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Homework Help: Physics Homework need help for answers/also explain how to do them

  1. Oct 11, 2007 #1
    Physics really stumps me, I try to study a lot but its not getting in my head... Anyway here are the questions i need help on. I tried doing the problem out but it confuses me.. SO far i'm going over velocity, displacement, like V=d/t and D=vt t=d/v

    1) You drive your Miata from home at an average velocity of 80 km/h for 3 hrs. Halfway to your destination, your Miata develops some engine problems, and for 5hrs you "nurse" the car the rest of the way. What is your average velocity for the entire trip?

    2) Two cars are headed in the same direction; one traveling 60 km/h is 20km ahead of the other traveling 80km/h.
    A)What would the graph look like showing the motion of the two cars?
    B) When does the faster car overtake the slower one?

    3) A bullet is fired with a speed of 720.0 m/s.
    A) What time is required for the bullet to strike a target 324 m away?

    B) What is the velocity in km/h
  2. jcsd
  3. Oct 11, 2007 #2
    You gotta know the distance from Miata to home to do that question. The word 'nurse' implies you are going slower and so need to calculate the speed after stopping.
  4. Oct 11, 2007 #3
    a) plot a graph on a piece of paper with speed/velocity on the side and time on the bottom (i'd just put 1hr, 2hr, 3hr etc for each of the velocities [unless you got given something])
    b) those 3 equations that you have are just different ways of writing the same thing.
    anyhow you have two velocities v1=60km/hr v2=80km/hr, when the cars pass each other t will be the same for both cars. so put both with respect to t (t=d/v). write out 2 equations so because the t's are the same d1/v1=d2/v2. you should be able to work this out from here.

    a) v=720m/s, d=324m -> you have an equation (t=d/v just sub the values in)
    b) v=720m/s
    1km=1000m -> 1km/1000m = 1
    1hr=3600s -> 3600s/1hr =1
    (you don't want to change the value so you need to multiply by 1)
    720m/s x 1km/1000m x 3600s/1hr (so 720m/s x 1 x 1)
    so this will cancel m and s and you'll be left with km/hr
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