Conservation of Momentum Its Not Just a Good

Conservation of Momentum It’s Not Just a Good Idea, It’s the Law

Law of Conservation of Momentum • Momentum is conserved for a system if there are no outside forces. – Conserved means stays the same – System a group of objects that interact – Outside force is a force that does not come from the objects in the system interacting • True for elastic collisions, inelastic collisions and explosions • Not true for falling bodies, simple pendulums and objects sliding or rolling down inclined plane

Cases where Momentum is Not Conserved • Falling body – Outside force of Earth’s gravity – Conserved if include Earth • The Simple Pendulum – Outside forces of gravity and tension on bob • Sliding or rolling down inclined plane – Outside forces of gravity and friction

Collisions Elastic Collisions Inelastic Collisions • Collisions involving bouncing • Momentum is conserved • Objects have different velocities after collision • Equation will not have parenthesis • Collisions involving sticking • Momentum is conserved • Objects have same velocity after collision • Equation will have parentheses on momentum after side

Explosions • Momentum before = momentum after = 0 • There is motion of objects in system. Momenta are equal and in opposite directions for different parts of system • Examples – Gun and Bullet – Rocket and Fuel

Check Question • You find yourself in the middle of a frozen lake with perfectly frictionless ice. This means that you cannot push off horizontally on the ice. How can you propel yourself to the side of the lake?

Momentum in 2 D and 3 D • Momentum is conserved in all directions and as a total amount in 2 and 3 D collisions and explosions • Pbefore = Pafter • Pxbefore = Pxafter • Pybefore = Pyafter • Pzbefore = Pzafter

Solving Momentum Problems 1. 2. 3. 4. Draw before and after picture. Write conservation of momentum equation Solve equation for unknown Substitute