Notes: Chapter 11.3 Newton’s Third Law of Motion and Momentum

Newton’s Third Law of Motion

• For every action force there is an equal and opposite reaction force.

• Called force pairs or action/reaction pairs

• Force pairs do not act on the same object, and thus don’t always have the same effect!

• Movie: Newton's third

Newton’s Third Law of Motion

• The equal forces don’t always have the same effect, though.

• According to the second law, F=ma. You will have different results depending on their masses.

Momentum• An extension of Newton’s Third

Law…tells what will happen from the action/reaction

• Is equal to mass x velocity• p = mv• units have no name, just kg . m/s

Momentum & Collisions

• In collisions, momentum is conserved for the system (both balls), that is, it is the same for all the objects before and after the collision.

• Called The Law of Conservation of Momentum

Law of Conservation of Momentum

• The system is both of the pool

balls. If they have the same mass:– the total momentum

before is mv + m0 = mv

– the total momentum after is m0 +

mv = mv

Law of Conservation of Momentum

• What if the red ball has twice the mass of the

white one? How fast will it move?

• What if the white ball has twice the mass of the red one? How fast will it move?

Momentum & Collisions

• Elastic collision: objects hit each other and don’t stick together (bounce off)

• Inelastic collision: objects hit each other and stick together

• Momentum and Collisions• Conservation of Momentum