# Is the friction always a bad force?

1. Oct 20, 2005

### maria curie

IF it is not,which situations does the presence of friction allow us?:grumpy:

2. Oct 20, 2005

### matt.o

not sure I understand the question, but friction is certainly good for stopping a car!

3. Oct 20, 2005

### maria curie

of course,but ı didn't mean that,

does the friction allow us to walk or cars to accelerate?

Last edited: Oct 20, 2005
4. Oct 20, 2005

### ZapperZ

Staff Emeritus
Try walking or driving on ice where there's very little friction.

Zz.

5. Oct 20, 2005

### maria curie

yeah..I am confused...in the high school teachers said' friction's and motion's direction are always opposite to each other'
now ı know it is not true.all right, when have they the same directions???

6. Oct 20, 2005

### FredGarvin

Think about what you are saying. In your opinion that friction is in the same direction of (edit: sliding) motion, that would mean that friction would accelerate the (sliding) motion.

Look at the simple idea of rolling a ball across the floor. Assuming the ball doesn't hit anything, what slows the ball down?

Edit: Good point Doc.

Last edited: Oct 20, 2005
7. Oct 20, 2005

### ZapperZ

Staff Emeritus
Why is that not true? Friction in the case of walking and driving a car helps you to grip the ground. It doesn't mean it is not in the opposite direction of motion. Look at the contact point when a wheel is rolling without slipping. You'll see that the frictional force is acting in the opposite direction of the lateral motion of the wheel.

When is friction in the same direction as motion? When you put an object on a conveyor belt to move it from one location to another. If the object doesn't slip, you have the contact friction between the object and the belt that pushes the object forward.

Zz.

8. Oct 20, 2005

### Staff: Mentor

What your teacher should have said is that friction always acts to oppose slipping between surfaces. It can certainly act in the direction of motion. A good example is the friction on the (nonslipping) tires of an accelerating car. Both friction and the car's acceleration have the same direction: forward.

9. Oct 20, 2005

### Staff: Mentor

Also remember that Newtonian physics requires balanced forces, so there will always be a force acting in the opposite direction of the motion(edit: when friction is involved)/acceleration. Ie, for a car (person running) accelerating to the right, the ground is being pushed to the left.

Last edited: Oct 20, 2005
10. Oct 20, 2005

### Danger

Life wouldn't exist without friction, if you get right down to it. The earliest (non-plant) forms wouldn't have been able to propel themselves in order to obtain food.