Tuesday, December 9, 2014

Newton's Laws and the Hybrid Car

I think the biggest reason I purchased a hybrid car was my fascination with the engineering that gives the car such good efficiency. Understanding the basic physics of the car possibly might increase the mpg that your car obtains, this past summer I had reached a 53.5 mpg average. The following post is my attempt to relate some basic physics to how the hybrid car functions. If there are great inaccuracies let me know.

Newton's First Law – object at rest will remain at rest unless acted on by an unbalanced force. An object in motion continues in motion with the same speed and in the same direction unless acted upon by an unbalanced force.   Law of Inertia

This means that a car that is not moving will take a force to get it moving, this normally is provided by converting chemical energy into mechanical energy by the burning of gasoline in an internal combustion engine. Explosions in the cylinders  of the engine rotate a crankshaft that connects to gears and shafts resulting the in drive wheels of the car rotating which moves the car.

Second Law – states that force is proportional to acceleration but acceleration is inversely proportional to mass. The formula Force = Mass x Acceleration relates to this law and the unit of force is the Newton.

Newton’s Second Law relates to how much force is needed to move the car for the mass of the car.  Greater force is needed to accelerate faster and with a larger mass. 

Normally a hybrid car is powered by both a internal combustion engine burning gasoline and an electric motor powered by rechargeable batteries.  Batteries are recharged by the internal combustion engine but also through capturing “kinetic energy” of the moving car, this is known as “regenerative braking”.  

In the hybrid car when it is coasting or braking, the internal combustion engine is turned off and the kinetic energy is converted to electrical energy and stored in batteries. This can be seen in the Energy Monitor display in a Prius hybrid car, the dashed line with the arrow points from the front wheels to the battery. 

Accelerating from a Stop in a Hybrid Car

Thinking of Newton’s First Law again when accelerating the hybrid car, when you gently press on the accelerator pedal the car will start moving with the electric motor. Some of the electricity supplied from the battery may have come from what was generated in the coast and the process of stopping before the car came to rest.

In the Energy Monitor diagram above the arrow points from the battery through the electric motor and towards the front wheels.

Once the car is moving less force is needed to keep it moving. Based on many factors the internal combustion engine will start to help power the car at some point. If the accelerator pedal is depressed hard from the start, the internal combustion engine starts right away along with the electric.

In this diagram the brown arrows indicate the internal combustion engine is helping to power the car and charge the battery.

Bill Kuhl


  1. As a normal man I have completely no idea what is relation in between Newton's Law and hybrid cars; but when scientifically think over it we can understand the relation in between these two concepts. Here also in this article we can get some useful description about hybrid car and its character; I am really glad to know the relation in between Newton's Law and hybrid vehicle. Thanks for such an informative article.

    1. Thanks for commenting Tim. If you understand the physics of what is going on you should be able to increase the average miles per gallon. I try to maximize the kinetic energy to charge the battery while coasting or braking and start up gently so the gas engine does not start right away when there are not other cars behind me.

  2. Batteries are recharged by the internal combustion engine but also through capturing “kinetic energy” of the moving car, hybrid

  3. I am responding to both this and your four year ownership update article. Thank you for sharing your observations with us. I have only recently had the opportunity to ride in a hybrid car. I agree with you that the behind the dash workings of the energy monitor was very interesting to me as well. I had the same thoughts concerning energy transfer and recovery. I hope that we can see the technology put the giant oil cartels out of business in our lifetime and see a major reduction of air pollution from automobiles. I had originally subscribed for your model aircraft ramblings. Please keep sending us all of these posts and thanks once again. Jim in Oregon

  4. Well we are getting bit confuse from the title of this article as "newtons laws and hybrid cars". According to my knowledge there will no direct relation in between these two concepts; but while following the above article I can get my answers in details that how newton laws are equivalent with hybrid cars. Thanks for such wonderful comparison.
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