Chap 04 SM - Chapter 4 Newtons Laws Conceptual Problems 1...

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295 Chapter 4 Newton’s Laws Conceptual Problems 1 While on a very smooth level transcontinental plane flight, your coffee cup sits motionless on your tray. Are there forces acting on the cup? If so, how do they differ from the forces that would be acting on the cup if it sat on your kitchen table at home? Determine the Concept Yes, there are forces acting on it. They are the normal force of the table and the gravitational pull of Earth (weight). Because the cup is not accelerating relative to the ground, the forces are the same as those that would act on it if it was sitting on your table at home. 2 You are passing another car on a highway and determine that, relative to you, the car you pass has an acceleration r a to the west. However, the driver of the other car is maintaining a constant speed and direction relative to the road. Is the reference frame of your car an inertial one? If not, in which direction (east or west) is your car accelerating relative to the other car? Determine the Concept No. You are in a non-inertial frame that is accelerating to the east, opposite the other car’s apparent acceleration. 3 [ SSM ] You are riding in a limousine that has opaque windows that do not allow you to see outside. The car is on a flat horizontal plane, so the car can accelerate by speeding up, slowing down, or turning. Equipped with just a small heavy object on the end of a string, how can you use it to determine if the limousine is changing either speed or direction? Can you determine the limousine’s velocity? Determine the Concept In the limo you hold one end of the string and suspend the object from the other end. If the string remains vertical, the reference frame of the limo is an inertial reference frame. 4 •• If only a single nonzero force acts on an object, does the object accelerate relative to all inertial reference frames? Is it possible for such an object to have zero velocity in some inertial reference frame and not in another? If so, give a specific example. Determine the Concept An object accelerates when a net force acts on it. The fact that an object is accelerating tells us nothing about its velocity other than that it is always changing.
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Chapter 4 296 Yes, the object must have acceleration relative to all inertial frames of reference. According to Newton’s first and second laws, an object must accelerate, relative to any inertial reference frame, in the direction of the net force. If there is only a single nonzero force, then this force is the net force. Yes, the object’s velocity may be momentarily zero in some inertial reference frame and not in another. During the period in which the force is acting, the object may be momentarily at rest, but its velocity cannot remain zero because it must continue to accelerate. Thus, its velocity is always changing.
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Chap 04 SM - Chapter 4 Newtons Laws Conceptual Problems 1...

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