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Homework Help: Iam really lost with this question

  1. Jul 6, 2008 #1
    1. The problem statement, all variables and given/known data
    A curling stone with mass 20.0 kg leaves the curler's hand at a speed of 0.885 m/s. It slides 31.5 down the rink before coming to rest.

    Find the Average force of friction acting on the stone.

    I have no clue as to what to do with this question. Can someone just guide and head me towards the right direction please? thank you.
  2. jcsd
  3. Jul 6, 2008 #2


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    Science Advisor

    Assuming constant friction force, there will be a constant acceleration. With constant acceleration, the average speed is just the average of the beginning and ending speeds. Here you are told the beginning speed and, of course, the ending speed is 0 so you can easily find the average speed. Knowing that, what time is taken to go 31.5 m? Once you know that, you can find the acceleration (negative- it deceleration) and then use F= ma, together with F= km where k is the coefficient of friction, to find k.
  4. Jul 6, 2008 #3
    ok when im finding the coeffeicient of friction, do i enter the Friction force as a negative or positive? Cause I dont think theres a negative coefficient of friction...unless if im wrong.
  5. Jul 6, 2008 #4
    By choosing the direction that will be positive at the beginning, and sticking to it, you can decide whether the friction force will end up positive or negative. It really does not matter, because you have to very carefully keep track of which parts of the motion it affects anyway. This is because the friction always acts to oppose the motion. In more complicated motions, the friction will happily reverse direction all by itself.

    Fortunately for us, this friction always gets to act in one direction!

    Suppose you deliberately state at the outset that directions positive for everything will be the direction the curling stone is headed when it is launched. The friction force will be opposing, so expect it might end up negative. The stone is decelerating to come to a halt. Expect the acceleration you get will be negative.

    First, you have to figure out the acceleration (negative) of that stone. HallsofIvy has given you quite a fast easy way to get to it, which takes advantage of knowing the force is constant.

    You can also use the basic equations of motion. There is one that provides the final velocity given the initial velocity, the acceleration, and the time it was applied. There is another which provides the distance covered, from also knowing those very same things. You can enter the information you know, and work the two together to end up knowing the acceleration.

    Regardless that it might have a minus sign (which tells you the direction), you can use the value to do the final step. You will need Newton's Law that describes the acceleration of a mass when a force is applied.

    This is the place where you get to show your attempt at a solution. Equations of motion and Newtons Law. This site, your own books and notes, or even a search engine will do. Get them first, and let us have it clear how they work. If you still do not understand them, we will try and help you
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