# Anyone here good at physics?

Discussion in 'General Off-Topic Chat' started by War, Dec 16, 2008.

# Anyone here good at physics?

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1. ### WarTake it easy~

Member
4
Oct 26, 2007
California
Hey guys... I have a Physics I project due tomorrow, and I need some help.

I need to demonstrate how the following can be related to Newton's First and Second Laws.

- Stopping distances for cars are considerably longer on wet pavements (slippery) than on dry pavements.

I have to explain why that is, and relate it to either the First Law, Second, or both. I also have to draw a picture, but that's something I have to do myself.

I know it's because there's less traction, I just need to explain it in-depth.

2. ### DensetsuPubic Ninja

Former Staff
11
Feb 2, 2008
Wouldn't YOU like to know?
Newton's first law: An object in motion (or at rest) tends to stay in motion (or at rest) unless a force acts upon it.
Newton's second law: Force is equal to the product of an object's mass and its acceleration (F=ma).

The force that acts on a car when you hit the brakes is friction. The force of kinetic friction (fk) is given by the following equation:

fk=µkmg

The coefficient of kinetic friction (µk) is a constant value that indicates how much friction there is between two surfaces (in this case, rubber tires and asphalt). The term "m" is the mass of the car, and the term "g" is acceleration due to gravity (which is 9.8 m/s2)

When you have water between the two surfaces of rubber and pavement, you lower the coefficient of kinetic friction µk, and so the force of kinetic friction fk goes down. So you have less frictional force acting upon the car, which is why the car takes longer to stop (this is how it's related to Newton's first law).

The equation fk=µkmg is actually just a modified version of F=ma. fk is just a type of force (F). µk is a unitless constant and therefore does not change the dimensions of the equation. Since "a" is acceleration, and "g" is acceleration due to gravity, they're the same thing. Essentially, fk=µkmg and F=ma are the same equation. So you have a frictional force fk acting against the force of the car F (this is how it relates to Newton's second law).

Hope this helps. If you have any other questions, let me know.

*EDIT*
I modified Newton's first law, but it really doesn't matter because the car isn't at rest in your example. I just like being technical.

If you want to be really technical, the force of kinetic friction initially comes from the brake pads and the tires are exerting a static force of friction against the pavement. Once the brake pads grab, the tires start skidding against the pavement (at which point it goes from static to kinetic friction). But I think for the sake of your assignment, it's safe to drop these assumptions.

The picture is really simple. I'm assuming you have to draw a force diagram. Basically all you have to do is draw a car on pavement and show the only four forces acting on the car.
Force Diagram

3. ### WarTake it easy~

Member
4
Oct 26, 2007
California
Thank you so much Densetsu, although you got a bit too technical for me! xD But I think I got the gist of it. If you dumbed it down a little, it would be perfect.

4. ### MartiinGBAtemp Fan

Member
2
Nov 4, 2008
California

GBATEMP!
A place where other people can do your homework -.^ b

5. ### WarTake it easy~

Member
4
Oct 26, 2007
California
Well, I always ask GBAtemp when I don't know something Anyway, thank you very much Densetsu, I finished the project, and hopefully I'll get an A! (Although my drawing sucked...)

6. ### DensetsuPubic Ninja

Former Staff
11
Feb 2, 2008
Wouldn't YOU like to know?
The drawing doesn't have to be fancy. You should get full credit as long as you draw a car, the pavement and two arrows: one arrow showing the forward force and the other arrow showing the backward force like I did. If you don't get full credit for that, your teacher is a douche

Oh, and you're welcome Anytime!

7. ### Talaria...

Member
2
Jan 31, 2007
...
Nice work Densetsu3000, beat me to it. Physics is one of the only subjects I excel at