Am I missing something? Basic acceleration

[jspek9]

Hey everyone, quick question from the new guy. I don't know maybe I'm just making it harder than it is.

Car A is traveling east at 15.7 m/s. As car A crosses the intersection, car B starts from rest 42 m north of the intersection and moves south with a constant acceleration of 1.2 m/s2.
What is the distance between A and B 5 s after A crosses the intersection?

So car A is obviously 78.5 meters east of the intersection. It seems like a simple a^2 +b^2=c^2 problem but for some reason I can't convert car B's acceleration of 1.2 m/s^2 to a correct displacement. It seems to me car B should be 36m from the intersection but that can't be correct.
So can someone please explain to me how to convert acceleration to displacement.

P.S. I tried using delta x=v_it+1/2a_xt² with no luck either.

 

[atyy]

I tried using delta x=v_it+1/2a_xt² with no luck either.

That's strange. I would have tried that. Could you show us the details of how you tried to use it?

 

[jspek9]

I must be using it incorrectly. I used,
(1.2)*(5)+1/2(1.2)(5)^2= 21

then ((21)^2 +(78.5)^2)^1/2 = 81.3

Is my initial velocity wrong and what causing the problem?

 

[LowlyPion]

I must be using it incorrectly. I used,
(1.2)*(5)+1/2(1.2)(5)^2= 21

then ((21)^2 +(78.5)^2)^1/2 = 81.3

Is my initial velocity wrong and what causing the problem?

What is the (1.2)*(5) ?

 

[jspek9]

acceleration multiplied by time

 

[LowlyPion]

acceleration multiplied by time

Yes. I know what it is. Just wondering why you put that term in there?

Wasn't car B at rest?

 

[jspek9]

I see what your saying. The initial velocity isn't 1.2 its 0. That makes sense and gives a correct answer.

thanks for the help. This is the first time I have used this site and you guys where great.:!) Thanks again.

 

[jspek9]

one more quick question though

the next question is asking what is the speed and magnitude of acceleration of car B relative to A

What exactly do they mean by relative?

 

[just.karl]

In relation to each other, I'm guessing.

 

[LowlyPion]

In relation to each other, I'm guessing.

Yes that is correct.

 

[jspek9]

can you be a little more specific? I can't seem to find anything in the text about how to go about seting these up.

 

[jspek9]

am I wrong assuming the final velociy of car B is 6 m/s at T=5?

 

[LowlyPion]

can you be a little more specific? I can't seem to find anything in the text about how to go about seting these up.

Start out by thinking about what is the equation for the distance car B travels relative to the origin (intersection) in terms of velocity and acceleration.

Likewise for the distance Car A travels from the origin (intersection).

Perhaps you can exploit the fact at any point in time the distance traveled by both is along the sides of a right triangle?