FORCES. Integrated Science Unit 8. I. Newton s Laws of Motion

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1 Integrated Science Unit 8 FORCES I. Newton s Laws of Motion A. Newton s First Law Sir Isaac Newton Lincolnshire, England 1. An object at rest remains at rest, and an object in motion maintains its velocity unless it experiences a net force 2. objects experience inertia *a. tendency of an object to resist a change in motion 1) their motion changes only when a net force is applied b. affected by the object s mass 1) more mass = more inertia & vice-versa B. Newton s Second Law 1. The unbalanced force acting on an object equals the object s mass times its acceleration 2. Formula: net force = mass x acceleration (F = ma) a. SI unit = newtons (N) b. 1 N = 1 kg x 1 m/s2 3. when forces are equal, objects with larger masses accelerate less 4. when more force is applied to an object, it accelerates at a higher rate

2 5. The 2nd Law can be looked at in terms of acceleration a. the acceleration of an object is in direct proportion to the net force on the object, and is inversely proportional to the object s mass b. formula: acceleration = net force / mass (a = F/m) II. Gravity, Weight, and Mass * A. Weight 1. a measure of the gravitational force exerted on an object 2. formula: weight = mass x free-fall acceleration (w = mg) a. SI unit = newton b. free-fall acceleration near a massive object = constant acceleration all masses near it experience c. near Earth = 9.8 m/s2 B. Mass *1. the amount of matter in an object 2. SI unit = kilograms 3. remains constant despite position in universe 4. relates to weight because gravity pulls on an object s mass

3 C. Law of Universal Gravitation 1. allobjects in the universe attract each other through the force of gravity a. all matter is affected by gravity 1) how much is due to size of objects & their distance from each other 2. formula: F = G(m1m2/d2) a. gravitational force increases as one or both masses increases b. gravitational force decreases as the distance between two masses increases c. G is a constant for gravitational pull D. Free Fall 1. when there is no air resistance, all objects (regardless of mass) falling near Earth s surface experience the same free-fall acceleration * a. the motion of a body when only the force of gravity is acting on it 2. the law of univ. gravitation causes free-fall acceleration to be constant a. gravity is constant b. near Earth, distance is about equal to the radius of the Earth c. mass 1 (Earth) is constant d. the only changing variable is mass 2 the object s mass 3. Air resistance acts on falling objects along with gravity a. falling objects stop accelerating at terminal velocity *1) the constant velocity of a falling object when the force of air resistance is equal & opposite to the force of gravity 2) reached when air resistance & weight are equal b. amount of air resistance can be changed by altering the shape of the object

4 E. Projectile Motion curved path an object follows when thrown, *1. the launched or otherwise projected near the surface of Earth 2. two components a. each acts independently but combine to create a curved path b. horizontal velocity force on object making it project forward 1) ex: throwing a ball c. vertical velocity force exerted by gravitational acceleration 1) w/out air resistance, 9.8 m/s2 toward Earth 3. Objects in orbit move by projectile motion a. forward motion & free-fall combine to create the circular path

5 III. Newton s Third Law A. Action & Reaction Forces 1. For every action force, there is an equal and opposite reaction force a. forces always occur in pairs 1) occur at the same time b. forces work on different objects 1) ex: kicking a ball action force acts on ball, reaction force acts on foot c. do not cause equal effects 1) size of objects being acted upon influences the results B. Momentum *1. a quantity of motion of a moving object, measured as a product of its mass & velocity 2. formula: momentum = mass x velocity (p = mv) 3. SI unit = kg x m/s + direction 4. the greater the mass, the greater the momentum 5. force is needed to change an object s momentum a. ex: applying brakes, catching a ball b. as time increases, amount of force decreases

6 6. Law of Conservation of Momentum a. the total amount of momentum in an isolated system remains constant 1) the total momentum of 2+ objects after a collision is the same as it was before the collision 2) some momentum may transfer from 1 object to another a) objects move in the direction of the greater momentum