can someone please explain to me the difference when it comes to a car moving down a road. i am doing Ek # 41 and i thought the rear tires were experiencing kinetic but it is static. when is it kinetic and not static. thanks again
The car scenario is a tricky one, because it seems like it would be kinetic because it's moving. But if you think about it, the tires are turning and making solid contact with the ground, rather than sliding across it. So it is still static friction, unless it starts talking about the tires skidding across the ground.
Here's a great real life example so you don't forget this.
Cars these days are built with ABS dynamics, anti-lock braking system.
static friction will always be greater than kinetic friction.
if your wheels lock while you step down on the brake real hard, real quick, your tires are stopping/skidding using kinetic friction. This causes your tire treads to wear out quicker and makes you stop in a longer distance.
ABS dynamics makes it so that when you step real hard on the brake, your brakes are pumped making sure your wheels don't start locking and skidding, allowing you to use entirely static friction to stop the car.
but don't take my word for it. try it out for yourself, and you've probably perceived this as well, just not realized it that your car pumps a couple of times to a stop if you're trying to brake in a really short distance.