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Calculate Tension Between Car and Trailer

  1. Jan 25, 2015 #1
    1. The problem statement, all variables and given/known data

    2. Relevant equations
    I have no idea how to start this question, I looked at my notes from class and the text book but just can't figure out how to start it!

    3. The attempt at a solution
    Any help would be greatly appreciated! Even if just to get started!

  2. jcsd
  3. Jan 25, 2015 #2
    Start with a free body diagram.
  4. Jan 25, 2015 #3
    Thanks, I have done that but this is as far as I can get before becoming stuck!

  5. Jan 25, 2015 #4
    Write down the force balance equation for the trailer. Also, in your free body diagram, you are missing a force between the ground and the car.

  6. Jan 25, 2015 #5
    Hi, thanks again for the help! I have added in a friction arrow for the car and one for the trailer (not sure how I forgot that!) but I'm not sure what you mean about a for balance equation for the trailer?

  7. Jan 25, 2015 #6
    It is unnecessary to include a friction arrow for the trailer because its tires are not driven; they are just idling. But including a friction arrow for the force the road exerts on the car is important. Which direction do you think this arrow is pointing?

    Getting back to the trailer, if you focus on the trailer as a free body, what is(are) the horizontal force(s) acting on it?

  8. Jan 25, 2015 #7
    I see, I have the arrow for the friction under the car pointing towards the trailer. As for the trailer, I think the forces acting on it are simply its weight due to gravity and the tension between the trailer and the car given that friction is unnecessary, is that right?

    Thanks again, hopefully I will have an epiphany soon!
  9. Jan 25, 2015 #8


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    A car engine is not a jet engine. If there were no friction between tyres and road, would the car go?
    In your diagram, you show a forward force propelling the car, but there are two things wrong with it. It has the wrong value (it takes 60s to reach 9m/s) and it is drawn in the wrong place.
  10. Jan 25, 2015 #9
    This is not correct. As haruspex was alluding to in the previous post, the frictional force exerted by the road on the car tires is pointing forwards, in the direction that the car is moving. The tires are driven by the car engine, and they are pushing backwards on the road surface. The road surface, in turn, is pushing forward on the car. This is where haruspex was saying that the forward arrow should go.
    Yes. So, given this information, what is the horizontal force balance on the trailer?

  11. Jan 26, 2015 #10
    You might be interested in newtons classic equation : f = m * a
    This would be involved in both solutions.
  12. Jan 26, 2015 #11
    Thanks for your help, I got the problem sorted today!

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