Car stopping distance using friction coefficient formula

[connormaphone]

I have been given a physics assignment which requires that I go over the physics of why 40km/h is a better speed to be followed in a school zone rather than 50km/h. (must include calculations for stopping distances etc.)

I used the formula Vo ^ 2 / 2 x μ x g.
It should give me the stopping distance if I am correct, Vo being the inital velocity, μ the friction coefficient and g for gravity. I first tried using 40km/h for Vo, .7 for μ and 9.8 for g.

40^2/2x.7x9.8

I use this and end up with 116.61...kilometres, this is evidently wrong and I am not sure what I am doing wrong as it should come out to 20-30 metres.
Can somebody please help?
Thanks.
(P.S I am in grade 11, so go easy on me )

 

[PhanthomJay]

You forgot to convert km/hr into m/s. It is essential that units are consistent.

 

[connormaphone]

Oh that gives me an answer of 13.72, I feel stupid now. Thanks for the help!

 

[PhanthomJay]

Oh that gives me an answer of 13.72, I feel stupid now. Thanks for the help!

13.72 m? Show how you arrived at that answer.

 

[connormaphone]

40000^2/2x.7x9.8
Just realized this is wrong as well. I don't even know.

 

[connormaphone]

Actualy, just made it 11.1 m/s which does equal 40km/h, I put it into the equation and ended up with approximately 9 metres.

 

[PhanthomJay]

Actualy, just made it 11.1 m/s which does equal 40km/h, I put it into the equation and ended up with approximately 9 metres.

Now, you're talking! Looks Good!
And welcome to PF!

 

[Chestermiller]

I have been given a physics assignment which requires that I go over the physics of why 40km/h is a better speed to be followed in a school zone rather than 50km/h. (must include calculations for stopping distances etc.)

I used the formula Vo ^ 2 / 2 x μ x g.
It should give me the stopping distance if I am correct, Vo being the inital velocity, μ the friction coefficient and g for gravity. I first tried using 40km/h for Vo, .7 for μ and 9.8 for g.

40^2/2x.7x9.8

I use this and end up with 116.61...kilometres, this is evidently wrong and I am not sure what I am doing wrong as it should come out to 20-30 metres.
Can somebody please help?
Thanks.
(P.S I am in grade 11, so go easy on me )

Your starting formula is incorrect. The μg should be in the denominator. That is the deceleration. The larger the deceleration, the shorter the stopping distance.

Chet

 

[PhanthomJay]

It is in the denominator. Parentheses would help, but I think it is in proper PEMDAS, per my recollect.

 

[connormaphone]

Got it, thanks for the help guys!