# Static Friction direction

1. Aug 26, 2008

### Dschumanji

My physics book shows the direction of static friction to be opposite to an applied force that is parallel to the surface in which an object rests with the same magnitude (if it is below or equal to its maximum value). If that is the case, when speaking about a car rounding a flat curve, why does it show a force of static friction being applied to the car without an opposite and equal magnitude force? How can a nonzero static friction force be exerted on an object that has no opposite and equal force to create the static friction force in the first place?

2. Aug 26, 2008

### Defennder

You need static friction in order for the wheels to turn. If there is zero friction between the wheels and the road, the the car would not move at all: The wheels would spin madly while the car remains stationary. And there is an equal and opposite force: The static friction exerts a force on the road, which exerts a reaction force to spin the wheels, thus moving the car.

3. Aug 26, 2008

### atyy

In the first scenario, I'm guessing that your book shows a force applied to an object and the force of friction on the object? In this case, the book is omitting the force that the object applies to the ground, that by Newton's third law, is equal and opposite to the force of friction on the object. Since the force is applied by the object to the ground, it doesn't affect how the object moves, but it affects how the ground moves, which it doesn't because the ground is very heavy.

In the second scenario, the force applied to the object, which is now the wheel, should come from the car's engine acting through the wheel's axel. They are probably not showing this because they are lazy. They are also again omitting the force that the wheel exerts on the ground, that Newton's third law predicts should be equal and opposite to the force of friction on the object.

4. Aug 26, 2008

### Staff: Mentor

Note that in this example, the object is in equilibrium: its acceleration equals zero.
In this example, the car is not in equilibrium: it's accelerating towards the center of the curve. The friction supplies the centripetal force.
When acceleration is involved, you don't need an "equal and opposite" force to produce static friction.

5. Aug 26, 2008

### Dschumanji

I fail at understanding Newton's Third Law!

Thanks for helping me get a clearer understanding.

6. Aug 26, 2008

### Staff: Mentor

I hope you are aware that static friction being equal and opposite to the applied force (in the example you gave above) is not an example of Newton's 3rd law. (The applied force and the friction are not 3rd law pairs.)

Here's how Newton's 3rd law would apply:

If I exert a force against some object, Newton's 3rd law says that the object will exert an equal (and opposite) force on me.

If a surface exerts a friction force against some object, Newton's 3rd law says that the object will exert an equal (and opposite) friction force on the surface.

7. Aug 27, 2008

### i_island0

Firstly, static friction acts indirection opposite to in which it has tendency to move.
I won't say it is a complete definition, but good enough for our case.
A proper definition would be:
Static friction acts in such a direction such as not to defy newton's law, viz,
F = ma, torque = I x angular acceleration. [vectorially pls]

This concept is very clear.
If you want to really understand the direction of friction, follow these sequence in your textbook.
First study pure rolling of wheels on rough surface.
Condition of pure rolling for free wheel - You will find that no friction acts in this case

Condition of pure rolling for car's wheel - for accelerating car, friction is in the direction of acceleration.
After this, come back to your question. acceleration is towards center of the circular path in which car is moving. The only force that can cause that acceleration is friction. So static friction, as said is self adjusting, adjust itself to act towards the center of the circle.

8. Aug 28, 2008

### sp1408

A very simple rule is that friction always opposes RELATIVE MOTION between two surfaces,when the car is accelerating,the point of contact of the wheel to the ground is trying to move backward,so friction acts in the forward direction to oppose it....

9. Aug 29, 2008

### i_island0

I was thinking other way round here. If the car is accelerating forward, then its friction that makes it move forward. So, why shall we say that friction is opposing relative motion, actually in this case, its friction that is promoting motion.

10. Aug 29, 2008

### Staff: Mentor

Friction propels the car forward, but there's no relative motion between the contact surfaces (unless the tires are slipping). I prefer to say that friction acts to prevent (or oppose) slipping between surfaces. In the case of the accelerating car, without friction the tire surface would slip backward. Friction prevents this and drives the car forward.

11. Aug 29, 2008

### i_island0

So we can conclude that:
a) kinetic friction opposes relative motion.
b) static friction opposes slipping. (there is no relative motion afterall)

12. Aug 29, 2008

### Dschumanji

I'm surprised that people are still posting in this thread. A lot has been cleared up

To go off topic a bit...

Can Newton's third law be applied in a non intertial frame of reference? My book states clearly that the first and second laws cannot, but doesn't mention anything about the third law.

13. Aug 30, 2008

### i_island0

Newton's laws: means all the three laws...not just 1st or second.

14. Aug 30, 2008

### Staff: Mentor

"Real" forces obey Newton's third law; inertial ("fictitious") forces do not.

15. Aug 30, 2008

### i_island0

very true, thats why when we write constraint equation, we do so in inertial frame always, else we end up with ambiguous results