Physics 218 Lecture 6

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Physics 218 Lecture 6 Dr. David Toback Chapter 4, continued Kinematics in Two Dimensions Today  Problems Projectile Motion The physics of the universe: The horizontal and vertical Equations of Motion behave independently This is why we use vectors in the first place Projectile Motion

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### Physics 218Lecture 6

Dr. David Toback

Physics 218, Lecture VI

Chapter 4, continued

Kinematics in Two Dimensions

TodayProblems

Physics 218, Lecture VI

Projectile Motion

The physics of the universe:

The horizontal and vertical Equations of Motion behave independently

This is why we use vectors in the first place

Physics 218, Lecture VI

Projectile Motion
• This is the heart and culmination of what we have learned so far
• You can rest assured there will be a problem like one in lecture today on the exam

Physics 218, Lecture VI

How to use the Tricks and Methods
• Next we’ll do an example problem like one of the homework problems in the text book
• Solve this problem using the right method
• Draw a diagram
• Convert the numbers to variables
• Solve to get a formula
• Plug in the numbers at the end
• Check
• Reasonable numbers?
• Silly numbers?
• Another way to do the same problem?

Physics 218, Lecture VI

How to Solve Problems

The trick for all these problems is to

break them up into the X and Y directions

Physics 218, Lecture VI

Firing up in the air at an angle

A ball is fired up in the air with speed Vo and angle Qo. Ignore air friction. The acceleration due to gravity is g pointing down.

What is the final velocity here?

Physics 218, Lecture VI

Important Definition ;-)

Word: Defenestration

Definition: The act of throwing someone or something out of a window

Physics 218, Lecture VI

Push BEVO out of a window

Reveille pushes BEVO directly out of a window that is a height h above the ground. Two Aggies observe that BEVO hits the ground a distance D from the base of the building.

Assuming BEVO was moving horizontally at the top (and ignoring air friction), how fast must Reveille have pushed her?

h

y=0

D

Physics 218, Lecture VI

Kick a football

A football is kicked at angle Q with respect to the ground with speed V0. Calculate:

• The maximum height
• The velocity at the maximum height
• The time of travel before the football hits the ground
• How far away it hits the ground
• What angle maximizes the distance traveled

Assume the ball leaves the foot at ground level and ignore air resistance

Physics 218, Lecture VI

Winner: Not my job award

Physics 218, Lecture VI

More Realistic Punt

h

A football is kicked at angle Q0 with a speed V0. The ball leaves the punters foot h meters above the ground.

How far does it travel, in the X direction, before it hits the ground?

Physics 218, Lecture VI

Rescue Plane

You are the pilot of a rescue plane and you are flying a plane with a horizontal speed of Vp. Your mission is to drop supplies for some mountain climbers on a ridge a vertical distance h below.

To do this you have to release the supplies a horizontal distance R in advance of the mountain climbers.

What vertical velocity should you give the supplies so that they arrive precisely at the the climbers position?

Physics 218, Lecture VI

Next Week

New Material: Not on the 1st exam

• Chapter 5:Read for Next time
• Newton’s Laws of Motion
• HW2 due Monday
• Cover CH. 3&4 in recitation, due the following Monday
• Lab 3 (posted on the website)

Physics 218, Lecture VI

End of Lecture Notes

Physics 218, Lecture VI

Next time…
• Reading: None, you’ve already finished Chapter 3 & 4 already
• Homework:
• HW1 was due yesterday
• HW2 covered in recitation this week; due this coming Monday
• Start working on HW for Chap 3 & 4
• Next time: More on kinematics in two dimensions

Physics 218, Lecture VI

Ball Dropping

Analyze Vertical and Horizontal separately!!!

• Ay = g (downwards)
• Ax = 0
• Constant for Both cases!!!

Vx = 0

Vx>0

Physics 218, Lecture VI

A weird consequence

Prove that an object projected horizontally will reach the ground at the same time as an object dropped vertically

Physics 218, Lecture VI

Firing up in the air at an angle

A ball is fired up in the air with velocity Vo and angle Qo. Ignore air friction. The acceleration due to gravity is g pointing down.

What is the final velocity here?

Physics 218, Lecture VI

Rescue Plane

You are the pilot of a rescue plane. Your mission is to drop supplies to isolated mountain climbers on a rocky ridge a height h below. If your plane is traveling horizontally with a speed of VO:

• How far in advance of the recipients (horizontal distance) must the goods be dropped?

Physics 218, Lecture VI

Rescue Plane cont.
• Suppose instead, that your plane can’t get to that position. Instead you must release the supplies a horizontal distance D in advance of the mountain climbers. What vertical velocity should you give the supplies so that they arrive precisely at the the climbers position.
• What is the speed of the supplies as they hit the ground?

Physics 218, Lecture VI

Announcements: Exam 1
• We will move the date of Exam 1 from Tuesday Feb 11th.
• Next week you need to do the reading for Chapter 4, Chapter 3-9 and Chapter 5. There will be quizzes on both.
• Note: You will get recitation on Chapter 3 and turn in your Homework before the exam.

Physics 218, Lecture VI

Next Week
• Chapter 4: (NOT ON 1st EXAM)
• Dynamics
• Newton’s Laws of Motion
• Finish HW3. Labs & Rec/Web as usual
• Exam Thursday: Here during regular time
• Bring a calculator and ruler
• Formula sheet will be provided (on the web already)
• Will cover:
• Chapter 1 (1-6)
• Calculus 1
• Chapter 2 (1-7)
• Chapter 3 (1-8, 10)

Physics 218, Lecture VI

Example: 3-8

Derive a formula for the horizontal range, R, of a projectile in terms of its initial velocity (v0 & Q) before the projectile travels before returning to its original height

Physics 218, Lecture VI