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Exam review: don't remember how to do this

  1. Jan 12, 2004 #1
    I have 2 problems I have forgot how to do.

    1. An Arrow is shot at an angle of 45 degrees abouve the horizontal at 69 m/s. (a) How high will the arrow go? (b) what horizontal distance will the arrow travel.

    these are the equations I know:

    Dx=Vxt
    (Dx: verticle distance, Vx: Verticle Velocity, t: time)

    Dy=Vyt+1/2at^2
    (Dy: Horizontal Distance, Vy: Horizontal Velocity, t: time, a: acceleration)

    Vx=VIcosø
    (Vx: Verticle Velocit, VI: I dont know what this means but it could mean initial velocity)

    Vy=VIsinø

    As far as this question goes... I have no clue... I think I was absent when my teacher went over this... but my exams are tomorrow and I don't have enough time time to ask him about it... please help.

    2. What is the Coefficient of Friction for a 2000 kg car that skids to a stop with an acceleration of -3m/s^2

    these are the equations I know for this problem... this problem isn't that hard but there doesn't seem to be enough information. I need a pushing force!!

    W=mg
    (W:weight, M: mass, g: accel. due to gravity)

    Fnet=ma
    (Fnet: Net Force, a: acceleration)

    Fnet=Fp+Ff
    (Fp: pushing force, Ff: Frictional force)

    Ff=µFn
    (µ: coefficient of friction, Fn: Normal Force)

    for this problem I know that the mass (2000 kg) and the acceleration (-3m/s^2) so Fnet=ma, Fnet= 2000*-3, Fnet=-6000.

    I need to find Ff so I can solve for µ (Ff=µFn) cuz I already know Fn: W=mg, 2000*9.8, W=19600. and Fn=W when on a flat surface. so Fn=19600

    to find Ff I'd use this equation: Fnet=Fp+Ff. I know Fnet=-6000 but I don't know Fp or Ff... Help

    any help would be greatly apreciated!!

    P.S. I already know the answers (1.) 121m and 484m (2.) .306. all I need to know is how to get those answers!!!

    Thanks
     
  2. jcsd
  3. Jan 12, 2004 #2

    Doc Al

    User Avatar

    Staff: Mentor

    Try this. Treat vertical and horizontal motion separately. (Find the components of the initial velocity in each direction.) First find the time it takes for the arrow to reach its highest point. (Use [itex]v_f = v_i + at[/itex], applied to the vertical motion.) Now find the height it reaches. (Use [itex]D_f=D_i+V_it+1/2at^2[/itex].) For part b, use [itex]D=V_it[/itex]. (I'll let you figure out what t must be.)
    Who says you need a pushing force? The ground is doing all the pushing: The only horizontal force on the car is the friction of the ground against the tires. You have the acceleration, so find the force ([itex]F=ma[/itex]). Then find the coefficient of friction: the normal force is the weight of car.
     
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