# Work Done and Force and Newtons Third Law of Motion

Peter G.
When a car is moving, the engine is transferring Chemical Energy to Kinetic Energy. He is doing work against the force of friction. If the force of friction is 210 N and the engine exerts a pull force for 10 M it transferred 2100J of Chemical Energy from its fuel to Kinetic Energy, considering no energy is lost in the process

Similarly, I guess, when we are skiing down a mountain we gradually convert the GPE energy we had uphill into Kinetic Energy (but we are doing no work right?) and some of the Kinetic Energy is doing work to deplete the snow sideways. So some of the Kinetic Energy is being lost, used to exert a force (push?) on the snow over a distance.

(Correct any of this if its wrong please!)

So my doubt is, when a car is moving it has a lot of kinetic energy. When it hits the wall, it stops moving, thus, loses all of the kinetic energy. But energy can't be destroyed, it was converted into another form right? So when the car hits a wall it exerts a pushing force over a distance (Since it transferred energy, did work on the wall) and due to Newton's Third Law the same force it exerted on the wall, the wall exerts it back on him in the opposite and therefore damages the car.

Thanks

## Answers and Replies

Homework Helper
Similarly, I guess, when we are skiing down a mountain we gradually convert the GPE energy we had uphill into Kinetic Energy (but we are doing no work right?)
The force of gravity is doing work on the skier, applying a downward force through the distance that the skier skis to accelerate the skier and/or to move snow around.
...some of the Kinetic Energy is doing work to deplete the snow sideways. So some of the Kinetic Energy is being lost, used to exert a force (push?) on the snow over a distance.
correct.
So my doubt is, when a car is moving it has a lot of kinetic energy. When it hits the wall, it stops moving, thus, loses all of the kinetic energy. But energy can't be destroyed, it was converted into another form right?
Correct. Mostly heat.

So when the car hits a wall it exerts a pushing force over a distance (Since it transferred energy, did work on the wall) and due to Newton's Third Law the same force it exerted on the wall, the wall exerts it back on him in the opposite and therefore damages the car.
If the wall is fixed to the ground, the car and the earth pushes back on the car and virtually all the kinetic energy of the car is lost as heat. If the car hits a moveable barrier the barrier will carry away some of the car's kinetic energy.

AM

Peter G.
But sometimes energy is transformed into other forms without we having to do work, right? I think I tricked myself into believing that Energy transferred (work) is the same as converting energy into a different form: To check if I am right, I will give an example:

Using the example of lifting a book up to a shelf: First, chemical energy from food is CONVERTED into kinetic energy. Then, we use the kinetic energy of our muscles to do WORK push the book upwards, TRANSFERRING energy from kinetic to GPE, correct?

Thanks.

Homework Helper
But sometimes energy is transformed into other forms without we having to do work, right? I think I tricked myself into believing that Energy transferred (work) is the same as converting energy into a different form: To check if I am right, I will give an example:

Using the example of lifting a book up to a shelf: First, chemical energy from food is CONVERTED into kinetic energy. Then, we use the kinetic energy of our muscles to do WORK push the book upwards, TRANSFERRING energy from kinetic to GPE, correct?
Better to stick to clear definitions. Energy is defined as the ability to do work. That does not mean "useful" mechanical work (as in a heat engine). Thermal energy, being the total kinetic energies of all the constituent molecules, may be considered as energy - capable of doing work - despite the fact that only a fraction of that energy may be converted into useful mechanical work.

Kinetic energy that is transformed into Gravitational Potential Energy certainly involves work. Work must be done to lift a mass m through a vertical distance h: W = mgh. The ability to perform that work may come from many types of energy: chemical energy in the muscles causing cells to contract against an opposing force is an example of chemical energy doing work. Those muscle contractions may be converted into kinetic energy of an object by throwing it, or into gravitational PE by lifting the object.

AM