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Forces SC Standards PS 5.7 Explain the motion of objects on the basis on Newton’s three laws of motion: inertia; the relationship among force, mass, and acceleration; and action and reaction forces. PS 5.9 Explain the relationship between mass and weight by using the formula Fw = mg. What is a Force? • Force – push or pull one body exerts on another – SI Unit of force: Newton (N) • Net force – the combining of forces when two or more forces act on an object • Balanced forces – forces that are equal in size and opposite in direction – Forces are acting on the same object – Do not cause the motion of the object to change • Unbalanced forces – forces that are not equal and/ or opposite in direction – Net force causes object to move in direction of force Check it out… http://www.batesville.k12.in.us/physics/PhyNet/ Mechanics/Newton1/The_Net_Force.html#Ne tForceQuiz Common Forces • Gravity – the attraction two objects have for one another • Air resistance – opposing force an object feels when traveling through air • Friction – resistance to motion caused by two objects in contact with one another. Inertia and Mass • Inertia – the tendency of an object to resist changes in its motion – The greater the mass of an object the greater its inertia • Newton’s First Law of Motion – ‘Stuff keeps doin’ what its doin’ until somethin’ else messes with it…’ F=mxa • Force, mass, and acceleration are all connected. – For any object, the greater the force applied to it, the greater its acceleration will be. Learning Checkpoint • If two equal forces act on an object in opposite directions, what is the acceleration? • According to balanced forces, the acceleration of the object would be zero. Learning Checkpoint • Calculate the force on an object that has a mass of 12 kg and an acceleration of 4 m/s/s. • F = mx a • F = 12 kg x 4 m/s/s • F = 48 kg x m/s/s = 48 N Learning Checkpoint What is the acceleration of a 200 kg object with 3,000 N of net force acting on it? a=F/m a = 3,000 kg x m/s/s / 200 kg a = 15 m/s/s Newton’s Third Law • When one object exerts a force on a second object, the second one exerts a force on the first that is equal in size and opposite in direction. – “To every action, there is an equal and opposite reaction.” – Example – when you jump on a trampoline the trampoline exerts the same force on you in the opposite direction. How are these not balanced forces? • Because the forces are acting on two different objects! Learning Checkpoint • When a force is exerted on a box, an equal and opposite force is exerted by the box. These forces are known as _____ forces. • These forces are action- reaction forces. Learning Checkpoint • What is the difference between weight and mass? • Mass is a measure of the amount of matter in an object, while weight is a measure of the force of gravity on the object’s matter.
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