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Homework Help: Physic Acceleration

  1. Aug 28, 2013 #1
    1. The problem statement, all variables and given/known data
    A car traveling 100 km/h is 200m away from a truck traveling 50 km/h (in the same direction)
    . Assuming constant braking acceleration, what is the minimum deceleration the car must have if it is not to hit the truck?

    2. Relevant equations
    I know acceleration is a = (Vf - vi)/t but I do not know how to get the acceleration/deceleration when there two object instead of just let a car moving.

    3. The attempt at a solution
    I know the Vi of the Car is = 27.8 m/s
    The Vf of the Car is 13.9 m/s (The acceleration of the truck is this so won't it be the final velocity of the car when it crash?)
    Distance of car travel = 200+x
    Distance of truck travel = x;

    Can someone guide me in the right direction? Thanks
  2. jcsd
  3. Aug 28, 2013 #2


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    Homework Helper

    Notice that the distance between the car and truck is decreasing at a rate of 100km/h - 50km/h = 50km/h.

    Also note that 200m = 0.2km. These will help you find time.

    Using this you can determine the proper acceleration.
  4. Aug 28, 2013 #3
    I understand when you say the decrease in the distance due to the different in the vehicle speed but I can't think of how to find the time when the truck and car is constantly moving. Thank you
  5. Aug 28, 2013 #4


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    Homework Helper

    Remember that when you're dealing with speed :

    $$v = \frac{Δd}{Δt}$$

    So that :

    $$Δt = \frac{Δd}{v}$$
  6. Aug 28, 2013 #5


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    Gold Member

    Put one car at an origin. Write its displacement as a function of time. Put the truck 200m along the x axis and then write its displacement as a function of time. Equate these to find the time to collision as a function of acceleration.
  7. Aug 28, 2013 #6
    Thank you for the replies guy.
    I came up with that since the truck will travel a certain distance before the car hit, the distance for the can can be dCar = 200m + x . Since x is the distance the truck travel, it can be represented as x = ((Vf + Vi)/2) * t - 200.
    since x = distance, x can be x = vt (velocity * time). Is this how to approach this problem without using relative velocity? I'm stuck after this part.
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