Mr. Rogers' AP Physics C: Mechanics (With IB Physics Topics) Objectives

Syllabus 1st Quarter 2nd Quarter 3rd Quarter 4th Quarter

Newton's Laws(4)

Friction(5) Mech Energy(7)  Momentum Semester Exam

Chapter 9: Linear Momentum and Collisions

AP Physics C Newtonian Mechanics:

D. Systems of particles, linear momentum 12% cumulative 70%

  1. Center of mass
  2. Impulse and momentum
  3. Conservation of linear momentum, collisions

 

Students conducting a conservation of momentum lab using air tracks and computer collected data.
Practice Test Study Guide

Objectives

Essential Question: Would a rail-gun have recoil?

Momentum and Collisions

  1. Define momentum 2 ways.

    1. How hard it is to stop an object
    2. P ≡ mv
  2. State Newton's second law in terms of momentum. S F = dP / dt
  3. Name two types of situations where momentum is conserved.
    1. Explosions - things flying apart
    2. Collisions - things flying together
  4. State one of the key reasons momentum can be conserved. - It's a vector!
  5. Solve 1 dimensional "explosion" problems using conservation of momentum.
  6. Describe the difference between elastic and inelastic collisions.
Collision Type Conservation  Momentum Conservation Energy Conservation Kinetic En. Particles Stick
Elastic Yes Yes Yes No
Inelastic Yes Yes No Yes
  1. Solve 1 dimensional inelastic collision problems using conservation of momentum.

Homefun: Read 9.1 & 9.3, Questions 1-10 p. 281; Problems 1, 3, 5, 19.

Homefun: Questions 1 - 11 p.44-45; prob 1, 3, 5, 7, 9

Read: Insultingly Stupid Movie Physics
Chapter 7, Movie Momentum: The Attractive Force of Glass, Rail-Gun Recoil, and Cosmic Toyotas, pp 99 - 115

Metacognition Problem Solving Question 9.1: Do objects collide or fly apart in the problem? If they do then the problem can often be solved using conservation of momentum. Solutions for final velocity of an object in one dimensional momentum problems usually end up with a mass ratio time the starting velocity.

 

Activities

Lesson 1

Key Concept: Conservation of momentum

Purpose: Understand how to use conservation of momentum in problem solving.

Interactive Discussion: Objective 1 - 4. Note that momentum is one of the great conservation laws of physics.

Demo 9.1:  Objective 3b. Explain that the spheres-on-a-rod will exhibit conservation of momentum when dropped. On the way down the total mass is the combined mass of the spheres. On the way up, only the small sphere will rise. Guess what happens to its velocity?

Video Clip: Show a video clip of the rail gun scene in Eraser.

In Class Problem Solving: Objective

  1. Arnie fires the rail gun.
  2. Bubba pushes Nancy.

Demo 9.2:  Objective 6. Demonstrate the difference between elastic and inelastic collisions using happy and sad spheres.

In Class Problem Solving: Objective

  1. A moving train car collides with a stationary car.
  2. Two meteors with different velocities collide and fuse.

Resources/Materials: spheres-on-a-rod, happy and sad spheres

 

Essential Question: Why is it essential that collisions between molecules of air be elastic ?
Elastic Collisions
Relevance: Elastic collisions are useful for analyzing numerous problems from gas dynamics (collisions between molecules) to billiards.
  1. Describe in qualitative terms what happens in one dimensional elastic collisions. 

  2. Solve 1 dimensional elastic collision problems.

  3. Solve problems inelastic collisions in 2 dimensions.

  4. Solve problems involving elastic collisions in 2 dimensions.

When a mass collides with a stationary mass of the same size in a glancing elastic collision, the velocity vectors form a 90 degree angle afterwards.(p.270)

 

Metacognition Problem Solving Question 9.2: Can I break the vectors into components? The most complex vector problem can always be broken up into simpler 1 dimensional problems. Hence, The sum of  momentums in each dimension after a collision has to be the same as the sum of momentums in the same dimension before the collision. - Think components!

Homefun: prob.15, 19, 21, 33, 37

 

 

Lesson 2

Key Concept: Conservation of Momentum With Elastic Collisions and in Two Dimensions.

Purpose: Expand the understanding of how conservation of momentum can be used for problem solving.

Demo 9.3:  Objective 9.  Demonstrate elastic collisions on an air track with combinations of various sized cars.

In Class Problem Solving: Objectives 9

  1. Derive an expression for the final velocities after the elastic collision of equal mass objects in one dimension.

Interactive Discussion: Objective 10 - 11.

In Class Problem Solving: Objectives 10 -11

  1. When Bob meets Jane at an intersection
  2. Bouncing billiard balls go boink.

Resources/Materials: Air track.

Essential Question: Why is the conservation of kinetic energy for elastic collisions different from conservation of energy?

Impulse Section 9.2

  1. Solve combined conservation of energy and momentum problems for inelastic collisions.

  2. Solve combined conservation of energy and momentum problems for collisions involving springs.

  3. Define impulse mathematically 2 different ways and explain in general terms what it indicates.

  • (imp) = F(t) ∙ dt
  • (imp) = ΔP  
  • Roughly speaking: an indication of how much effect a force will have on changing an object's momentum.

Note: impulse is a vector.

 

Metacognition Problem Solving Principle 9.3: Remember that impulse obeys Newton's third law! In other words, during a collision, the change in momentum caused by the impulse one object exerts on a second is exactly the same as the change in momentum caused by the impulse the second object exerts on the first.

Homefun: Read 9.2, prob. 6, 27, 57, 61 

 

Lesson 3

Key Concept: Impulse

Purpose: Understand what happens during a collision.

Interactive Discussion: Objective 12. How much force can I take?

In Class Problem Solving: Objectives 12

  1. Ballistic pendulum.

  2. Bob and Jane play chicken with bumper cars (collisions with springs).

  3. Bozo burns the rope on the bozomobile (releasing springs).

Interactive Discussion: Objective 13 . How much force can I take?

Demo 9.4:  Objective 12, 13.  Demonstrate the effects of elastic and inelastic collisions on a block of wood using happy and sad sphere pendulums. Which has the higher impulse?

In Class Problem Solving: Objectives 9

Resources/Materials: Happy and sad sphere pendulums.

Essential Question: Why is center of mass important to martial artists, dancers, and acrobats?

Center of Mass Section 9.6

  1. Define center of mass

Discrete objects
Xcm

=

∑ Xi mi
∑ mi
Ycm

=

∑ Yi mi
∑ mi
General case Xcm = 1/m X dm Ycm = 1/m Y∙ dm
English

the average spatial location of an object's mass

  1. Find the center of mass of various objects.

  2. State whether an object will rotate, translate, or both by looking at the location of forces relative to the center of mass.

  3. Explain how center of mass relates to the  stability of an object.

  4. State what happens to the center of mass of an object if it exploded in outer space. 

Metacognition Problem Solving Principle 9.4: After calculating the center of mass coordinates for an object, does it look like the object could be balanced by placing a thin support under the center of mass? Center of mass is like a balance point. By thinking of it in this manner, it's often possible to judge if a center of mass calculation makes sense by asking if the object would balance if suspended from

 

Homefun: Read 9.4 thru 9.6, prob. 39, 41 

 

Lesson 4

Key Concept: Center of mass

Purpose:  Calculate the center of mass and use it as a problem solving concept in momentum problems.

Interactive Discussion: Objective 15. All of an object's mass can be considered to exist at it's center. How can the center of mass be found experimentally?

Demo 9.4:  Objective 16. Throwing a meter stick. Throw a meter stick so that it rotates and so that it translates. What is the difference in how the force is applied?

Demo 9.5:  Objective 16. Attempting to stand on one leg. Stand with the feet spread apart shoulder width. Attempt to raise one foot and stand on only one foot without changing the position of your center of mass. (It can't be done)

Demo 9.5:  Objective 16. Throwing an opponent who attempts to punch you (Aikido). Have a subject stand on one foot with the back leg extended backwards and extend his fist as far forward possible. This simulates the act of rushing forward while throwing punch. Have a second person very lightly pull forward on the wrist of the first person's extended arm. (This will very easily unbalance the  first person. If the first person were actually rushing forward while throwing a powerful punch, he would be violently thrown to the ground with both translational and rotational motion)

In Class Problem Solving: Objectives

Essential Question: Why was the Apollo Rocket so huge?

Rockets and Explosions Section 9.6

Relevance: Want to learn rocket science? Here it is. Conservation of momentum is the basis principle that makes rockets go.

  1. State what happens to the center of mass of an object when the parts of an object are pulled together by an internal force (assuming no external resistance forces). 

  2. State what happens to the center of mass of a rocket (including its fuel) when it is accelerating in outer space.

  1. Calculate the thrust produced by a rocket.

    FT = vm dm/dt

     

  2. State two reasons why a rocket must have a very large supply of fuel in order to move from the surface of Earth into outer space.

 

Metacognition Problem Solving Principle 9.4: Remember that the location of an object's center of mass of a system is only affected by external forces. Any form of explosion in an unrestrained object will generate internal forces.

 

Homefun: Read section 9.7, prob. 43, 51 

Lesson 5

Key Concept: Propulsion using momentum and center of mass 

Purpose:  Understand how rockets work.

Interactive Discussion: Objective 15. 

Demo 9.4:  Robot which propels itself by changing the location of its center of mass.

In Class Problem Solving:

Tsiolkovsky's rocket equation, or ideal rocket equation is named after Konstantin Tsiolkovsky who independently derived it and published in his 1903 work,[

\Delta v\ = v_e \ln \frac {m_0} {m_1}

where:

m0 is the initial total mass, including propellant, in kg (or lb)
m1 is the final total mass in kg (or lb)
ve is the effective exhaust velocity in m/s or (ft/s) 
\Delta v\ is the delta-v in m/s (or ft/s)

Some typical values of the exhaust gas velocity Ve for rocket engines burning various propellants are:

Formal Lab Investigation

Title Investigation of a Collision on an Air Track
Category Linear momentum
Purpose Determine if conservation of momentum can be observed in the collision of 2 sliders on an air track
Models Pbefore Pafter conservation of linear momentum
Overview Momentum is conserved in a collision as long as there is not a friction force acting on the objects in a way that significantly reduces the velocities. Since an air track simulates a friction free environment, it should be an ideal test platform for conservation of momentum investigations.

Level the air track so that the sliders remain stationary unless they are pushed.

Place 2 sliders of equal mass on the track. Just before the collision, the one in back will be sliding toward the stationary one in front.

Set up a pair of photogates so that the first measures the moving slider's velocity just before the collision and the second measures the velocity of the 2nd cart just after the collision.

Gently push the slider in back, measure the results of the collision, and determine if conservation of momentum is a good model for the system.

Safety Issues Air track motors can overheat if the air inlet is blocked.
Equipment Limitations Air tracks an their sliders are much more delicate than they look. Do NOT drop or strike them
Resources/Materials: Air track and slider, photogates, computer system set up with Vernier LabPro software and Lab Pro units
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Insultingly Stupid Movie Physics is one of the most humorous, entertaining, and readable physics books available, yet is filled with all kinds of useful content and clear explanations for high school, 1st semester college physics students, and film buffs.

It explains all 3 of Newton's laws, the 1st and 2nd laws of thermodynamics, momentum, energy, gravity, circular motion and a host of other topics all through the lens of Hollywood movies using Star Trek and numerous other films.

If you want to learn how to think physics and have a lot of fun in the process, this is the book for you!

 

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