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Chapter 12: Forces & Motion 12.1 (pg’s 356-362) Forces Force: can cause a resting object to move, or it can accelerate a moving object by changing the object’s speed or direction. Force is measured in newtons (N). 1 N is the force that cause a 1-kg mass to accelerate at a rate of 1 meter per second each second (1N = 1kg x m/s2 ) A Newton (N) a unit of force, explains how force, mass, and acceleration are related. Representing Force: an arrow to represent the direction and strength of force. Combining Forces: o Net Force: the overall force acting on an object after all the forces are combined. Forces in the same direction add together Forces in opposite directions subtract from one another. Balanced Forces: when the forces on an object are balanced, the net force is zero and there is no change in the object’s motion. Unbalanced Forces: an unbalanced force acts on an object, the object accelerates. Friction: a force that opposes the motion of objects that touch as they move past each other. o Static Friction: force that acts on objects that are not moving. o Sliding Friction: force that opposes the direction of motion of an object as it slides over a surface. o Rolling Friction: force that act on rolling objects. o Fluid Friction: force that opposes the motion of an object through a fluid. Gravity: is a force that acts between any 2 masses. o Gravity is an attractive force that pulls object together. o Earth’s gravitational force exerts a force of attraction on every other object that is near Earth. o The force of gravity does not require objects to be in contact for it to act on them. o Earth’s gravity act downward toward the center of Earth. o An upward force or supporting force balances the fownward force of gravity. Falling Objects: Gravity causes objects to accelerate downward, where as air resistance acts in the direction opposite to the motion and reduces acceleration. 12.2 (pg’s 363-371) Newton’s First & Second Laws of Motion: 1st Law: the state of motion of an object does not change as long as the net force acting on the object is zero. o Sometimes called the Law of Inertia: the tendency of an object to resist a change in its motion. o An object at rest tends to remain at rest, and an object in motion tends to remain in motion with the same direction and speed. 2nd Law: The acceleration of an object is equal to the force acting on it divided by the object’s mass. Acceleration = Mass = Force Mass Force Acceleration Force = Mass x Acceleration Doubling the mass of an object cuts its acceleration in half. The acceleraion of an object is always in the same direction as the net force. Mass & Weight: o Mass is a measure of the inertia of an object, how much “stuff” something is made up of. o Weight is a measure of the force of gravity acting on an object. Weight = Mass x Acceleration due to gravity 12.3 (pgs. 372-377) Newton’s Third Law of Motion & Momentum Newton’s 3rd Law: whenever on 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) Momentum: is the product of an object’s mass and its velocity. An object has a large momentum if the product of its mass and velocity is large. Momentum = Mass x Velocity Conservation of Momentum: if not net force acts on a system, then the total momentum of the system does not change. Closed system: other objects and forces cannot enter or leave a system. In a close system, the loss of momentum of one object equals the gain in momentum of another object, momentum is conserved. 12.4 (pgs. 378-382) Universal Forces: Electromagnetic Forces: are associated with charged particles Nuclear Forces: the strong nuclear force and the weak nuclear force, act within the nucleus to hold it together Gravitational Force: every object in the universe attracts every other object. o It takes a huge mass such as Earth’s to exert a large gravitational force. o Gravity is the weakest universal force, but it is the most effective over long distances.