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Homework Help: Two questions help please

  1. Oct 9, 2005 #1
    finding friction coefficient without a mass??

    A box slides down a 37.1o ramp with an acceleration of 1.29m/s2. (acceleration of gravity is 9.8m/s2) Determine the coefficient of kinetic friction between the box and ramp.

    I just have no idea where to start with this since it doesn't give mass, so i can't draw that triangle to figure out weight, parallel force, or perpendicular force.

    and another question..
    1. A car is parked near a cliff overlooking the ocean on an incline that makes an angle of 25.7o with the horizontal. The negligent driver leaves the car on neutral, and the emergency brakes are defective.The car rolls from rest down the incline and has a velocity 20m/s when it reaches teh edge of the cliff. The cliff is 21.2 meters above the ocean. (acceleration of gravity is 9.8m/s2) How far is the car from the base of the cliff when the car hits the ocean?

    I attempted to do this and somehow got to a quadratic equation adn then i used the quadratic formula and redid it about five times and still got it wrong. we haven't really finished learning this in class, so i'm not sure of how to do it.

    it'd be great if someone felt like helping me...thanks a bunch :smile:
    Last edited: Oct 9, 2005
  2. jcsd
  3. Oct 9, 2005 #2


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    Homework Helper

    Draw a diagram of the box sliding down the ramp.

    Label the box with the forces acring on it.

    What is the net force on the box in its direction of motion ?

    Use newton's 2nd law.

    You don't need to know the value of the mass for this problem.
  4. Oct 10, 2005 #3


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    You are given the initial velocity in both size and direction. That is, 20 m/s at an angle of 25.7° below the horizontal.
    Split this velocity into vertical and horizontal components (velocities).

    Vertical movement
    Set up an eqn of motion to find out how long it takes to fall from 21.2 m high to the gound, when it's initial velocity downwards is -- whatever you work it out as.

    Horizontal movement
    Now that you know how long it takes to reach the ground, find out how far it has travelled horizontally using this time and the horizontal component of its initial velocity.
  5. Oct 10, 2005 #4
    i still don't get that first one

    how do you find out what the net force is??
  6. Oct 10, 2005 #5


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    As Fermat said right in the beginning: Draw a diagram. Do this before you do anything else.

    Take the direction along the ramp as x (and perpendicuar to that: y). Now divide the forces into x and y components.
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