Impulse & Momentum


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Impulse & Momentum

  1. 1. Some brainteasers to get you back in the physics mood <ul><li>1) If a semi truck and a convertible have a head-on collision, which will experience the greater impact force? </li></ul><ul><ul><li>The semi </li></ul></ul><ul><ul><li>The convertible </li></ul></ul><ul><ul><li>Both the same </li></ul></ul><ul><ul><li>Need more info </li></ul></ul><ul><li>2) Which would cause more damage; driving into a solid concrete wall or having a head-on collision with an identical car traveling at the same speed towards you? </li></ul><ul><ul><li>The wall </li></ul></ul><ul><ul><li>The car </li></ul></ul><ul><ul><li>Both the same </li></ul></ul><ul><ul><li>Need more info </li></ul></ul>
  2. 2. Let’s start with the semi and the car…. <ul><li>Both will have the SAME impact force </li></ul><ul><ul><li>Newton’s 3rd Law! </li></ul></ul><ul><ul><li>For every action there is an equal and opposite reaction </li></ul></ul><ul><li>So what’s different? </li></ul><ul><ul><li>The acceleration (deceleration) </li></ul></ul><ul><ul><ul><li>F=m*a </li></ul></ul></ul><ul><ul><ul><li>So same force, but a smaller mass on the car means the deceleration is more….and hurts more! </li></ul></ul></ul>
  3. 3. And now for car vs. the wall… <ul><li>Both will cause the SAME amount of damage because in both cases the car is coming to a complete stop in the same amount of time </li></ul><ul><ul><li>This has to do with momentum, our next unit </li></ul></ul><ul><li>What if the car you hit was moving slower? </li></ul><ul><ul><li>You’d keep going after the collision so you would have a smaller change in momentum and get hurt less </li></ul></ul><ul><li>What if the car you hit was moving faster? </li></ul><ul><ul><li>You’d immediately change directions (go backwards) causing a much greater change in momentum and much greater damage to yourself </li></ul></ul>
  4. 4. Impulse & Momentum Welcome back!! Wednesday January 7th, 2009
  5. 5. Momentum <ul><ul><li>Where have you heard momentum before? </li></ul></ul><ul><ul><li>What does it mean? </li></ul></ul><ul><ul><li>Once again, physics and real life have slightly different definitions </li></ul></ul><ul><ul><li>Momentum: product of an object’s mass and velocity </li></ul></ul><ul><ul><li>p = m*v THIS IS A VECTOR </li></ul></ul><ul><ul><li>units: kg m / s </li></ul></ul><ul><ul><li>Practice: A 2250 kg pick-up truck has a velocity of 25 m/s to the east. What is the momentum of the truck? </li></ul></ul>
  6. 6. Momentum & Force <ul><ul><li>How can you change the momentum of something? </li></ul></ul><ul><ul><li>Push/Pull it (change in acceleration) </li></ul></ul><ul><ul><li>Slow it down (change in time) </li></ul></ul><ul><ul><li>Where have we seen acceleration and time before?? </li></ul></ul><ul><ul><li>F = m*a we know: a =  v /t </li></ul></ul><ul><ul><li>F= m*  v /t multiply both sides by t </li></ul></ul><ul><ul><li>F*t = m*  v have you seen m*v before? </li></ul></ul><ul><ul><li>BUT m*  v ≠ m*v </li></ul></ul><ul><ul><li>SO F*  t ≠ p F*  t =  p </li></ul></ul><ul><li>What the heck is  p? Impulse!! </li></ul>
  7. 7. Impulse <ul><li>For a constant external force, the product of the force and the time over which it acts on an object </li></ul><ul><li>I= F*t </li></ul><ul><li>Example: A 1400 kg car moving westward with a velocity of 15 m/s collides with a utility pole and is brought to rest in 0.30 seconds. Find the magnitude of the force exerted on the car during the collision. </li></ul>
  8. 8. Impulse in Real Life <ul><li>A magician yanks a tablecloth out from </li></ul><ul><li>under some dishes. Briefly explain why </li></ul><ul><li>the dishes were not given much impulse </li></ul><ul><li>by the tablecloth. </li></ul>
  9. 9. Impulse in Real Life Part 2 <ul><li>Two identical balls (1 kg) are dropped from the same height onto the floor.In each case they have velocity v (3 m/s) downward just before hitting the floor. In case 1 the ball bounces back up, and in case 2 the ball sticks to the floor without bouncing. In which case is the magnitude of the impulse given to the ball by the floor the biggest? </li></ul><ul><li>A. Case 1 </li></ul><ul><li>B. Case 2 </li></ul><ul><li>C. The same </li></ul>Happy Ball I =  p = mv final – m v initial = m( v final - v initial ) = 1 (3 - (-3)) = 6 kg m/s Sad Ball I =  p = mv final – m v initial = m( 0- v initial ) = 1 ( 0- (-3)) = 3 kg m/s V i = -3 m/s V f = +3 m/s
  10. 10. Even more “real life” <ul><li>You drop an egg onto 1) the floor 2) a thick piece of foam rubber. In both cases, the egg does not bounce. </li></ul><ul><li>In which case is the impulse greater? </li></ul><ul><li>A) Floor </li></ul><ul><li>B) Foam </li></ul><ul><li>C) the same </li></ul><ul><li>In which case is the average force greater </li></ul><ul><li>A) Floor </li></ul><ul><li>B) Foam </li></ul><ul><li>C) the same </li></ul>I =  P Same change in momentum  p = F * t F =  p/t Smaller t = large F
  11. 11. Stopping times & distances depend on the Impulse-Momentum Theorem
  12. 12. Momentum is conserved <ul><li>Momentum is “Conserved” meaning it can not be created nor destroyed </li></ul><ul><ul><li>Can be transferred </li></ul></ul><ul><li>Total Momentum does not change with time. </li></ul><ul><li>This is a BIG deal! </li></ul>
  13. 13. Last example… <ul><li>Movies often show someone firing a gun loaded with blanks. In a blank cartridge the lead bullet is removed and the end of the shell casing is crimped shut to prevent the gunpowder from spilling out. When a gun fires a blank, is the recoil greater than, the same as, or less than when the gun fires a standard bullet? </li></ul><ul><li>A. greater than </li></ul><ul><li>B. same as </li></ul><ul><li>C. less than </li></ul><ul><li>If there is no bullet then p bullet = 0 so p gun = 0 </li></ul>p gun = -p bullet