• SDN Site Updates

    Hey everyone! The site will be down for approximately 2 hours on Thursday, August 5th for site updates.

SaintJude

Full Member
Jan 4, 2012
1,479
5
Status (Visible)
  1. Pre-Medical
The passage gives you a value of car's mass, 1000 kg, and coefficient of static friction, 0.6 & also that of kinetic friction, 0.2.

Q: The car is traveling at a velocity of 24 m/s along a level road. What is the shortest time it will take for the car to come to rest? (Assume that the car does not skid, and neglect air resistance.)



Explanation:

The question asks for the least time, which means that the static force of friction is at its maximum value

What's the reasoning behind this?

Thus the acceleration is found from F= ma= ustatic * N & then one uses v = vo + at
 
Last edited:

pm1

Full Member
Mar 21, 2012
323
8
Status (Visible)
  1. Pre-Medical
The passage gives you a value of car's mass, 1000 kg, and coefficient of static friction, 0.6 & also that of kinetic friction, 0.2.

Q: The car is traveling at a velocity of 24 m/s along a level road. What is the shortest time it will take for the car to come to rest? (Assume that the car does not skid, and neglect air resistance.)



Explanation:



What's the reasoning behind this?

Thus the acceleration is found from F= ma= ustatic * N

The only force applied to the car in order to stop it will be friction. Thus, Fnet = Ffriction, hence ma=Ffriction. Note that this acceleration is actually deceleration.
Since they ask for the shortest time for the car to come to a stop we want a max a, in order to get a max a we use max friction.

If you want to stop an object from moving as quickly as possible, it makes sense to use the max force against its current movement, right?
 

SaintJude

Full Member
Jan 4, 2012
1,479
5
Status (Visible)
  1. Pre-Medical
The only force applied to the car in order to stop it will be friction. Thus, Fnet = Ffriction, hence ma=Ffriction. Note that this acceleration is actually deceleration.
Since they ask for the shortest time for the car to come to a stop we want a max a, in order to get a max a we use max friction.

If you want to stop an object from moving as quickly as possible, it makes sense to use the max force against its current movement, right?

But why not use kinetic friction? In this quantitative case it wouldn't make a difference as it will be the same numbers. But if there was an answer choice that asked you which value you needed from the choices below, why would you choose a? I mean the car is moving...

a. determine maximum static friction
b. determine kinetic friction.
 

MrNeuro

Full Member
7+ Year Member
Mar 17, 2010
606
5
Danger Zone
Status (Visible)
  1. MD/PhD Student
i use impulse....i think you get the same answer

Favg t = delta mv

delta mv = 1000(24) = 24000

Favg = total net constant force applied Ffriction = us(N) = 6000

t= 4 seconds

the whole F=ma route w/ kinematics seemed a little odd to me but i think you should still get the same answer
 
About the Ads

MrNeuro

Full Member
7+ Year Member
Mar 17, 2010
606
5
Danger Zone
Status (Visible)
  1. MD/PhD Student
But why not use kinetic friction? In this quantitative case it wouldn't make a difference as it will be the same numbers. But if there was an answer choice that asked you which value you needed from the choices below, why would you choose a? I mean the car is moving...

a. determine maximum static friction
b. determine kinetic friction.

question says the cars not sliding...meaning its us also remember that when you slam on the breaks on your car your tires are actually still rolling thats why you have ABS (anti lock braking system) because as your wheels turn your tires are exhibiting static friction which exerts a larger deceleration than would a kinetic friction...
 

SaintJude

Full Member
Jan 4, 2012
1,479
5
Status (Visible)
  1. Pre-Medical
Ah, yes impulse is a great approach too.

I never understood why "Assume that the car does not skid" = indicates static friction. Why?

P.s. the passage was about "anti lock brake system..." but I just skimmed it..
 

ymartino

Full Member
Jul 29, 2013
49
9
Status (Visible)
  1. Pre-Medical
You don't need to use impulse. You can use the equation F=us(N)=m(v/t). I'm replacing acceleration with (v/t) since we're looking for time. Using the coefficient of friction of 0.6*1000kg*9.81m/s^2, you get 5886=m(v/t). Multiplying mass by velocity gives 24000kgm/s. Then simply divide 24000kgm/s by 5886kgm/s^2 to get 4.1 seconds. Hope that helps!
 
This thread is more than 7 years old.

Your message may be considered spam for the following reasons:

  1. Your new thread title is very short, and likely is unhelpful.
  2. Your reply is very short and likely does not add anything to the thread.
  3. Your reply is very long and likely does not add anything to the thread.
  4. It is very likely that it does not need any further discussion and thus bumping it serves no purpose.
  5. Your message is mostly quotes or spoilers.
  6. Your reply has occurred very quickly after a previous reply and likely does not add anything to the thread.
  7. This thread is locked.