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Section 3 Newton’s Third Law of Motion and Momentum Key Concepts • What is Newton’s third law of motion? • What is needed for an object to have a large momentum? • How is momentum conserved? When this bumper car collides with another car, two forces are exerted. Each car in the collision exerts a force on the other. Newton’s Third Law • A force cannot exist alone. Forces always exist in pairs. • According to Newton’s third law of motion, whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object. • Action and Reaction Forces: These two forces are equal in size and opposite in direction. Video 1 Newton’s Third Law • Action-Reaction Forces and Motion • The action force causes the water to move in the direction of the action force. • However, the water also exerts its equal and opposite reaction force on the swimmer. • The reaction force acts on the swimmer and pushes her forward through the water. Video 2 Newton’s Third Law • Action-Reaction Forces Do Not Cancel • The reason is that the action and reaction forces do not act on the same object. • The action force acts on the water, and the reaction force acts on the swimmer Newton’s 3rd Law of Motion For example, consider the interaction between a baseball bat and a baseball. The baseball forces the bat to the right (an action); the bat forces the ball to the left (the reaction). Momentum • the product of an object’s mass and its velocity. An object with large momentum is hard to stop. • An object has a large momentum if the product of its mass and velocity is large. • The momentum for any object at rest is zero. Momentum • Momentum Formula • Momentum = Mass x Velocity • units of kilogram-meters per second or kg•m/s Momentum • Which has more momentum, a 0.046kilogram golf ball with a speed of 60.0 meters per second, or a 7.0-kilogram bowling ball with a speed of 6.0 meters per second? Conservation of Momentum • collisions obey the law of conservation of momentum. • law of conservation of momentum, if no net force acts on a system, then the total momentum of the system does not change. • In a closed system, the loss of momentum of one object equals the gain in momentum of another object— momentum is conserved. Momentum Momentum Reviewing Concepts • 1. Using Newton’s third law, explain what is meant by action and reaction pairs of forces. • 2. State in your own words the formula for momentum. • 3. What is a necessary condition for the conservation of momentum? • 4. If an eagle and a bumblebee are traveling at 8 km/hr, which has more momentum? Explain.