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Force (Chapter 3) Sep 29 Write everything in the yellow font
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Acceleration is the change in velocity per unit time Acceleration = v final - v initial /time Any change in speed and/or direction causes an acceleration Example Units: m/s/s (m/s 2 ), km/h/s, mph/s
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1. A car accelerates from rest to 35mi/h in 18s. What is its acceleration? 2. A car accelerates from 22m/s to 35m/s in 12s. What is its acceleration? 3. A car slows down from 60km/h to 35km/h in 11s. What is its acceleration?
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Objects dropped will free fall to earth. acceleration due to gravity is 9.8 m/s 2 For every second that passes an object will gain 9.8 m/s in speed. All objects fall at the same acceleration when air resistance is minimal See fig 3.4, p. 36
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As we have discussed, all object must be acted upon to change their state. When a force is applied, the object experiences acceleration. This means that the net force is not zero during accelerations!! Acceleration is directly proportional to net force, and accelerations are always in the direction of the net force. See fig 3.5, p. 37
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Mass is a measure of inertia. The greater the mass of an object the greater its inertia. More force is required for any given acceleration. Mass ≠ weight weight is dependent upon gravity, mass is not Acceleration is inversely proportional to mass. fig. 3.11, p. 40
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Mass and weight are not the same thing! Mass is the amount of matter, weight is how much gravity pulls on that mass, so it is a force weight = mass(kg) x 9.8 m/s 2 mass must be in kg
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Force equals mass times acceleration. Units: Force = Newtons (kg∙m/s 2 ) Mass = kilograms Acceleration = m/s 2 So, what is weight actually a measure of? Force triangle
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4. Consider a 1000 kg car pulled by a cable with 2000 N of force. What will be the acceleration of the car? 5. What force is needed to accelerate a 85kg bowling ball 13m/s/s? 6. A 25N force is applied to a toy that then accelerates 5m/s/s. What is the mass of the toy? 7. What is the weight of the toy?
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When two objects are in contact with each other friction is present. Friction is always in the opposite direction of motion. If the object is not accelerating, the force causing the motion and the force of friction are equal. When objects fall through the air we call the friction air drag.
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Friction is dependent upon the types of materials and the incline of the surface. When you pull or push an object across the floor with a constant force of 75 N the friction force is also 75 N, but in the opposing direction. (∑F = 0)
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When a free falling object’s air drag and the weight force of the object are equal then acceleration stops and the maximum speed has been reached. (All forces cancel!!) Lighter, less uniform objects reach terminal velocity sooner than heavier, more uniform objects. Feathers vs. rocks, golf ball vs. Styrofoam ball, etc. See fig 3.18, p. 47
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