# Friction homework problem help

1. Jul 3, 2006

### MalayInd

1) We cannot drive a car on ice because there is no force on the car which acts horizontally. It means that when we press the accelertor, we provide only the rotational motion to the wheels. Now, when we drive on a road, friction provides the necessary force to enjoy the drive. We can have different accelerations while driving the car. How is it possible when the same force of friction is responsible for different accelerations?
2)Contact force between two surfaces is divided into two components, namely force of frictions and normal reation. Why the contact force is neither horizontal nor vertical but oblique to the surface?
3)Why the limiting friction(betwween two given surfaces) is dependent on the normal reaction?
4)Why does the value of coefficient of friction(ratio of force of friction to normal reaction) mostly remain less than 1?

Keep Smiling
Malay

2. Jul 3, 2006

### vijay123

hey,
1) ok, let us clarify doubts. there r actually 2 kinds of friction present. one is static friction while the other is kinetic.kinetic friction happens between the surface of the road and the tire, while the static friction(the larger force) acts at thecenter of a rotaing wheel, just like a gyroscope.i think, kinctic friction has only got to do with translation motion whiule staticequalises other forces like centripetal and centrifugal.
makes sense?....that wus jus a breif idea anyway...it may not be true, but according to my understand, that concept makes sense to me...

3. Jul 3, 2006

### vijay123

3) the limting force is only done to the normal force. this is because that is the only force that can vary. mass * gravity is always constant and done not vary. but the normal can be vary. for eg. if you press hard on somefin, it is just the normal force that is increasing, its not you mass*gravity which increases.hence, friction depends on how hard to press at the contact area. the hard yoou press and tranlate, the greater the friction. but if it wus your mass*gravity, friction would be constant to all particles of a given weight.:tongue2: hope dat makes sense.....

4. Jul 3, 2006

### Hootenanny

Staff Emeritus
Just some slight corrections here. Actaully there is some friction present between the tyres an ice (although it is very very small). You are quite right, if we were to apply the same force to an object, it would accelerate at the same rate. However, the horizontal force applied by the tyres is variable. What is not variable is the maximum frictional force, this is fixed as the product of the coefficient of friction and the normal reaction force. So long as the applied force is less than or equal to the product of the coefficient of friction and the normal reaction force, the tyres will not 'spin'
Friction is a result of the interaction between the intermolecular forces between the atoms of the two surfaces, it therefore follows that the more they are 'pushed together' by the normal reaction force, the greater these IM interactions will be.
One answer would be that the coefficient of friction is defined as the ratio between the frictional force and the normal reaction force, since the frictional force is generally less than the normal reaction force, the coefficient of friction is less than than one.

5. Jul 3, 2006

### vijay123

4) there are values of friction higher than one. but in our society, you like to have flat, regular particles....like smooth steel and so on...then it can be, jus that its not a necessity and is a nuisanse....
that question wus quite silly, i say....nonetheless.....worthy to be answered.

6. Jul 3, 2006

### Hootenanny

Staff Emeritus
I think you may have your concepts confused here. As long as the tyre is not slipping it is static friction which occurs here since the surface of the tyre is stationary relative to the ground (it has an equal linear velocity). Kinetic friction would occur only if the tyre began to slip (such as during a 'lock up' or when you do doughnuts

7. Jul 3, 2006

### Staff: Mentor

Careful! The friction force between the ground and the tire can be either kinetic or static. The difference is that kinetic friction occurs when the surfaces slip against each other while static friction occurs when the surfaces do not slip. Since kinetic friction is weaker than static friction--especially on wet surfaces--when we drive we try to make sure the tires do not slip so that we have greater control on the car.

In any case, without friction on the tires we couldn't accelerate the car. That means we couldn't even get the car out of the driveway, much less stop it or turn it. Any acceleration--speeding up, slowing down, changing direction--requires a force; friction supplies that force.

[Looks like Hoot beat me to it!]

8. Jul 3, 2006

### vijay123

ah dang it....knew it....thansk for that talk guys. think i took it over hand...lol

9. Jul 3, 2006

### Hootenanny

Staff Emeritus
Makes a change

10. Jul 3, 2006

### MalayInd

How is the applied force by the tyres variable?
What do you mean by 'spin' here?
What will happen if applied force is greater than maximum force of friction?
Both of them arise from IM interactions. Why has the ratio to be a constant?
Why the frictional force is generally less than normal reaction force?

Keep Smiling
Malay

11. Jul 3, 2006

### Hootenanny

Staff Emeritus
The torque supplied to the wheels can be varied by varying the output power of the energy (using the accelerator pedal).
'Wheel spin', as in when you leave skid marks on the tarmac. This occurs when the applied force is greater than the maximum static frictional force and the linear velocity of the tyres is greater than (or less than) the velocity of the road. When a wheel is rolling normally, there is static friction between the tyre and the road surface, since the tangental velocity of the tyre is equal to the linear velocity of the road (relative to the car), therefore the surface of the road and tyre are at rest relative to each other. Do you follow?

12. Jul 3, 2006

### MalayInd

When we provide more torque, the wheel rotates faster. Does the angular speed of rotating wheel has to do something with the force applied by tyres?

Keep Smiling
Malay