right
Vertical Motion (y-direction)
Acceleration
Down

8. Reading
Projectile Motion and Inertia Many students have difficulty with the concept that the only force acting upon an upward moving projectile is gravity. Their conception of motion prompts them to think that if an object is moving upward, then there must be an upward force. And if an object is moving upward and rightward, there must be both an upward and rightward force. Their belief is that forces cause motion; and if there is an upward motion then there must be an upward force. They reason, "How in the world can an object be moving upward if the only force acting upon it is gravity?" Such students do not believe in Newtonian physics (or at least do not believe strongly in Newtonian physics). Newton's laws suggest that forces are only required to cause an acceleration (not a motion). Recall from the Unit 2 that Newton's laws stood in direct opposition to the common misconception that a force is required to keep an object in motion. This idea is simply not true! A force is not required to keep an object in motion. A force is only required to maintain an acceleration. And in the case of a projectile that is moving upward, there is a downward force and a downward acceleration. That is, the object is moving upward and slowing down.

9. Horizontal and Vertical Motion

10. Example 1: Kicking a soccer ball
A soccer ball is kicked horizontally with a velocity of 25 m/s off the roof of an 18m tall building.
How long does it take for the soccer ball to hit the ground?
How far does the soccer ball land from the base of the building?

12. Thought Question 1
Suppose a rescue airplane drops a relief package while it is moving with a constant horizontal speed at an elevated height. Assuming that air resistance is negligible, where will the relief package land relative to the plane?
below the plane and behind it
Directly below the plane
Below the plane and ahead of it

13. Plane

14. Thought Question 2
Supposing a snowmobile is equipped with a flare launcher that is capable of launching a sphere vertically (relative to the snowmobile). If the snowmobile is in motion and launches the flare and maintains a constant horizontal velocity after the launch, then where will the flare land (neglect air resistance)? in front of the snowmobile b. behind the snowmobile c. in the snowmobile

15. Snowmobile and Flare Launcher

16. Why are these ideas counter-intuitive?
Friction slows down the horizontal motion of projectiles

17. Example 2: Horizontally Launched Projectile
A cannon fires a cannonball horizontally off of a cliff that is 92 m high with an initial velocity of 19 m/s.
How long does it take for the cannonball to hit the ground?
How far does the cannonball land from the base of the cliff?

19. Example 3
3.A pool ball leaves a 0.60-meter high table with an initial horizontal velocity of 2.4 m/s. Predict the time required for the pool ball to fall to the ground and the horizontal distance between the table's edge and the ball's landing location.

21. Example 4
4.A soccer ball is kicked horizontally off a 22.0-meter high hill and lands a distance of 35.0 meters from the edge of the hill. Determine the initial horizontal velocity of the soccer ball.  

23. Tips for Solving Projectile Motion Problems
Write down all known quantities in the x- dimension.
Write down all known quantities in the y-dimension.
Write down all unknown quantities.
Determine which dimension you must apply equations to in order to solve for unknown quantity
Apply appropriate equation to the dimension that you have selected.