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Impulse and Momentum

Impulse and Momentum. Have you ever wondered…. Why golfers and bowlers “follow through”? Why skydivers bend their knees upon impact? Why falling on a wood floor hurts less than falling on a cement one?. Have you ever wondered….

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Impulse and Momentum

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  1. Impulse and Momentum

  2. Have you ever wondered… • Why golfers and bowlers “follow through”? • Why skydivers bend their knees upon impact? • Why falling on a wood floor hurts less than falling on a cement one?

  3. Have you ever wondered… • What’s the best strategy to win the “water balloon toss” or the “egg toss”? • Why we make auto air bags and padded dash boards? • How the police can figure out how fast you were going BEFORE an accident by measuring your skid marks?

  4. To answer all of these questions--and more--you need to understand: Impulse and Momentum! • Recall, Newton’s first law…The Law of Inertia • The study of inertia for moving objects is the study of “momentum”

  5. Consider the following: • When a baseball bat hits a baseball, a force is applied OVER A SHORT PERIOD OF TIME. • How would a graph of Force vs. Time look?

  6. Impulse • Definition: A force applied over a period of time is called an IMPULSE. • The SI units of Impulse: N ∙ s I = F · ∆t

  7. Back to baseball...Which will have the greater speed afterward? • A large or a small ball? • A fast or a slow ball? • A large or a small bat? • A fast or a slow swing?

  8. Momentum • Definition: The MOMENTUM of a body is equal to the product of its mass and its velocity. • Is momentum a vector or a scalar? • SI Units are: kg ∙ m/s p = m· v

  9. Making the Connection: • Look at the units of Impulse and Momentum…Do you notice anything? • Recall, F = m·a…can you rearrange the equation to say something about Impulse or Momentum?

  10. The Law of Conservation of Momentum • Internal forces are forces that act within a system, external forces are forces that act on a system from outside… • If a system is not affected by any net external force, it is said to be “ISOLATED”. • In an isolated system, the total linear momentum is always conserved!

  11. Elastic and Inelastic Collisions • Collisions in which the bodies crash and stick together are called “INELASTIC” Collisions • Collisions in which the bodies crash and bounce are called “ELASTIC” Collisions

  12. Example - Inelastic Collision • In an accident, two cars travelling in opposite directions collide and stick together. The first car has a mass of 1000 kg and a speed of 20 m/s, the second has a mass of 1750 kg and a speed of -15 m/s. What is the speed of the two cars immediately after the crash?

  13. V01 = 15 m/s V02 = 0 m/s Example - Elastic Collisions • In a pool game, two identical balls of mass 0.75 kg collide as shown below. Find their speeds right after the collision. • How far will the one on the right go if µ=0.65? • How would your answer change if the ball on the right had a speed of 12 m/s to the left?

  14. Example 2 - Elastic Collisions • Two identical blocks of mass 1.0 kg collide elastically, as shown below. Find their speeds right after the collision. • How far will the one on the right go if µ=0.65? • How would your answer change if the ball on the right had a speed of 12 m/s to the left? V01 = 15 m/s V02 = 12 m/s

  15. The Ballistic Pendulum • A “ballistic pendulum” is made from a block of wood of known mass. It is used to find the speed of a bullet. If the bullet has a mass of 7.5 g and the pendulum has a mass of 2.5 kg and rises to a height of 65 cm, find the speed of the bullet (Hint: use conservation of momentum and energy).

  16. The End...

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