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A motor is dragging a log up a slope with a cable. When the cable breaks, what happens to the forces? There is no more tension, but does the direction of friction change since the log is now moving downwards? Why else would the speed be decreasing?

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  • $\begingroup$ What do you mean "...the speed be decreasing"? $\endgroup$
    – Bob D
    Dec 9, 2020 at 21:22
  • $\begingroup$ shouldnt the resultant force and motion be in different directions for the speed to be decreasing $\endgroup$
    – borns
    Dec 10, 2020 at 8:30
  • $\begingroup$ Are you talking about before or after the cable breaks $\endgroup$
    – Bob D
    Dec 10, 2020 at 8:42

3 Answers 3

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You need to know that friction acts in a way to oppose relative motion of object

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1st case

Since the log is moving up relative to incline friction acts downward working as an opposition

2nd case

Now there is only a component of mg which causes log to move downwards relative to incline so friction changes its direction by same logic.

The speed is decreasing because the force whi h caused it to move up is no more there and mg developed is in a direction opposite to velocity attained so retardation occurs and object eventually stops

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The direction of the dynamic friction for is always opposing the direction of motion. If it would not to that, the friction force would accelerate your object, which does not make sense.

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    $\begingroup$ It's not direction of motion rather relative motion :] $\endgroup$
    – imposter
    Dec 9, 2020 at 19:59
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    $\begingroup$ Every motion is relative motion. $\endgroup$
    – Bernhard
    Dec 9, 2020 at 20:00
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    $\begingroup$ In case of rotational motion it might be possible that friction opposes relative motion but at the same time supports motion think about it $\endgroup$
    – imposter
    Dec 9, 2020 at 20:05
  • $\begingroup$ Here is an example where the relative motion is important. physics.stackexchange.com/questions/598523/… $\endgroup$
    – John Darby
    Dec 9, 2020 at 21:01
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    $\begingroup$ I don't think this is very relevant in the context of the level of this question. $\endgroup$
    – Bernhard
    Dec 9, 2020 at 21:24
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If the log is moving after the cable breaks, the direction of static friction will be opposite to its velocity, so it will change direction.

But it is also possible that the log stays still after the cable breaks. In that case you will determine the direction of friction by saying that it should compensate the other forces. This direction will most certainly change in this case also. Note that the magnitude of this friction should remain below a certain value $\mathbf{T} \le \mu \mathbf{N}$, otherwise the log starts sliding.

There are other factors that could make the speed decrease, but it mostly depends on what you include in your model.

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