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Unformatted text preview: 51APPLYING NEWTONS LAWS5Q5.1. Reason: For an object to be in equilibrium, the net force (i.e., sum of the forces) must be zero. Assume that the two forces mentioned in the question are the only ones acting on the object. The question boils down to asking if two forces can sum to zero if they arent in opposite directions. Mental visualization shows that the answer is no, but so does a careful analysis. Set up a coordinate system with the xaxis along one of the forces. If the other force is not along the negative xaxis then there will be a y(or z) component that cannot be canceled by the first one along the xaxis. Assess: In summary, two forces not in opposite directions cannot sum to zero. Neither can two forces with different magnitudes. However, three can. Q5.2. Reason:Objects in static equilibrium do not accelerate and remain at rest. Objects in dynamic equilibrium do not accelerate and move with constant velocity. (a)The girder is moving at constant speed. We assume its being lifted straight up. If so, its in dynamic equilibrium. (b)Since the girder is slowing down it is accelerating, and therefore not in static or dynamic equilibrium. (c)Since the barbell is not accelerating and remains at rest it is in static equilibrium. (d)Once the plane has reached its cruising speed and altitude the plane is moving with constant speed and direction. It is in dynamic equilibrium. (e)A rock in free fall is accelerating due to gravity. It is not in equilibrium. (f )The box is accelerating since the truck is accelerating and the box is not sliding relative to the truck. The box is not in equilibrium. Assess:For an object in equilibrium!Fnet=!0.Q5.3. Reason: Assume you are sitting in a chair, and that you are at rest. (Parts of your body may be moving, but if you model your body as a particle, then you generally arent moving much as you read.) The two forces that act on you are the gravitational force of the earth on you, directed down; and the normal force of the chair pushing up on you. These two forces are equal in magnitude and opposite in direction and so the sum (the net force) is zero. Assess: When you arent accelerating Newtons second law says you experience a zero net force. So this analysis would apply equally to the case of you reading this while sitting in a chair on a smoothly moving (constant velocity) train. Q5.4. Reason: Weight is due to the pull of gravity on an object. Mass is a measure of an objects tendency to resist being accelerated. (a)The mass of an object is independent of location. Whether the object is on the earth or the moon or in space away from sources of gravity, its mass is the same. (b)The weight of an object depends on the acceleration of gravity at the location. The weight of an object on the moon is less than the weight of an object on the earth....
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 Fall '10
 Shawhan
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