_ THE _5B_2007 V10 B

_ THE _5B_2007 V10 B - T A K E - H O M E E X P . # 5 B St a...

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TAKE-HOME EXP. # 5B Standing Still: A Special Case of Equilibrium, Newton’s Laws and “Balanced “Forces Many objects in the world stand still, at rest with respect to a frame of reference attached to the Earth’s surface, like the horizon frame pictured to the right. Relative to this frame, although skyscrapers may sway in a wind and roll with earthquakes, most buildings, bridges, trees, parked cars, and our sitting selves are usually at rest. It’s easy to see: look around you right now. The buildings, parked cars and sidewalks are not changing their motion. They are not accelerating. Consider a parked car. Newton’s 1 st law says that an object will stay at rest unless a “resultant” force acts it on. A resultant force is produced by the vector addition of all the forces acting on the object. Newton’s 2 nd law says an object will accelerate —change its motion—if the sum of all the forces acting on it is some value other than zero. However, on each building, parked car, and sidewalk, the summed forces must be zero, since we can see that none of them are accelerating. We can find all the forces acting on an object by reasoning with Newton’s 3 rd law , which states that all forces occur in equal-magnitude pairs. If all the forces acting on an object sum to zero, the object is said to be in “equilibrium”. For a parked car, the vertical forces on the object are in equilibrium. This equilibrium— the forces in the vertical direction sum to zero—means that the car does not accelerate vertically. You can use this idea to weigh your car. N S equator Observer and perceived horizon plane The sum of the forces on a car at must be zero in any direction. CAR AT REST
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TAKE-HOME EXP. #5B: EQUILIBRIUM, BALANCED FOTCES, AND NEWTON’S LAWS THE#5B-2 ____________________________________________________________________ Observing a Car at Rest. Imagine a car of mass, m, at rest in a parking lot or in a driveway. The surface is horizontal and flat. The Earth and car interact gravitationally. Drawing A , shown below, pictures the pair of forces created by gravity. The Earth pulls on the car with a force of magnitude F = mg. Newton’s third law calls attention to its partner, the force on the Earth due to the car. A. B. C. Drawing B shows a different pair of third-law forces, the ones due to short-range contact of the tires to the pavement (where the rubber meets the road). Note the important difference in the two drawings. The forces in drawing A are due to a long-range force. The pair of forces in drawing B are due to short-range contact forces. Drawing C summarizes the two vertical forces acting on the car. Since four tires are in contact with the pavement, we can redraw the
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_ THE _5B_2007 V10 B - T A K E - H O M E E X P . # 5 B St a...

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