How can a car reverse directions without using any energy?

Question

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?

In: 5

Answer 1

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.