# Why don't we consider internal forces for motion? [duplicate]

I read an example in my physics book and was not satisfied with that.

Suppose there is a car initially at rest. If the car accelerates then my book says that it is the friction force only which caused this acceleration since it is the only external force acting on the car. But I was not satisfied with that.

What I think is that the motion of the car was the result of both friction as well as the force exerted by the engine on the wheels. If there was no friction then the wheels would rotate only and will not move forward. But what if there was no engine and no external force was given except friction. The car would not move in both the cases. Doesn't it mean that for acceleration both internal as well as external force is needed?

If it is correct then why don't we count the internal force?

If I am wrong, then correct me.

• Does this answer your question? Does a car use friction to move?
– user87745
May 17, 2020 at 5:41
• But the wheels rotate due to the torque which was internal force
– user262060
May 17, 2020 at 5:47
• Friction can't accelerate any body. It just holds the wheels.
– user262060
May 17, 2020 at 5:48
• So why don't we count that internal force ?
– user262060
May 17, 2020 at 5:48

If you want to count the force from the engine on the wheels, then by Newton's third law you should also count the force from the wheels back on the engine. Again by Newton's third law, these two forces have equal magnitude and opposite directions.

When we want to find the acceleration of the car, we use Newton's second law and must sum all the forces that act on the car. But all these internal forces will come in pairs that cancel when summing them, like the case with the engine <> wheel pair.

The key point is that if you want to start counting internal forces, you cannot be selective and only count one of them :)

Fundamentally, what is required to move the car is an external force which is applied on it. It doesn't matter if that force is classified as friction, air pressure, someone kicking the car, gravity. What matters is that there exists an external force. Some of these involve something inside the car some do not. If the car were on a red carpet, and a strong-man pulled the red carpet along, the car would be accelerated by the friction forces between the wheels and the carpet, without any engine at all.

What is really important about internal forces is that if you have an internal force, its equal and opposite reactionary force points in the opposite direction, and sums to zero. This is why they cannot cause acceleration. If you are in the car, and push on the seat as hard as you can with your feet, it will not cause you nor the car to go anywhere.

If the internal force of the car was all it took, then "spinning your wheels" would not be a thing. If someone guns their engine on a wet road, the engine is free to put as much force/torque on the wheels as they please. It may spin the wheels faster and faster, but it won't make the car move until they apply a force on the environment.

What makes some of this tricky is that, in a normal situation, a car goes forward. We think about cars operating in a setting where the physics works to make them go forward, and we make sense of cars there. But put them in more unusual enviornments, such as on wet roads, and we quickly find out what was the important part.

Your understanding of what an internal force is is the actual. problem.

Internal forces does not mean forces generated inside a body. If it was so, what you said is entirety correct. An "external'' force can only be created by an "internal'' force and both are necessary. But that is not what internal forces mean.

Internal forces are those which act only within (inside) a body. Say you trying to push the car from inside : Here the force you exert is experienced by an object (car) which is within the system.

When the car engine runs, ultimately the wheels push the ground. The ground is not a part of the system. Similarly, the friction from ground which prevents the wheels from slipping is acting on the wheels, which is not a part of the ground. Thus, both these forces - engine's force and frictional force are acting on an object that is not a part of its system. Hence both these forces are external.

Now comes the point of perspective. From the ground's perspective, it is the engine that is exerting a force on ground and for you in the car, it is the ground that is pushing. That is why the book says frictional force is the only external force.

Just for knowledge

I would say both are external but then comes consequences- if both are external, the net external force is zero.. so nothing should move! To prevent such misconceptions, they keep it simple in the beginning. The answer to this is that the forces act on different bodies - (friction on the car and engine on ground) and at a time, only one force is observable. If I watch everything from outside. space, with just you, your car and earth, I would see your car moving over earth as well as earth very slightly move back keeping the centre of mass of the whole system at the same point.

Just think of it as

• "external" = "between object A and object B"
• "internal" = "between object and itself"

Using Newton's laws there will only be acceleration in the first case.

For something to accelerate itself, yes, there have to be internal and external forces. This applies to cars, walking, skating, etc.