I’m no expert in cars and how they work, which is definitely why I don’t know the answer to this question. What I don’t get is how cars, even when going down a slope at high speed, can gradually slow down and freeze, even when on a steep slope. Why does it not roll down? I don’t get how the tires freeze the car like glue, like shouldn’t it move slightly at least?
EDIT: I definitely worded this badly lmao. I was more interested in how cars on steep slopes don’t move on slopes, because wouldn’t an object without wheels still slide downwards when on a slope?
EDIT 2: Thanks for answering the question. It makes sense now that the friction of the rubber tires and the material on the road is what keeps it so still when brakes are applied on slopes.
In: Physics
Cars can move on their wheels in two ways: either the wheels roll, or they don’t. If the wheels don’t roll, then the car will have to slide on its tires.
Braking applies some kind of friction to the wheels (e.g. by pushing brake pads against the rims) in order to slow their rolling. This will have one of two effects. As long as there is sufficient friction between the tires and the road, the forward momentum of the car over the road means that the road surface will push backward on the tires, and thus keep them rolling. Applying brakes that slow the rolling applies a force in the opposite direction against the road surface, i.e. a force that pushes backward on the car, against its direction of travel. If you apply this force long enough, the car will slow to a stop.
If there isn’t enough friction between the tires and the ground, then another scenario may occur. If the brakes apply more friction against the wheels than the road does, then the wheels may lock up (stop rolling) before the car has stopped moving forward. This sends the car into a skid, where it will slide over the driving surface. Eventually, the car will still come to a stop, due to friction between the tires and the ground.
Skidding is more likely to happen when there is little friction between the tires and the ground, such as when driving on worn-out tires, or when driving on slippery surfaces (e.g. sandy, wet or icy roads, or various off-road surfaces like sand or mud). It presents a risk, because when the wheels lock up, you cannot steer the car – it will (mostly) happily keep sliding in the same direction regardless of how the front wheels are oriented. To maintain steering, the wheels have to roll. This is why modern cars are equipped with anti-lock braking systems (ABS), which stop the wheels from locking up. If a speed sensor detects that the wheels have slowed to a stop too quickly, ABS will briefly and rhythmically release the brakes, intermittently, even as the driver keeps applying the brakes. This allows the wheels to keep rolling enough to maintain steering. In vehicles without ABS, you can achieve a similar effect by rhythmically “pumping” the brakes (i.e., intermittently taking your foot off the brake pedal), though human drivers can’t do this as efficiently as an automated system can.
Okay, now let’s say you’ve come to a halt, and you’re still applying brakes (either the regular brake, or a parking or handbrake). This provides friction on the wheels. Any force that tries to get the car moving again (like gravity pulling it down a slope) would have to either overcome this friction (in which case the car, or rather the wheels, will start rolling), or the friction between the tires and the ground (sending the car into a skid). As long as it does neither, the car will stay where it is and not even move a little.
If the car is on a level, non-slippery surface, then parking brakes may not even be strictly necessary to keep the car from moving, as there is always some internal friction on the wheels, even if the car isn’t in gear.
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