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Help refresh my memory

  1. Aug 28, 2003 #1
    Hi everyone,

    It's been a while since I've had physics. I can catch on quickly once pushed in the right direction but a friend is in a college level physics class that I once had and she needs help. I've tried to answer some questions but I don't know if my logic is right, or if I'm even close. Please help me figure out how to help her with this problem:

    A car that weighs 3200 lbs is traveling at a constant 68mph down a piece of straight, flat interstate highway. Use Newton's 1st Law to calculate the net force on the car. Use Newton's 2nd Law to calculate the car's acceleration.

    My thinking is this: The net force is 0 because there is no force acting against the car. Am I wrong in thinking this? Should the net force be 3200 lbs because of gravity? For the second question, I'm assuming the car's acceleration is also 0 because it's moving at a constant speed. Please help correct my thinking. I have the formula acceleration = force/mass.

    Second question: In the above problem, the force of air drag on the car is 300lbs. What is the forward force produced by the action of the car's engine and drive train?

    My thinking is this: the forward force is -300lbs. I know I'm totally wrong I just want to start somewhere.

    Please help. I made an A in Physics two years ago but now I need a refresher course and I'd like to be able to help my friend with these questions.
  2. jcsd
  3. Aug 28, 2003 #2


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    Seems to me that you have it pretty well under control. You may want to consider the direction the forces are acting. That 3200lb acts down there is a equal reaction force acting up so your net Y force and there for acceleration is 0

    You are correct since the speed is constant the X acceleration is also 0.

    To hold a constant speed the car must balance the drag force so will provide and equal and opposite force. The algebraic sign will dependend upon your choise of coordinate system.
  4. Aug 28, 2003 #3


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    I confess that that first problem sounds very strange. If you are told that the car is "traveling at a constant 68mph down a piece of straight, flat interstate highway", you shoudn't need to use "Newton's second law" to determine that the acceleration is 0!

    Now, the fact that the acceleration is 0 tells us that the NET force is 0 but that may be because of opposing forces- the air resistance and the wheels force on the road. You say that you are given that the air resistance force is 300 lbs and say that, therefore, the force forward is -300 lbs. That looks fine to me. You may be concerned about that "-" for a forward force.

    Well, you were given +300 for a backward force so that may be the way they are taking the coordinate system! Another possibiity is that they are just talking about the magnitude of the force and expect "300 lbs." as the answer.
  5. Aug 28, 2003 #4
    Well the net force is zero, since the car continues to move at a constant rate. If there were a net force, there would be acceleration. So, there is no net force and no acceleration. It's worded strangely with the references to Newton's laws..
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