When exiting off of an interstate, why does the car seem to go from 75mph to 50mph faster than 25mph to 0mph? The road composition, grade of the road, or the tires on the vehicle never changed, yet the car seems to maintain its velocity at the slower speeds.
In: Physics
For one, automatic transmissions will provide power to the wheels while in gear and idle (as in not pressing the gas). This will naturally keep the car moving forward at *some* speed, though it tends to be pretty low for most vehicles.
But a big factor is air resistance. Your car meets a whole lot more air in a given period of time at 75, and 50, than it does at 25. That air pushes against the car trying to move through it (or rather the car pushes into the air in front of it). The higher the speed the more resistance your car will meet, if you stop providing power (let off the gas) the high resistance of having a high speed will work to lower the speed more quickly than low resistance.
Go outside in some high winds and walk into the wind. Now run into the wind. Now turn around and walk/run with the wind. Which one is easier? You car goes through a similar effect, and the more speed it has the “faster” the wind becomes.
Air resistance is helping you far more in the 75-50 example. Air resistance is a function of speed^2, so going twice as fast gives you 4x the air resistance, which always helps you slow down.
So I’ll preface that I’m NOT a physics professor or anything. That being said – I know that the faster something is going, the more energy it takes to KEEP it going that speed. That means the faster something with car like conditions is going (friction, air resistance) the more it “wants” to slow down.
There’s a lot of forces working against a car maintaining 75 mph where at 25 mph those slowing down forces are in effect at a much lower level.
Hopefully that makes sense.
In addition to what previous posts have said about air resistance, the brakes also play and important role. Brakes are fairly efficient at converting your kinetic energy into heat slowing you down quickly, while most cars use ICEs. That car can only accelerate at its quickest at peak power, which takes time for the engine to wind up.
Now, to directly answer why brakes help, is that they convert kinetic energy (movement) to heat via friction. Your brakes clamp down on of the rotor which spins with the wheels. Which means the faster the rotor spins the more heat generated therefore a faster slowdown.
I am not going to go into the physics via equations and stuff, but simply, when you are breaking the forward moment force stops. When you press the brakes the faster the speed of the wheels the faster you can remove the energy, so therefore you decelerate quicker because you can remove that energy quicker. Plus you also have tire resistance.
It’s been a while since I’ve done the actual math for this but if I remember correctly this should all be correct in a general sense
There are several things going on here. It sounds like you are not the driver, is that correct? Maybe you’re describing your own experience? Not sure.
First, the driver is the one in control of the brakes. When you are exiting the highway, you often are traveling into a more restricted situation: low visibility curves, lower ramp speed limits, unfamiliar exits. This makes the driver more proactively want to slow down. The driver will brake to a speed where they feel they can control the car. For me this is an automatic response, I don’t “think” to slow down fast.
As others have mentioned, automatic transmissions provide some non-zero force to the wheels, even when the accelerator is not pressed. The only way to counter this force is to apply the brake. As the speed slows, the “impulse” provided by the transmission becomes more of a factor. AT cars will never come to a complete stop until the brakes are applied strongly enough, except perhaps on a steep enough uphill.
Drivers intuitively know how their cars will react as they approach a stop, so they let the car do its thing until the stop is required. They apply the brake to allow the car to stop smoothly, and this takes some muscle memory. Driving someone else’s car will take some getting used to for this reason.
I find all these sciency answers quite interesting. I’m thinking it has more to do with the amount of break you apply. At slower speeds you break alot more gently to come to a stop than you do to rapidly reduce speed leaving the interstate.
The limit of braking is tire grip. If you lose grip and start to slide, you’re not slowing down efficiently. Under heavy braking at high speed, the car leans forward a lot and puts weight on the front tires. The extra weight gives the front tires more grip. As the car loses speed, it loses weight in the front giving you less grip. If you continued braking hard, the car would want to slide. You hopefully aren’t near the grip limit on public roads but, faster cars can brake harder without sliding.
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