IF it is not,which situations does the presence of friction allow us?:grumpy:
not sure I understand the question, but friction is certainly good for stopping a car!
of course,but ı didn't mean that,
does the friction allow us to walk or cars to accelerate?
Try walking or driving on ice where there's very little friction.
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???
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.
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.
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.
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.
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.
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