Collisions Unit A Momentum

Objectives You will be able to state and apply the Law of Conservation of Momentum to linear collisions. You will be able to define an isolated system, and be able to determine if a system is or is not isolated.

Collisions There are two types of collisions we study: Elastic Collisions – objects collide and bounce back without deforming, there is no energy lost (pool balls) Energy and momentum are conserved. http://www.youtube.com/watch?v=pZqkaJDaz2A Inelastic Collisions – objects collide but deform in some way, losing energy (volleyball) Only momentum is conserved http://videosift.com/video/Golf-ball-deformation-at-70-000-fps

Collisions and systems A collision is an interaction between two objects where a force acts on each object for a period of time. A group of two or more objects that interact is known as a system. A system where the system’s mass is constant, and no external net force acts on the system is an isolated system.

Isolated System Matter, energy and information do not enter or leave the system - the system’s total energy and mass stay the same There is no such thing as an isolated system in real life, it is completely theoretical (it is the model we use in Physics 30)

Mometum is conserved in an isolated system The sum of the momentum before an event is the same as the sum of the momentum after

Step 1: Determine if the system is isolated, if it is you can use the conservation of momentum Step 2: Write the conservation statement. Step 3: Note the initial momentum is zero because there is no movement Step 4: Sub in the values for the momentum and watch your negatives.

Law of Conservation of Momentum. 1) Use when objects do not stick together. 2) Use when objects stick together. 3) Use for explosions (momentum total is zero before). Note: These formulas are not on your formula sheets and you can use the first formula exclusively if you like. *

Using “Conservation of Momentum” Colliding Objects Stick Together Example : A 1.50 g pellet is fired into a 12.3 g wood block. The block and imbedded pellet fly off at 2.78 m/s E. What was the pellet’s velocity before impact? Answer is “+” *

Example : A 980 kg Toyota going at 52 Example : A 980 kg Toyota going at 52.8 km/h N collides with and becomes entangled with a 738 kg Honda going 79.3 km/h S. What is the velocity of the wreckage immediately after impact? Note: It does not matter what units of mass or velocity are used - just be consistent! Answer was “–” *

Example A 50 g bullet is fired into a 650 g block of wood, which sits on a frictionless surface. After impact the bullet and block move right at 30 m/s. Find the bullet’s velocity just before impact.

Solution Before: After

Solution

“Explosion” Problems Example : When a 960 g plate is dropped, the first 370 g piece flies off at 2.63 m/s  S. What is the velocity of the second piece? Note: In order for the total final momentum to be “0”, if one piece flies north, the other must fly south. *

Example: Fred sits in a stationary, frictionless wheelchair and throws a 3.0 kg ball outward at 8.0 m/s. If the mass of Fred and his chair is 75 kg, what will be the velocity of Fred and the chair immediately after the ball is thrown?

Solution Before: After Because both velocities are zero.

Solution

http://www. coolschool. ca/content/junk/showcase/content/physics/crash http://www.coolschool.ca/content/junk/showcase/content/physics/crash.swf

Example A 0.25-kg volleyball is flying west at 2.0 m/s when it strikes a stationary 0.58-kg basketball dead centre. The volleyball rebounds east at 0.79 m/s. What will be the velocity of the basketball immediately after impact? (Textbook p. 478 #1) Is the system isolated? Write the conservation statement. Plug and chug. (watch your negatives)

Example 3 A 125-kg bighorn ram butts heads with a younger 122-kg ram during mating season. The older ram is rushing north at 8.50 m/s immediately before collision, and bounces back at 0.11 m/s [S]. If the younger ram moves at 0.22 m/s [N] immediately after collision, what was its velocity just before impact? (Textbook p. 479 #2)

Assignment: Workbook p. 51 #2 – 5, 8 Reading from the textbook: p. 468-485

Slo Mo for fun http://www.youtube.com/watch?NR=1&feature=endscreen&v=rU7iYYpSrlo