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Welcome Back. Today’s Schedule: Class structure Concept of momentum Quiz tomorrow. Class Structure. The purpose of this course is to familiarize you with the basic principles of physics, preparing you for AP Physics or college level physics. Schedule (subject to change) ‏ Website

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Welcome Back

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  1. Welcome Back Today’s Schedule: • Class structure • Concept of momentum • Quiz tomorrow

  2. Class Structure • The purpose of this course is to familiarize you with the basic principles of physics, preparing you for AP Physics or college level physics. • Schedule (subject to change)‏ • Website • Academic Integrity • Rigor • Six Idea system

  3. A car moving at a constant speed travels past a valley in the road, as shown below. Which of the arrows shown most closely approximates the direction of the car’s acceleration at the instant that it is at the position shown? (Hint: draw a motion diagram.)‏

  4. This Week: Momentum Today Jan. 3: • What is momentum? • p = mv Jan. 3/4: • How does momentum change? (impulse)‏ • I = mΔv = fΔt Wednesday, Jan. 4: • Quiz! • Conservation of momentum • pinitial=pfinal • Thursday, Jan. 5: • Elastic and inelastic collisions • Friday, Jan. 6: • Quiz! • Momentum conservation in 2D • Monday, Jan. 9: • Problem Solving • Integrating our knowledge

  5. Today: What is Momentum?

  6. Momentum in the Vernacular • In everyday experience, momentum is the amount “unf” an object has So what factors affect the momentum of an object?

  7. What affects Momentum? Which has more “unf”? • A biker going at 20 mph • A car going at 20 mph

  8. What affects Momentum? Which has more “unf”? • A biker going at 20 mph • A car going at 20 mph A car will certainly hurt more, why? Because it is more massive (more mass)‏

  9. What affects Momentum? Which has more “unf”? • A car going at 10 mph • A car going at 2 mph

  10. What affects Momentum? Which has more “unf”? • A car going at 10 mph • A car going at 2 mph The faster car will have more “unf”, why? Because faster things are harder to stop

  11. Momentum Defined Momentum is the product of mass and velocity This is normally written p = m x v Bolded letters denote vectors What are the units of momentum? p = m x v m: kg v: m/s p: kg • m/s :kilogram meters per second

  12. p=mv What is the momentum of a bee that weighs 10 grams and flies at 2 m/s? How does that compare to a tortoise that weighs 1kg and moves at .05m/s?

  13. p=mv What is the momentum of a bee that weighs 10 grams and flies at 2 m/s? 10g=.01kg p=mv=.01x2= .02kgm/s How does that compare to a tortoise that weighs 1kg and moves at .05m/s? p=mv=1x.05= .05kgm/s The tortoise has more momentum.

  14. p=mv Which has more momentum, my car or me?

  15. p=mv Which has more momentum, my car or me? vcar=0 vme= something like 1m/s pcar = mcar * vcar = mcar *0 = 0 pme = mme* vme = mme * something like 1m/s = more than 0

  16. Homework: • Review for quiz tomorrow • This powerpoint is on the website • Visit the physics classroom for more practice on momentum

  17. Today’s Schedule (Jan 3/4)‏ • How does momentum change? (impulse)‏ • I = mΔv = fΔt

  18. How Does Momentum of an Object Change? p=mv Consider Δp=Δmv What does this mean? Why is this not a change in momentum of the object?

  19. How Does Momentum of an Object Change? p=mv Consider Δp=mΔv What does this mean?

  20. How Does Momentum of an Object Change? p=mv Consider Δp=mΔv This means that velocity is changing. Unlike Δm, Δv does not imply that the object is falling apart or clumping together

  21. Introducing Impulse • Δp is know as impulse (I)‏ • Think of impulse as a change from the default path. (Momentum would keep carrying me this direction but I changed it)‏ • I had a sudden impulse to “” (I was suddenly did something that was not planned)‏

  22. Δp = I = mΔv A bowling ball (5kg) moving at 5m/s bowls through a set of bowling pins. Right before the ball falls down the shoot it is going 3m/s What is the impulse the bowling pins provide to a bowling ball?

  23. Δp = I = mΔv A bowling ball (5kg) moving at 5m/s bowls through a set of bowling pins. Right before the ball falls down the shoot it is going 3m/s What is the impulse the bowling pins provide to a bowling ball? I = mΔv I = m*(vfinal – vinitial)‏ I = 5*(3-5)=-10kgm/s

  24. Other Ways to Find Impulse I = mΔv Remember: a = Δv/Δt therefore: Δv = aΔt Substituting in we get: I = maΔt Remember: F=ma Substituting in we get: I=FΔt I = Δp = mΔv = FΔt

  25. I = Δp = mΔv = FΔt In football, a field goal is kicked with the football initially at rest. The football (300g) is kicked at 25m/s.  What was the impulse? 300g=.3kg I = mΔv = m*(Vfinal-Vinitial) = .3*(25-0) = 7.5kgm/s The player's foot was in contact with the ball for .1 seconds.  What is the average force during the time of contact? I = FΔt 7.5 = F * .1 F= 75 N

  26. I = Δp = mΔv = FΔt I want to open a cracked door by throwing a ball at it. If I have two balls of equal mass, one bouncy, and the other clay, which of the two should I throw to achieve my goal? The bouncy one The clay one It doesn’t matter, they are the same Z) I have no clue

  27. Clay ball Bouncy ball

  28. Homework: • Period 3: • Prepare for the quiz tomorrow • Review the physics classroom “The Impulse-Momentum Change Theorem” (link on website)‏

  29. Today’s Schedule (Jan. 4)‏ • Quiz on "What is momentum" • Conservation of momentum • pinitial=pfinal

  30. Quickie Quiz (Recall what we did yesterday)‏ You have 10 minutes You may use a standard calculator No phone or computer calculators If you have no calculator, there are extras at the front.

  31. Two-Minute Problem A loaded truck collides with a car causing a huge damage to the car. Which of the following is true about the collision? A. The force on the truck is greater than the force on the car B. The force on the car is greater than the force on the truck C. The force on the truck is the same in magnitude as the force on the car D. The car and truck accelerate in the same direction E. During the collision the truck has greater acceleration than the car Z. I have no idea

  32. Conservation of Momentum Recall Newton's Third law: Every action has an equal and opposite reaction: F1=-F2 When I push on the desk it pushes back on me with equal force in the opposite direction.

  33. Steel ball demo I = Δp = mΔv = FΔt Compare the force of Ball1 on Ball 2 to Ball 2 on Ball 1 Compare the time of contact for Ball 1 with Ball 2 to the time of contact for Ball 2 with Ball 1

  34. Steel ball demo Compare the force of Ball1 on Ball 2 and Ball 2 on Ball 1 F12 = -F21 Compare the time of contact for Ball 1 with Ball 2 to the time of contact for Ball 2 with Ball 1 Δt12 = Δt21

  35. Derivation of Conservation of Momentum Recall from last class that I = Δp = mΔv = FΔt F1 = -F2 Δt1 = Δt2= Δt I1= F1 Δt I2 = F2 Δt = - F1 Δt I1=-I2 In any interaction, momentum gain of one object is equal to the loss of momentum from another

  36. Quick check An astronaut (80kg) in space kicks off of his space shuttle at 1.5m/s. What is the impulse that he provides to the space shuttle? (Assume that away from the space shuttle is the positive direction)‏ What is the impulse that the space shuttle provides to him? What is the total change in momentum of the space shuttle and the astronaut together? 120 kgm/s 240 kgm/s 0 kgm/s -120 kgm/s Z) I have no idea

  37. Total Momentum of a System is conserved If there are no outside forces acting on an system, the momentum of that system remains constant; it is conserved This is the property momentum conservation. ptotal = Σp = p1 + p2 + p3 + … ptotalinitial = ptotalfinal

  38. Momentum Conservation ptotal = Σp = p1 + p2 + p3 + … ptotailnitial = ptotalfinal A 10 kg object moves at a constant velocity 2 m/s to the right and collides with a 4 kg object moving at a velocity 5 m/s to the left. Which of the following statements is correct? A. The total momentum before and after the collision is 20 kg·m/s B. The total momentum before and after the collision is 40 kg·m/s C. The total momentum before and after the collision is 10 kg·m/s D. The total momentum before and after the collision is 30 kg·m/s E. The total momentum before and after the collision is zero

  39. Momentum Conservation ptotal = Σp = p1 + p2 + p3 + … ptotailnitial = ptotalfinal A freight car A with a mass of 24,000 kg moves at a constant velocity of 8 m/s on a horizontal railroad track and collides with an empty stationary car B with a mass of 24,000 kg. After the collision the car A is moving at 3 m/s in the same direction. What is the velocity of car B after the collision? A. 1 m/s B. 3 m/s C. 5 m/s D. 7 m/s D. 11 m/s

  40. Momentum Conservation ptotal = Σp = p1 + p2 + p3 + … ptotailnitial = ptotalfinal The same situation as before: A freight car A with a mass of 24,000 kg moves at a constant velocity of 8 m/s on a horizontal railroad track and collides with an empty stationary car B with a mass of 24,000 kg. However, after the collision the car A is moving at 3 m/s in the opposite direction. What is the velocity of car B after the collision? A. 1 m/s B. 3 m/s C. 5 m/s D. 7 m/s D. 11 m/s

  41. ptotal = Σp = p1 + p2 + p3 + … ptotailnitial = ptotalfinal Time to get tricky, you may need a calculator: A loaded freight car A with a mass of 24,000 kg moves at a constant velocity of 8 m/s on a horizontal railroad track and collides with an empty stationary car B with a mass of 8,000 kg. After the collision the cars stick to each other and moves like one object. What is the velocity of two cars after the collision? A. 2 m/s B. 4 m/s C. 6 m/s D. 8 m/s D. 12 m/s

  42. Homework: • Periods 2 and 3: • We do not have a quiz tomorrow, however I expect you to be able to do the practice quiz on the website. If you don’t feel comfortable with it take extra care reviewing the physics classroom • Review the Physics Classroom • “Momentum Conservation Principle” • (link on website)‏

  43. Today’s Schedule (Jan. 5)‏ • Summary of Equations • Schedule for the next week and a half • Test on Jan. 13th (Friday next week)‏ • Today’s material: Review momentum equation uses

  44. Momentum in a Nutshell • p = mv • Δp = I = mΔv = FΔt • ptotal = Σp = p1 + p2 + p3 + … • ptotailnitial = ptotalfinal

  45. p=mv What is the momentum of a bee that weighs 10 grams and flies at 2 m/s? 10g=.01kg p=mv=.01x2= .02kgm/s How does that compare to a tortoise that weighs 1kg and moves at .05m/s? p=mv=1x.05= .05kgm/s The tortoise has more momentum.

  46. I = Δp = mΔv = FΔt In football, a field goal is kicked with the football initially at rest. The football (300g) is kicked at 25m/s.  What was the impulse? 300g=.3kg I = mΔv = m*(Vfinal-Vinitial) = .3*(25-0) = 7.5kgm/s The player's foot was in contact with the ball for .1 seconds.  What is the average force during the time of contact? I = FΔt 7.5 = F * .1 F= 75 N

  47. ptotal = Σp = p1 + p2 + p3 + … • An astronaut (80kg) in space kicks off of his space shuttle at 1.5m/s. What is the impulse that he provides to the space shuttle? (Assume that away from the space shuttle is the positive direction)‏ • What is the impulse that the space shuttle provides to him? • What is the total change in momentum of the space shuttle and the astronaut together? • 120 kgm/s • 240 kgm/s • 0 kgm/s • -120 kgm/s • Z) I have no idea

  48. ptotailnitial = ptotalfinal ptotal final = m₁f x v ₁f + m ₂f x v₂f ptotal initial = m₁i x v ₁i + m ₂i x v₂i ptotailnitial = ptotalfinal

  49. No Official Homework We will be working on more difficult problems from here on out If you are not comfortable with the concepts covered today you will get lost and not benefit from our practice problems Review kinematics if the projectile motion was difficult

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