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Acceleration questions

  1. Jul 5, 2004 #1
    Of my 10 questions on this prelab these were the 3 I had difficulty on.

    Why will a car accelerate on an incline?

    I said because of the cars positioning it will have a better pick-up due to an incline.

    Suppose a car is traveling along a perfectly horizontal surface and begins to slow down. Is the car accelerating?
    I said it is deccelerating, but i wasnt sure if this was correct, or how to explain.

    Suppose he car travels at a constant speed on a horizontal surface. Would you expect the distance to be directly or inversely proportional to the time? Wasnt sure about this one.


    THX!
     
  2. jcsd
  3. Jul 6, 2004 #2
    Gravity pulls the car downhill with a force equal in magnitude to the component of gravity that lies parallel to the hill.

    Yes, deceleration is a form of acceleration. Acceleration is any change in velocity, that can be in magnitude (faster or slower), or it can be in direction alone (like in uniform circular motion), or it can be both. Because the car is slowing down, the car's velocity is changing, and it is therefore accelerating.

    If A is "directly" proportional to B, then A = KB, where K is some constant. For example, if I have to use two pieces of cheese on every cheeseburger I make, then the number of slices of cheese I use is "directly proportional" to the number of cheeseburgers I make. Number of slices of cheese = two X number of cheeseburgers. Here the constant of proportionality (K) has a value of 2.

    If A is "inversely" proportional to B, then A = K/B. This means that, as B becomes larger, A becomes smaller. For instance, the strength of gravity is inversely proportional to the square of the distance between the object in question and the object it's moving away from. If I put in a larger distance, I'm dividing K by a larger number, and so A (the strength of gravity), becomes smaller. If I put in a smaller distance, I'm dividing K by a smaller number, and so A is larger.

    If the distance travelled by the car was inversely proportional to time, that would mean that the distance approached zero as the time became large. If the distance travelled by the car was directly proportional to time, that would mean that the distance became large as the time became large. That's the one that would make sense. The longer you drive, the further you go. Therefore, the distance is directly proportional to time.
     
  4. Jul 6, 2004 #3
    RE: "Why will a car accelerate on an incline?"

    The question is vague. My car is at rest on an incline as we speak. It is not accelerating. I can certainly drive up or down an incline at constant speed.

    Bad, bad, bad question.

    Now, if we ignore friction and other extraneous forces, then the forces that act on the car cannot cancel, because two forces (in this case, the gravitational and normal force) can only cancel if they point in opposite directions (which they do not in this case). Since the vector sum of the two forces cannot be 0, the car must accelerate according to Newton's second law.


    RE: "I said because of the cars positioning it will have a better pick-up due to an incline."

    This is circular logic.
     
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