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Chapter 10 – Forces Chapter 10 – Forces Section 3 – Newton’s 1st and 2nd Laws Standards 2.e – Students know that when the forces on an object are unbalanced, the object will change its velocity (that is, it will speed up, slow down or change direction) 2.f – Students know the greater the mass of an object, the more force is needed to achieve the same rate of change in motion Newton’s 1st Law of Motion Inertia (in ur shuh) – the tendency of an object to resist change in its motion Newton’s First Law of Motion – an object at rest will remain at rest, and an object that is moving at constant velocity will continue moving at constant velocity, unless acted upon by an unbalanced force (called the law of inertia) Newton’s 2nd Law Newton’s 2nd Law – the net force on an object is equal to the product of its acceleration and its mass Force = Mass x Acceleration Acceleration = Force / Mass Mass = Force / Acceleration F m a Units Mass – kilograms (kg) Acceleration – meters per second per second (m/s2) Newton – the force required to accelerate one kilogram of mass at 1 meter per second per second (kg.m/s2 which is a Newton) Force Problem Example A 52 kg water skier is being pulled by a speedboat. The force causes her to accelerate at 2 m/s2. Calculate the net force that causes this acceleration. Force = Mass x Acceleration Force = 52 kg x 2 m/s2 Force = 104 kg.m/s2 Force = 104 N Force Problem Example What is the net force on a 1,000-kg object accelerating at 3 m/s2? Force = Mass x Acceleration Force = 1000 kg x 3 m/s2 Force = 3000 kg.m/s2 Force = 3000 N Force Problem Example What net force is needed to accelerate a 25-kg cart at 14 m/s2? Force = Mass x Acceleration Force = 25 kg x 14 m/s2 Force = 350 kg.m/s2 Force = 350 N Chapter 10 – Forces Section 4 – Newton’s 3rd Law Standard 2.e – Students know that when the forces on an object are unbalanced, the object will change its velocity (that is, it will speed up, slow down or change direction) Newton’s 3rd Law of Motion Newton’s 3rd Law of Motion – 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 on the first object Newton called one force an action, and the other he called a reaction Action-Reaction Pairs Momentum Momentum of an object is the product of its mass and velocity (p = m * v) Momentum is the ‘quantity of motion’ The unit for momentum is kg * m/s Conservation of Momentum When two objects collide in the absence of friction, momentum is not lost Law of Conservation of Momentum states that the total momentum of the objects that interact does not change The total momentum of any group of objects remains the same unless outside forces act on the objects (friction is an example of an outside force) Momentum Problem Example Which has more momentum: a 3.0-kg sledgehammer swung at 1.5 m/s, or a 4.0kg sledgehammer swung at 0.9 m/s? Momentum = Mass x Velocity Momentum = 3.0 kg x 1.5 m/s Momentum = 4.5 kg.m/s Momentum = Mass x Velocity Momentum = 4.0 kg x 0.9 m/s Momentum = 3.6 kg.m/s Momentum was greatest for 3.0 kg hammer Momentum Problem Example A golf ball travels at 16 m/s, while a baseball moves at 7 m/s. The mass of the golf ball is 0.045 kg and the mass of the baseball is 0.14 kg. Which has greater momentum? Momentum = Mass x Velocity Momentum = 0.045 kg x 16 m/s Momentum = 0.72 kg.m/s Momentum = Mass x Velocity Momentum = 0.14 kg x 7 m/s Momentum = 0.98 kg.m/s Momentum was greatest for baseball Momentum Problem Example What is the momentum of a bird with a mass of 0.018 kg flying at 15 m/s? Momentum = Mass x Velocity Momentum = 0.018 kg x 15 m/s Momentum = 0.27 kg.m/s Chapter-10 Forces Conservation of Momentum In the absence of friction, momentum is conserved when two train cars collide. BRAIN BREAK! With your partner, discuss each of Newton’s 3 Laws and write a simple example of each