Review! What direction is friction occurring in reference to movement? What is the unit of force, a newton, equal to? What causes an object’s motion to change? When can forces be added together? What affects gravity between objects?

Chapter 10 Section 4: Newton’s Third Law Key concepts: What is Newton’s third law of motion? How can you determine the momentum of an object? What is the law of conservation of momentum? Key terms: momentum, law of conservation of momentum

Newton’s third law States that if one object exerts a force on another object, then the second object exerts a force of equal strength in the opposite direction of the first object

Easy way of saying it (for every action there is an equal and opposite reaction) http://www.youtube.com/watch?v=i3az2Yx9c2s

Problem Wait! "If A pushes B, then B pushes A with an equal and opposite force. If these forces are equal and opposite, they cancel, producing a net force of zero. This means that neither object can accelerate, which means that Newton's Laws predict that nothing can ever move.“ Right?

Solution? Newton's Third Law really does say that if A pushes B, then B pushes A with an equal and opposite force. However, these forces DO NOT CANCEL because they influence the motion of different objects. The force that A exerts on B influences B's motion, and the force that B exerts on A influences A's motion. The force on B can cancel with other forces on B - but NOT with forces on A(and vice versa).

Momentum Momentum is a characteristic of a moving object – it is determined by multiplying the mass and velocity Momentum = mass x velocity The unit is kgm/s The more momentum an object has, the harder it is to stop

Law of conservation of momentum Total momentum of any group of objects remains the same, or is conserved, unless outside forces act on the objects. Look at figure 18, page 358. Or remember that Newton’s Cradle thing.