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Warm-up: Which is harder to stop– A Mack truck or a roller skate going down a hill at the same velocity? Explain your choice. Impulse and Momentum Day 1. WHAT DO THESE ARTICLES TITLES MEAN?.
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Warm-up: Which is harder to stop– A Mack truck or a roller skate going down a hill at the same velocity? Explain your choice.
We’ve seen that momentum involves some sort of inertia (mass) and some sort of movement (velocity). What do you think the equation for momentum could be?? THINK-PAIR-SHARE (30 Seconds)
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?
Back to baseball...Which will have the greater speed afterward? • A large or a small ball? • A ball hit off of a fast or a slow pitch? • 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. • Is momentum a vector or a scalar? • What are the SI units of momentum? kilogram-meter per second (kg-m/s) p = m · v
Turn And Talk CONCEPTUAL Questions (3 min to discuss) • Is the momentum of a car traveling south different from that of the same car moving north at the same speed? • Will a truck always have a greater momentum than a roller skater? • Can you think of a case where the roller skater and the truck would have the same momentum?
Turn and Talk Question #1 • Is the momentum of a car traveling south different from that of the same car moving north at the same speed? YES! Momentum is a vector quantity, and the momenta of the two cars are in opposite directions!!!
Turn And Talk: Question #2 • Will a truck always have a greater momentum than a roller skate? • No! A large truck @ rest has NO MOMENTUM!! • A moving roller skate does!
Turn and Talk Question #3 • Can you think of a case where the roller skates and the truck would have the same momentum? • What Do You Think?
Determine the following momentums… • Ex #1 - A 1200lb car moving at 60mph • Ex #2 - A 5.0g bullet moving at 900m/s • Ex #3 - A 3 ton elephant sitting down • Ex #4 - A 150g hummingbird flying at 40mph • Could the momentum of the hummingbird ever be equal to that of the elephant?
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?
Impulse • Definition: A force applied over a period of time is called an IMPULSE. • What are the SI units of Impulse? I = F · ∆t
The Impulse-Momentum Theorem • When a net force acts upon a body for a period of time, the Impulse applied by the forceis equal to the body’s change in Momentum! • This is similar to the Work-Energy Theorem… F∆t = m∆v
Applying the Impulse-Momentum Theorem #1: Follow Through etc… • If your Force is limited, how can you MAXIMIZE the impulse you apply? • By increasing the time of contact! • Golfing…Baseball…
Applying the Impulse-Momentum Theorem #2: Maximize ∆time • If your change in momentum is constant, how can you MINIMIZE the force applied to you? • By increasing the time of contact! • Bending knees upon impact, rolling with the punches, crumple zones, running shoes, air bags…
Momentum Changes and Bouncing • Is it more dangerous when a flower pot falls on your head and breaks or stays in one piece and bounces back up? • When an object bounces, what happens to it’s velocity? How does that effect it’s change of momentum? • Since ∆p is greater in bouncing situations, the Impulse applied will be greater!
Bouncing Examples: • Rain vs. hail • Lester Pelton • Broken Bat Home runs? • Can you think of any more?
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 • 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?
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 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. • 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
