How is power of engine transferred to kinetic energy of car?

Question

Okay, I understand that via the torque from the engine, the wheels push back on the ground, and static friction pushes forward, thus accelerating the car.

However, the force of static friction does no work as it doesn't move at the point of application. The work is said to come from the engine. The engine does work on the car. But how does it “work” really?

Is it through this force balance mentioned above that the power from the engine is thus applied/transformed into the car’s kinetic energy? I don't see how the power delivered to the wheels gets to be kinetic energy of the car without friction doing work.

Probably overthinking or confusing something. Appreciate the help in advance just confused

Answer 1

The friction just provides the grip. The torque of the engine does the work. Its the same when you start running- your legs exert the force and friction just prevents your feet slipping backwards.

Answer 2

Work has two different but equivalent definitions. One is that it is a force applied over a distance, and the other is that work is a transfer of energy. I prefer the second definition, as it is a bit clearer in circumstances like this.

The car accelerates, and therefore there is clearly an increase in KE. But was work done by the road? The road is stationary as are the tires where they meet the road, so according to the first definition of work there was no work. But let’s look deeper, the KE increased, so where did that energy come from? It came from the fuel. So no work was done by the road according to the second definition either. The car’s KE was increased and its PE was decreased, so no energy was transferred by the road.

I find that looking at transfer of energy helps to identify work easier. When you jump the floor does not transfer energy to you, chemical energy changes to KE and gravitational PE. When you slide down a frictionless slide the slide does not transfer energy, but gravity does. Analyzing the forces can be tricky, but analyzing the energy transfer is less so.

If you subdivide the car into separate components then you can see work being done and energy being transferred between the components. The energy from the fuel is first converted to mechanical energy at the crankshaft. At that point there is a force and there is a displacement, so that is where the work is being done first.

Now the energy of the crankshaft is relatively constant (slight rotational KE and increase in temperature, but much smaller than the amount of energy from the engine) so where does that input energy leave the crankshaft? It immediately goes to the rotational motion of the gear box and then the wheels. Finally, the wheel converts the rotational motion of the gears into a linear motion. That linear force goes up from the contact patch to the axle where it is finally converted into KE of the car.

The purpose of the ground is not to do work (it doesn’t) but to convert the rotational motion into linear motion. In that sense, it is just another part of the overall drive train.

Answer 3

The wheels exert a backwards force on the ground, so the ground exerts a forward force on the wheels. That's what actio = reactio tells us.

Since the center of the wheel is (largely) stationary relative to the car, the impulse (force times time) delivered from the ground must be forwarded at the axis of the wheel. Here we have again the axis of the wheel pushing on the car, and the car pushing back on the axis.

Since the two forces that ground and car exert on the wheel are not applied at the same place, they result in a torque that tries to stop the wheels from rotating. Again, this torque is the reaction to the torque provided by the engine (via the gearbox, obviously).

So, we have torque from the engine that causes the wheel to push on the ground and the axle on the car. Each force is countered by an equal reaction force/torque.

Now, lets look at the energies. Energy is force times distance.

• The engine/gears/axle/wheels rotate while transfering torque, so we have work delivered through the drive train (work = torque times angle of rotation).

• The car moves in the ground's frame of reference, so the force that the axle exerts on the car does work on the car.

• The ground does not move, so there's no work done on it within its frame of reference.

You see, while the wheel pushes on both the ground and the car, it only does work on the car, simply because the car can move while the ground is stationary.

If you would put the car on a trailer (one of the trailers which are used to transport cars) and forgot to secure the trailers brake, trying to drive the car off the trailer would instead shoot out the trailer from under the car. In this case, the "ground" would not be stationary, but rather moveable and lighter than the car. So the force the wheels apply on the trailer would make the trailer accelerate faster than the heavier car, which experiences the opposite force. As such, the wheels will do work on the trailer, much more work than on the car. It'll be the trailer that takes away the majority of the energy delivered by the car's engine. Oh, and don't try this at home!

Answer 4

I don't see how the power delivered to the wheels gets to be kinetic energy of the car without friction doing work.

Forces don't have to do work.

Imagine a compressed spring with a mass. The compressed spring has a bit of energy in it. When we release the spring, it can accelerate (do work) on the mass. This seems pretty simple.

But for the mass to move right, the spring has to be held in place. That means the wall is providing the force that moves the mass. It does (almost) zero work, but without the wall, the spring could not do work on the mass, either.

In the case of the spring, the forces from both ends; and in the case of the car, the friction from the road is what couples or binds the two masses together. It allows the energy in the system to do work by separating the masses.

So the engine power delivered to the axle is creating a force on the car (forward) and the ground (backward). The car is trying to do work on both (but because the mass of the ground is so large, we usually ignore the minuscule amount that can be done and assume all the work is done on the car).