# Homework Help: Maximum speed of a car

1. Jan 2, 2010

### alessandro

Friction provides the force needed for a car to travel around a flat, circular race track. What is the maximum speed at which a car can safely travel if the radius of the track is 80 m and the coefficient of friction is 0.40 ?

I'm totally lost...hope u guys can help me

2. Jan 2, 2010

### rock.freak667

It is traveling around a circle, so centripetal force comes into play. What provides the centripetal force?

3. Jan 2, 2010

### lovemake1

ok. to start off this question.
you need to think of the equation and plot in the knowns.
what equation do you use in this case ? ( hint: Force equation)
now this is in a circular race track, so it must be a centripeteal force.

the ultimate question in this question is what really supplies the neccesary force for the car to stay on the track ?

4. Jan 2, 2010

### yamugushi

This is a supply demand problem:
Supply force: .4 friction
demand = v^2
Fnet = MV/r

solving algebraically:
V=$$\sqrt{}Force of friction (radius)$$

note you may want to get the equation yourself, because I'm doing this by memory and it may be wrong.

5. Jan 2, 2010

### alessandro

the agent provides the centripetal force...the agent proivdes also the acceleration right?

6. Jan 2, 2010

### alessandro

the friction provides the force necessary right? But usually to find this force i need also the mass...

7. Jan 2, 2010

### rock.freak667

No you do not need the mass, what is the normal reaction in this case equal to?

8. Jan 2, 2010

### alessandro

i don't really know..i'm lost..

9. Jan 2, 2010

### vela

Staff Emeritus
First, draw a free-body diagram of the car, and identify the forces on the car. Tell us what forces you think are on the car and which way they point.

10. Jan 2, 2010

### alessandro

the forces are the gravitazional force which points downward, friction force which acts in opposite direction of the car.......

11. Jan 2, 2010

### vela

Staff Emeritus
You're still missing a force. If gravity pulls the car down, and there's no counteracting force, the car will accelerate downward (through the road), which doesn't happen, so there must be another force on the car that balances the car's weight. What is that force and which way does it point?

Also, for simplicity, you can assume that the car isn't speeding up or slowing down, so there's no frictional force parallel to the velocity of the car, but there is still another frictional force, the one that keeps the car from sliding off the curve. Which direction does that frictional force point?

12. Jan 2, 2010

### alessandro

that frictional force point downward ?

13. Jan 2, 2010

### vela

Staff Emeritus
What's your reasoning for saying it points downward? It would help if you explained your answers, otherwise we can't tell if you're just making random guesses.

14. Jan 2, 2010

### alessandro

because the gravitazional force points upward and the frictional force which is against the gravitazional force...it would be pointing downward

15. Jun 1, 2010

### emdk

1 force of gravity > down
2 Centrifugal force > outwards
3 Lift (aerodynamic force)(like a wing of a airplane) > up
4 Drag (air resistance) > backwards
5 Trust > forwards

so if trust is to much your car will lift more, therefore centrifugal fore will prevail and your car will want to go straight forwards.

so you would need the value for how much lift the car has, the weight, the and also take in consideration the friction (on soap the car will slide faster than on concrete.) of the road.

it all depend of how accurate you want the result to be.