Something I don’t quite understand from a physics standpoint. Imagine a car going down a road approaching a roundabout, on neutral; it can follow the roundabout 180 degrees and start going in the reverse direction while only losing a little speed/energy. But the car did a lot of “work” in the physics sense — a multi-thousand-pound vehicle completely reversed direction in a few seconds. How is that energy redistributed (force diagram, etc) to show where the energy for all that work came from?
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It loses quite a lot of energy and speed. The straighter the line the less speed you lose. The more change to the line you introduce the more speed will be shed. The lateral forces exerted on the car during turns and moves is where the speed is lost. I would also add that you have ‘circled back’ rather than reversed direction. The energy for all that ‘work’ was present at the starting conditions. The loss in speed post-turn is dependant on the traction and amount of directional change.
The main force is applied perpendicularly to the movement, so no work is actually done. You’re basically deflecting the velocity, not really changing it, in a simplified model.
Mind you, in real life, there are a ton of factors screwing everything up, including how the rubber on the wheels behave, as well as the differential, plus a bunch of elasticity eveywhere.
Firstly, as was mentioned, there’s no physics-work done if the acceleration is perpendicular to the movement, which is what happens when you go in a circle.
Secondly, the energy doesn’t go anywhere in an idealized setup, because energy isn’t a vector quantity. A car going the same speed in the opposite direction still has the same kinetic energy.
If you’re looking for where the momentum went, there’s a force exerted on the road by the tires to keep the car going in a circle. The force on the car is toward the center of the circle, like the orbit of a moon or when you swing a mass on a string. So the road and the planet it’s attached to will have their momentum changed very slightly in the opposite direction.
Imagine a ball swinging on a string. Obviously no work is done, since it is easy to keep the ball swinging. The car is the ball.
The energy before and after is the same – while speed comes with a direction energy does not. If the force on it pushes sideways, neither speeding it up nor slowing it down, then no work is done.
The energy stored in the car = the mass of the car times it’s speed
The energy got there by burning fuel or rolling down a hill
Coasting around a roundabout in neutral changes the direction of the car
The tyre’s warm up as they respond to the turn due to the friction of the road
The amount of heat generated in the tyres is a tiny fraction of the total energy stored in the moving car.
TL;DR It might look like the car changed direction without using energy but it did cost a tiny bit of momentum in that the tyres warmed up due to friction
Well, we know where it came from; the existing momentum of the car and the newly added chemical energy of the wondrous engine. The force is “normal” to the direction of travel during the turn (think ball on string twirling around or planetary motion).
If you go 180 around a roundabout, your speed is going to be cut in half if not more. You lose tons of energy through heat and sound of the tires and the drive train turning sharper. Most modern CV joints are extremely tight and inefficient while turning.
Think of it like this.
When you skateboard down a U ramp, you come up the other side right? That is because as you go around the ramp, you are constantly changing that downwards momentum into sideways momentum at the cost of some of the energy.
Now think how much sideways momentum you would get if you just dropped from a building, NONE. That energy has to be FULLY stopped and then re added in the opposite direction rather than deflecting the energy around a curve.
Think about if when running in soccer, instead of running and then turning 45 degrees mid run, you had to stop running, turn 45 degrees, and then get back up to speed.
Ok- let’s start with a ball going down one of those gravity toys. They don’t lose any speed when they go around turns, right? Because there’s no where for the energy to go except into keeping the ball moving forward. Steel rolling on steel leaves very little friction- it just rolls.
Energy is never “lost”, remember, conservation of energy isn’t just a rule, it’s the law.
So now let’s compare a car on the road and a ball on a track. The ball turns because the track turns. The force of the ball pushes against the track but the track is stronger and guides the ball. With nowhere for the energy to go, the ball keeps rolling.
The rolling car also has energy- kinetic energy. Instead of a track, it rolls on wheels- where does the force come from to make it turn?
Friction- specifically between the ground and the tires. Wheels spin in one direction and when you turn them, they push the ground and the body “apart” until the car is lined up with the wheels direction and there’s no force being applied on the sides of the wheel, just forward momentum. It’s easier for the wheels to “push” the car to the side than for the car to continue forward, because of the friction of the tires.
But it *is* using energy- that is, energy is changing state. The tires generate heat in the turn. Go touch a set of tires after a car has been driving- they’re hot! Racecar tires can get so hot the rubber melts.
So the reason it looks like there isn’t much energy taken away from the cars momentum is because cars moving have *a lot* of momentum. They’re very heavy and can move very fast. They don’t lose a lot of energy in turns because the system has a lot of lubrication- wheel bearings- to make it roll very easily and maintain that energy, and because tires are very good at minimizing losses to heat.
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