Inelastic vs Elastic Pg 233 239 Pg 240 248 Momentum vs Energy All interactions conserve momentum They do not necessarily conserve kinetic energy Obvious example Explosions Kinetic energy before is zero ID: 595834
Download Presentation The PPT/PDF document "Collisions" is the property of its rightful owner. Permission is granted to download and print the materials on this web site for personal, non-commercial use only, and to display it on your personal computer provided you do not modify the materials and that you retain all copyright notices contained in the materials. By downloading content from our website, you accept the terms of this agreement.
Slide1
Collisions
Inelastic vs. ElasticPg. 233 – 239Pg. 240 - 248Slide2
Momentum vs. Energy
All interactions conserve momentum.They do not necessarily conserve kinetic energy.
Obvious example: Explosions
Kinetic energy before is zero.
Kinetic energy after is non-zero.Slide3
Momentum vs. Energy
All interactions conserve momentum.They do not necessarily conserve kinetic energy.
Head-on collisions also do not conserve energy: the kinetic energy is transformed into work done on the car.Slide4
Analyzing Collisions
This feature divides all collisions into two classes:Collisions in which kinetic energy is conserved = elasticCollisions in which kinetic energy is not conserved = inelastic
Elastic InelasticSlide5
Analysing Collisions
Elastic collisionCollision in which momentum and kinetic energy are both conservedInelastic collision Collision in which momentum is conserved but not kinetic energy** you can determine whether a collision is elastic or inelastic by calculating both the kinetic energy before and after the collision. Since momentum is always conserved, the total kinetic energy before and after a collision are the same, the collision is elastic. If not, the collision is inelastic
noteSlide6
Inelastic Collisions
A collision in which kinetic energy is lost is called an inelastic collision.A collision in which the maximum possible energy is lost is called a perfectly inelastic collision.Slide7
Completely Inelastic Collisions
The maximum possible energy loss (if no work is done on the objects) occurs when the two objects stick together after colliding so that they have the same final velocity.Slide8
Elastic or Inelastic?
1. A 0.50 kg object (A) is moving at 5.0 m/s [E] when it collides, head-one, with a stationary 1.0 kg object (B). If the 0.50 kg rebounds directly backward at 1.2 m/s, was the collision elastic? (hint: you will need to first find the velocity of the 1.0 kg object after the collision )Slide9
Practice
2. Car A, with a mass of 1800 kg, was travelling north at 46 km/h and car B, with a mass of 2500 km, was travelling east at 38 hm/h when they collided.A) Would the cars be located more to the North or EastB) Was the collision elastic or inelastic?Slide10
Perfectly Elastic & Inelastic Collisions
Most real collisions fall somewhere between elastic and inelasticHowever, it is useful to consider perfectly elastic and perfectly inelastic collisions as ideal examples of Newton’s LawsSlide11
Perfectly Inelastic Collisions
Pg. 236 - 239Slide12
Perfectly Inelastic Collisions
We concluded that when objects collide, become deformed, and stick together, the collision is inelasticSlide13
Perfectly Inelastic Collisions
noteSlide14
Practice
1. The two objects shown collide head-on and stick together in a perfectly inelastic collision. What is their combined velocity after the collision?2. A CSI expert needed to find the velocity of a bullet fired from a gun. He fired a 5.50 g bullet into a ballistic pendulum with a bob that had a mass of 1.75 kg. The pendulum swung to a height of 12.5 cm as shown. What was the velocity of the bullet just before it hit and become embedded in the pendulum bob? (hint: start with conservation of energy and then use conservation of momentum)
V1+2 = -9.8 m/s
Vb
= 500 m/sSlide15
3. A block of wood with a mass of 0.50 kg slides across the floor toward a 3.50 kg block of wood. Just before the collision, the small block is travelling at 3.15 m/s. Because some nails are sticking out of the blocks, the blocks stick together when they collide. Scratch marks on the
flloor indicated that they slide for 2.63 m cm before coming to a stop. What was the magnitude of the force?Slide16
Textbook
pg. 239, #2, 6, 7Slide17
Perfectly Elastic Collisions
Pg. 240 - 248Slide18
Perfectly Elastic Collisions
As suspected, when hard objects such as billiard balls collide, bounce off each other, and return to their original shape, they have undergone elastic collisionsVery few collisions are perfectly elastic, but in many cases, the loss of kinetic energy is so small that it can be neglectedBecause both kinetic energy and momentum are conserved, an analysis of this type of collision yield two very useful equationsSlide19
Perfectly Elastic Collisions
noteSlide20
PracticeSlide21
Consider the two carts shown below. What do you suppose happens to the total mechanical energy during an elastic collision?
When the two gliders collide, the spring bumper compresses initially and then springs back to its original shape. This occurs because some of the kinetic energy of the moving gliders is converted into elastic
potential energy
in the spring bumper during the compression which is
then converted
back into kinetic energy during the rebound.Slide22
Elastic Collisions and Mechanical EnergySlide23
Practice
3.
3.Slide24
Textbook
Pg. 248, #3, 5