Impulse, Momentum, and Collisions

Impulse, Momentum, and Collisions Sections Covered – Physics Chapter 7: pages 86 – 99 *Chapter 10: pages136 - 146 Topics Covered Linear Momentum Impulse Conservation of Momentum Collisions Elastic Inelastic Total inelastic *Center of Mass

*Review What is inertia? How much resistance an object has against a change in motion. Mass = numerical amount of inertia Large mass = lots of resistance Small mass = not a lot of resistance

Momentum Momentum is a quantity of movement or a measurement of how difficult it is to bring an object to a stop. Two Kinds of Momentum: Linear Momentum- Momentum in a straight line Angular Momentum- Rotational or Circular Momentum

Linear Momentum p = linear momentum m = mass SI Units: v = velocity Linear momentum is a vector that points in the same direction as the velocity. v p

Which Has More Momentum? A truck with a mass of 9000 kg moving 50 m/s? OR A moped with a mass of 180 kg moving 10 m/s?

Impulse For a constant force, the product of the force and time over which the force acts is the Impulse J = Impulse F = force SI Units: Δt = time of contact Impulse is a vector with the same direction as the force applied. F J

Impulse-Momentum Theory The impulse exerted on an object changes the momentum of the object. The larger the impulse, the greater the change in momentum

Conceptual Question When you fall a significant distance and hit the ground, why do they tell you to tuck and roll when you hit the ground rather than just landing right on your feet?

Defining the System – Internal vs. External Forces Define your system (in this chapter – this will normally involve 2 objects) Any forces acting within your define system are internal forces The two skaters above are in a closed system

Defining the System – Internal vs. External Forces Forces that act on an object outside of the defined system are called external forces Examples: Case 1: Meteor = External Force. Outside of the system Case 2

Defining the System – Internal vs. External Forces Forces that act on an object outside of the defined system are called external forces Examples: Case 1: Meteor = External Force. Outside of the system Case 2 Earthquake= External Force. Outside the system – changes how the problem is evaluated

Conservation of Momentum The total momentum of an isolated or closed system remains constant. Isolated/Closed Systems – systems with only internal forces, no external Total Initial momentum = Total Final Momentum Two Object Conservation of Momentum (For Objects 1 and 2):

Conservation of Momentum For two objects, object A and object B: Example: A B mA = 2.3 kg vAo = 1.4 m/s mB = 0.8 kg vBo = 0 m/s vAf = 0.2 m/s vBf = ? m/s

Warm-Up #1 A tennis player places a 55.0 kg automatic ball machine on a virtually frictionless tennis court. Both the machine and tennis balls inside are initially at rest. The machine fires a 0.0570 kg tennis ball with a positive velocity. The machine moves backwards with a velocity of -0.0373 m/s. What is the velocity of the tennis ball? What are the two objects in this system? vf = 36.0 m/s

Warm-Up #2 An ice skater (mass = 65.0 kg) moving to the right with a velocity of 2.50 m/s, holding a snowball (mass = 0.150 kg) which is moving with the skater. She then throws the snowball with a velocity of 32.0 m/s with respect to the ground. What is the velocity of the skater after she throws the snowball? What are the two objects in this system? vf = 2.43 m/s

Warm-Up #3 A boy on a skateboard is initially at rest holding an 8.00 kg jug of water. He tosses the water, giving it a positive velocity of 3.00 m/s. The boy and skateboard move backwards with the magnitude of their velocity being 0.600 m/s. What is the mass of the boy and skateboard together? What are the two objects in this system? m = 40.0 kg

Collisions!! Three different kinds of collisions IMPORTANT: In ALL collisions, momentum is conserved. Elastic Collisions Inelastic Collisions Total Inelastic Collisions

Collisions! Elastic Total Inelastic Inelastic Perfect rebound, no energy loss Some energy loss Objects are stuck together after collision Momentum conserved Momentum conserved Momentum conserved KE conserved KE NOT conserved KE NOT conserved KEo = KEf

Collision Summary YES YES NO YES NO YES Collision Kinetic Energy Conserved Momentum Conserved Elastic Collision Inelastic Collision Total Inelastic Collision YES YES NO YES NO YES