1. Unit 3: Projectile & 2D Motion

2. Projectiles
Airborne objects that move as a result of their own inertia and gravity.
They exhibit two dimensional motion.
Horizontal and vertical motion
Horizontal and vertical motion are independent of one another.
The resultant motion is a combination of horizontal and vertical motion.

3. 2 Dimensional Motion Projectiles Riverboats & Airplanes
Gravity affects outcome
Riverboats & Airplanes
Gravity does not affect outcome

5. If a cannonball is projected horizontally from the top of a cliff
If a cannonball is projected horizontally from the top of a cliff. In the absence of gravity, the cannonball would continue its horizontal motion at a constant velocity.
Horizontal velocity is constant (ax = 0)
If dropped from rest in the presence of gravity, the cannonball would accelerate downward.
Vertical velocity gains speed (ay = -9.8 m/s2)

6. Monkey In the Absence of Gravity

7. Why Does the Hunter Miss the Monkey?

8. The Hunter Now Aims

9. Strategies for Applying Kinematic Equations in 2 Dimensions
Make a drawing of the situation.
Decide which directions to be called positive and negative. Do this for both the x and y directions.
Remember that the time variable is the same for both x and y.
Make note of all the assumed information.
Starts at rest.
Stops
When motion is divided into two segments, remember the final velocity for one segment becomes the initial velocity for the next segment.
Make notice of the situation…do you multiply or divide the time by 2… or leave it?

10. Anytime the object starts at a given height
Do not multiply time by 2….since starts at max height already

11. Anytime an object starts at an angle from the ground!
Vy at the highest peak is always 0
Remember when use max height and Vy=0….it’s only half the time the object stays in the air
So…to find total distance (dx)…multiply time by 2
Launch
velocity
Launch
angle
Ending
velocity

12. Steps to working a problem
Draw the picture
Label with variables and numbers
Write the givens
Write what you are looking for
Write equation
Plug and chug
Answer with number and unit!

13. EX 1: A football is kicked at an angle of 45º to the ground with a speed of 25 m/s. Calculate …
A. The initial horizontal velocity
B. The initial vertical velocity
C. Maximum height reached

14. EX 2: A launched projectile has a horizontal displacement of 53 meters and total time 7 seconds. Calculate …
… note – think of various ways this question could be formed on a test (cannon fires, football thrown, baseball hit, golf ball hit, soccer ball kicked, etc.)
A. Maximum height of projectile
B. Launch Velocity
C. Launch angle

15. A launched projectile has a horizontal displacement of 53 meters and total time 7 seconds. Calculate …
A. Maximum height of projectile.
Answer …
-Begin with equation
-Use the fact that at top of trajectory Vyi is zero.
-Use second half of trajectory.
-Plugging in values:

16. A launched projectile has a horizontal displacement of 53 meters and total time 7 seconds. Calculate …
B. Launch Velocity.
Answer …
Need X and Y velocities (Vx and Vy).
Vx is found using:
Vy is found using half of trajectory and:
Knowing that Vy is zero at half the time …
So …

17. A launched projectile has a horizontal displacement of 53 meters and total time 7 seconds. Calculate …
C. Launch Angle. θ

18. Ex 3: An object is launched horizontally from a height of 350 meters and lands 900 meters horizontally. Calculate …
Velocity of object.

19. Ex 4 An object is travelling at 37◦ from vertical at velocity 8 m/s
Ex 4 An object is travelling at 37◦ from vertical at velocity 8 m/s. How long will it take to go 38 m horizontally?
NOT A PROJECTILE!!!!
Need X component velocity.
Note that (similar to projectiles) time is shared between two motions. Each component is independent of the other.
θ=37

20. Review Questions
In the absence of gravitational force and friction, describe the motion of an object.
How do you describe horizontal motion?
Is vertical motion influenced by gravitational force?
How far is an object being thrown horizontally given an initial speed of 20 m/s from a 45 m tower?