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Homework Help: A deceleration problem

  1. Jun 9, 2004 #1
    I have a question that I have been working on, but I am stuck and not quite sure if i am approaching the problem right.
    The question asks: a person is driving 30.0 m/s 23 degrees north of east when a person walks out in front of them. The person slams their brakes and turns the cars wheels. The result is a deceleration(constant) that is 5.00 m/s2 oriented 53 degree north of east. How long does it take to stop? B)How far does the car go before stopping? C)What is the car trajectory?

    What I first did was draw it up,and assumed to take the 30 m/s 23 degrees north of east and finding the components. Vx and Vy. They were Vx=27.6m/s and Vy=11.7 m/s. Then itried to find the time, hoping too find the distance. But the whole thing is I do not know what to do with the deceleration part and the 53 degrees that was given in the question. Any help would be helpful! :smile:
  2. jcsd
  3. Jun 10, 2004 #2


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    At first glance, I would say that rather than resolving it into Vx and Vy, resolve it into its velocity in the direction of acceleration (53 degrees north of east) and it's velocity perpendicular to that direction. You could forget about direction and treat the things as scalars, where in one direction, the car would be undergoing constant acceleration and in the other, constant velocity. However, noting that second bit, that in one direction it will be undergoing constant velocity, I had to look over the problem again. What I think they're saying is that as soon as the driver noticed the person, he started heading 53 degrees north of east, i.e. the same speed, 30.0 m/s, but a different direction. So, at first he's going at a constant velocity in one direction, and then he suddenly starts heading in another direction but this time at constant acceleration (and obviously decreasing speed). So, what I just said basically describes, in words, the car's trajectory. As for A and B, you can just use your basic kinematic equations, and again, you can treat the stuff in scalars because the velocity and acceleration would be colinear. You know initial speed/velocity (30.0), final velocity (0), and acceleration (-5.00). All you have to do is calculate time elapsed and distance travelled, and I'm sure you know the equations to solve those.
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