Impulse and Momentum Have you ever wondered Why

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… • • • 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?

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”

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?

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

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?

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

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?

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!

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

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?

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. V 01 = 15 m/s V 02 = 0 m/s • • How far will the 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?

Example 2 - Elastic Collisions • Two identical blocks of mass 1. 0 kg collide elastically, as shown below. Find their speeds right after the collision. V 01 = 15 m/s V 02 = 12 m/s • • How far will the 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?

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).

The End. . .