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Calculating the tension in rope of accelerating object

  1. Dec 13, 2015 #1
    • Member reminded that the formatting template is required in the homework areas.
    I have attached a picture so that it is easier to visualize this problem. The information is all in it. The number scribbles are just my attempts to solve it:

    The current topic we are on in class is Newton's Laws and we have covered moments and equilibrium previously, as well as resolving forces. So I assume that F = m * a may be involved, possibly equilibrium if my attempt is correct, but it does not tell you what to use. This is for AS-Level Physics.

    I used F = m*a to get the force in the opposite direction of the friction like 70 * 3.4 and 850 * 3.4 (it doesn't say the boat is accelerating but I assumed so as the person is accelerating something has to make him accelerate). Then I said that the forces to the right must equal the forces to the left, but I am not sure if this applies to accelerating objects, but anyway I said that 3128 (sum of the 2 m*a) = x + 325 and calculated x (The tension) from there. But I have a feeling its incorrect.
  2. jcsd
  3. Dec 13, 2015 #2


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    Start with a free body diagram for the man.
  4. Dec 13, 2015 #3
    Not really sure why I'd do that? The question is 3 marks, so not a lot of work is required.

    Would the friction force acting on the skier equal the resultant force of the boat?
  5. Dec 13, 2015 #4


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    The tension is the same at both ends of the rope so you only need to think about what's going on at one end. The acceleration of the man and all the forces on him except the tension are given or can be calculated.

    Got to go out for a few hours now.
  6. Dec 13, 2015 #5
    Ok well I calculated the tension to be 313N, as if the resultant force must equal 238N, then the T - 238 must equal 75, so 75 + 238 = 313.

    If this is correct, then to calculate the total driving force produced by the boat I would need to add 238 and the resultant produced by the boat?
  7. Dec 13, 2015 #6


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    Yes 313N is correct for the tension.

    I can only see the question (i) in the image you posted. The rest are off the bottom of the image.
  8. Dec 13, 2015 #7


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    What do you mean by that term?
    What are the forces acting on the boat?
  9. Dec 16, 2015 #8
    Thanks guys for your help! I got the question sorted! :)
    I can post what I did for the driving force when I get it handed back, because I've handed it into school.
  10. Dec 16, 2015 #9
    You asked why you would need to draw a free body diagram. This question is symptomatic of the difficulty you have had with solving this problem. I hope you don't feel you have advanced beyond the point of needing to use free body diagrams. The part of the course you are studying now requires their use even more intently than before. Repeat after me: "ALWAYS USE FREE BODY DIAGRAMS."

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