Car, Coffee, and static friction

[KingNothing]

Here is a problem from a worksheet I have..I'm having a little trouble with it. My teacher neglected to explain most of it:

A coffee cup on a dashboard of a cars slides forward if the driver decelerates from 40 km/h to rest in 3.5 seconds, but doesn't slide if it takes any longer.

Now, I know everything is done in m/s, so that's 100/9 m/s. I know what the problem is asking and everything..but I don't have any formulas that I can use (I don't think)...in this case, can acceleration be used the same as deceleration? Is it okay to just put it in an equation as a negative?

 

[ShawnD]

What is the question even asking? An even better question is, what is the question? What you quoted was a statement.

Acceleration and deceleration can be used the same way.

Here are some formulas you can try using:
F = ma
Fd = (1/2)mv^2
F = uN

 

[KingNothing]

Sorry. It's a very outdated book. That's what the question says...I'm 99% sure we are supposed to figure the coefficient of static friction.

Is it impossible? That just kinda came to me...we don't know the mass of the cup.

 

[ShawnD]

Originally posted by Decker
I'm 99% sure we are supposed to figure the coefficient of static friction.

Ok that makes things easier

A coffee cup on a dashboard of a cars slides forward if the driver decelerates from 40 km/h to rest in 3.5 seconds, but doesn't slide if it takes any longer

setup a formula for equilibrium and the answer writes itself.

F = F
uN = ma
umg = ma
ug = a
u(9.8) = (40km/h * 1h/3600s * 1000m/km)/3.5s
u = 0.324