Am I on the right track?

BuBbLeS01

Question:
A. A car that weighs 12000.0 N is initially moving at a speed of 57.0 km/hr when the brakes are applied and the car is brought to a stop in 2.6 s. Find the magnitude of the force that stops the car, assuming it is constant.

B. What distance does the car move during this time?

Okay so I drew the diagram and I am planning on using the f=ma equation but I think I need to solve for a first using Vf=Vo + at. Am I on the right track or am I completely off??

learningphysics

Yes, you're on the right track.

BuBbLeS01

Okay so I did...
(57 km/hr)^2 = a x (2.6s)
and I got 1249.6 m/s^2...which way wrong...where am I going wrong?

BuBbLeS01

Wait I wrote that backwards sorry...

I got...

0 = (57 km/hr)^2 +a(2.6s)

learningphysics

Why are you squaring 57?

PhanthomJay

Why are you squaring the velocity? And you've got to be consistent with your units...you can't have seconds on one side of the equation and hours on the other without introducing a conversion factor.

BuBbLeS01

oh my gosh I don't know why I am squaring it...I am looking at the wrong equation. Okay I changed 57 km/hr to 0.01583 km/s. Now I have...
0 = 0.01583 km/s + a (2.6s)...
(-0.01583 km/s) / (2.6s) = a

so now I can use f=ma
f= (12000.0 N) x (0.0060897 km/s^2)

learningphysics

careful... you should use m/s for velocity and m/s^2 for acceleration (because your force equation requires kg for mass and m/s^2 for acceleration... to get the result in N)...

and also 12000.0N isn't the mass, it's the weight... get the mass of the object.

BuBbLeS01

okay so this is what I have now...

0=15.83 m/s + a x (2.6s)
(-15.83m/s) / (2.6s) = a = (-6.08846)

w=mg
12000/9.8=1224.49

f=ma
1224.49 * -6.08846 = -7455.26 N
That is really big!!

learningphysics

Looks good. That's the right answer.

BuBbLeS01

It says its incorrect....I don't know what is wrong.

learningphysics

I don't think you carried enough decimal places... I get -7456.8N

BuBbLeS01

should I be entering it as positive since its the magnitude?

learningphysics

Yes! I didn't notice that it asked for magnitude!

BuBbLeS01

Yay it's right!! Hehe...Thank you so much for your time!! I really do appreciate it!!

learningphysics

no prob.

BuBbLeS01

How do I start the second part of this question?