# Friction and Newton's Third Law

1. Jan 28, 2012

### Zoolog

Hello,

I have a question from a conceptual problem in my textbook as follows:

A car is moving north and speeding up to pass a truck on a level road. The combined contact force exerted on the road by all four tires has vertical component 11.0 kN downward and horizontal component 3.3 kN southward. The drag force exerted on the car by the air is 1.2 kN southward. What is the net force acting on the car?

The answer given in the textbook has the normal and gravitational forces cancelling out; and the net force being 2.1 kN northward on the car; with static friction exceeding the drag force by that much. My confusion is with the source of the friction. If the tires exert 3.3 kN south on the road, and the road exerts a force of 3.3 kN north on the tires where does the friction come in? Is the friction a force in addition to the equal and opposite force? Does the static friction on the car come from moving with the tires to resist this force? Or is the static friction on the car from the road resisting the tires motion?

Thank you

2. Jan 28, 2012

### AlephZero

The "friction" is the pair of opposite 3.3 kN forces. As you said, one force acts on the road and the other force acts on the tires, i.e. on the car.

The question asks you about fhe net force acting on the car.

The force acting on the road does actually change the motion of the earth by a tiny amount, but it's so small that you can ignore it.

3. Jan 28, 2012

### Zoolog

O.K, so if I understand you correctly the friction IS the equal and opposite force from the third law?

4. Jan 28, 2012

### AlephZero

I'm not sure what you mean by "the equal and opposite force" (singular, not plural).

Suppose you hold a block of wood in each hand and rub them together. You can't really say that the "friction force" acts on one block and some other force acts on the other block. I would say there are two equal and opposite friction forces (consistent with the third law), with one force acting on each block.

5. Jan 28, 2012

### Zoolog

I think I understand now. So the normal force is the force equal and opposite to weight and perpendicular to the contact force. "Friction" is the pair of forces parallel to the contact surface?

6. Jan 28, 2012

### Ken G

No, the normal force is a pair of equal and opposite forces on the car and ground, and so is gravity. They are the same magnitude here.
Yes.

7. Jan 28, 2012

### Zoolog

Alright, thank you