You are in your car and are driving in a horizontal, straight line at a constant speed. Is your mechanical energy conserved?

Question

gebooors · Answer

Which forces are acting in this situation?

anonymous · Answer

friction, gravity, normal force, and the car's engine

gebooors · Answer

And maybe air resistance, but in can be including in friction.

So, what happens to force engine produces?

anonymous · Answer

it pushes back on the earth, and the earth pushes back on it

gebooors · Answer

Yes, and it is equal to frictional force

anonymous · Answer

they cancel each other out

gebooors · Answer

And which form energy is converted by frictional force?

anonymous · Answer

kinetic

gebooors · Answer

Tired warm up, so friction produces heat.
It is not possible to convert back to kinetic energy.

gebooors · Answer

I means tires of the car warm up

anonymous · Answer

so its not conserved because of friction? Thats what I thought! but i got it wrong :(

gebooors · Answer

Yes, you're right! Friction is a noin-conservative force.

anonymous · Answer

hmm then I wonder why then the answer is that it is conserved?

gebooors · Answer

If there is friction it is not conserved.
If car rises up to hill, kinetic energy is converted to potential energy and then
Energy is conserved ( if there is no friction). In small speeds air resistance is small.

anonymous · Answer

The question states " you are in your car" and asks for "your" mechanical energy. Since you are not doing any work yourself and the systems under consideration is only you and not the entire car. You can safely assume that since the motion is horizontal and linear your mechanical energy is not changing at all thus it has to be conserved. The discussion above is all correct but deals with the energy of the car while nothing is said about the internal object (you inside the car) asked about.

anonymous · Answer

mechanical energy should be conserved as the person is not doing any work

anonymous · Answer

This is what my professor said: The condition for the conservation of mechanical energy is that the sum of kinetic and potential energies, defined as the mechanical energy, stay the same. In the problem there is energy loss through friction but it is being replaced by the conversion of the chemical energy in the car's gasoline into kinetic energy, replacing what is lost through friction.

anonymous · Answer

exactly