kinetic vs static

[reburbia]

If the rear wheels of a truck drive the truck forward, the frictional force on the tires by the road is static and in the direction of the moving truck
(from examcrackers general physics)


Huh? I'm so confused. If a truck is moving, is the friction on the tires necessarily kinetic? And doesn't friction always oppose motion?

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[theseeker4]

If the rear wheels of a truck drive the truck forward, the frictional force on the tires by the road is static and in the direction of the moving truck
(from examcrackers general physics)

Huh? I'm so confused. If a truck is moving, is the friction on the tires necessarily kinetic? And doesn't friction always oppose motion?

The rubber of the tires is not sliding along the concrete (ie the tires are not spinning and "burning rubber") so the frictional force between those two surfaces is static. Friction between the air and truck, axle and truck, etc is kinetic friction.

Which way is the tire trying to move? Imagine looking at a truck from the driver's side as it drives forward. The wheels are moving counter-clockwise, so the motion of the wheel surface where it touches the road is backward in relation to the motion of the truck. The friction is opposing the motion of the TIRE, which is the interaction we are talking about. Just like when you walk, you are trying to slide your foot backward, and the static friction between your foot and the ground opposes this movement, moving your body forward.

You just have to focus in on exactly what the friction interaction they are talking about is, and not let the fact that the truck is moving one way distract you from the actual interaction.

 

[reburbia]

Ahh ok. The direction part made sense immediately but the kinetic/static distinction still feels tricky even after you explained it. Thanks

Also got a real kick out of your mdapp profile.. welcome back from the law world

 

[theseeker4]

Ahh ok. The direction part made sense immediately but the kinetic/static distinction still feels tricky even after you explained it. Thanks

Also got a real kick out of your mdapp profile.. welcome back from the law world

LOL, thanks, but luckily I was never actually in the law world, or I would probably have a ton more debt than I do now. I had abandoned that path by the time I graduated.

 

[MedPR]

If the rear wheels of a truck drive the truck forward, the frictional force on the tires by the road is static and in the direction of the moving truck
(from examcrackers general physics)


Huh? I'm so confused. If a truck is moving, is the friction on the tires necessarily kinetic? And doesn't friction always oppose motion?

Yes, friction always opposes motion. When you drive a car in the forward direction, the tires push backwards on the road. Hence, friction is in the same direction as your car but in the opposite direction as the tires, which are actually in contact with the road.

Static friction prevents sliding. The tires aren't actually moving at every instant where static friction from the road acts on them.