# Friction on a car

I wanted to ask why doesn't the car move on a frictionless surface. Suppose you are on a frictionless surface When you press the accelerator the wheels start rotating and apply a force to the ground. Then the ground should also apply force in the opposite direction, regardless of presence or absence of friction.(Third law) Or the rotating wheels don't apply any force to the ground? But the force by the wheels on the ground should not depend on the friction of the road right? The same thing I can ask for moving on ice. Suppose the ice offers no friction. You are standing on it. You apply some force. The ice applies the same force to you( Third law ) and you move on a frictionless ice. Where is the flaw in my logic ?

Edit 1: Many users are pointing out that I won't be simply able to apply a horizontal force to the ground. (because there is no friction). Now suppose there is some friction and I am walking on the ground. Can I apply some force greater than the limiting value of the friction of the ground? What will happen then? Please explain the physical situations and also how third law holds in each case.

• Ever walk on ice, or on loose dirt? If you try to apply more force than the limiting friction of the surface, your foot slips. Commented Jul 17, 2021 at 16:39
• "Suppose you are on a frictionless surface When you press the accelerator the wheels start rotating and apply a force to the ground.", sorry, right there you have already fallen off the bus. On a frictionless surface the wheels will just spin, and spin, and spin, and spin Commented Jul 17, 2021 at 17:27

Without friction, neither the car’s tyres nor your foot can apply a horizontal force to a horizontal surface. They simply slide across the surface. So there is no horizontal reaction force from the surface either.

• Why the force applied by me depends on the friction of the surface ?
– Ali
Commented Jul 17, 2021 at 7:05
• @Ali Friction is a force that opposes relative motion between two surfaces. If there is no friction then there is no force resisting relative motion between your foot and the ground and so no action-reaction pair. You can move your leg, but this will not move your centre of mass. Commented Jul 17, 2021 at 8:15

That's a good question, and I once had that question in my head when I was younger.

To explain you very very briefly, First, forget about the road. Think about the air. Air friction (or air resistance) makes things to slow down, right? When is it air-frictionless? When there is no air. Hence, frictionless means absence of something to interact, or simply no interactions with other things. And everything is made of atoms so I was talking about atoms. Friction happens when atoms interact. So don't even think that we can push the ground, or even touch the ground (this is why it's a thought experiment). Pushing or touching is collisions (interactions) between atoms. If atoms don't interact with each other, the car should be just passing through the ground. And this is impossible.

My advice is, don't take things too seriously in high school. Just focus on the math. If it makes sense, then that's good enough. More will be answered in university levels. I hope this somewhat answered your question.

• I think you are right on this. Now let's say the ground has some friction. What happens if I apply some force greater than the limiting one. Now, the ground should apply equal and opposite force right ? But to do so it has to exceed the value of limiting friction. What will happen then ?
– Ali
Commented Jul 17, 2021 at 7:14
• That's also a good question. So friction is also kind of normal force. Only the friction equal amount of the applied one will be present. You apply 10N, the friction is 10N. You apply 20N, the friction is 20N. But then the ground's limiting friction is 100N, and you put 110N. That's when you start moving with the rest 10N . If you weight 1kg, then 10N/1kg= 1m/s^2 . You will start moving at 1m/s^2 acceleration.
– user237964
Commented Jul 17, 2021 at 7:20
• I put 110 N on the ground. The ground puts 110 N on me which is greater than 100 N(the limiting value ). How is it possible ? If the ground puts 100 N then the third law gets violated.
– Ali
Commented Jul 17, 2021 at 7:22
• Lol. If I don't take it too seriously, I will end up with no college. I live in India , it is not like in the US.
– Ali
Commented Jul 17, 2021 at 7:25
• Same with the walls. Say a wall had a limiting normal force of 1000N. You push the wall at 100N. The wall will push you back at 100N. You push at 900N, so will the wall. But if you push 1001N, the wall will break (or be pushed by the rest 1N, if the wall has wheels under it)
– user237964
Commented Jul 17, 2021 at 7:28

Your understanding of the third law is correct. So there is only one question left. What kind of forces can you apply on the ground?

1. vertical. this fine, gravity can press you down on the ground
2. Horizontal. This you can ONLY do with friction. In fact this is almost the definition of friction, it’s the horizontal applied along the surface.

So the reason you can’t move sideways without friction is because that is what friction means. When you say there is no friction , what you are really saying is there are no horizontal forces along surfaces.

• One user mentioned I will begin sliding. Can you tell me what is the horizontal acceleration by which I slide ? What is it equal to ?
– Ali
Commented Jul 17, 2021 at 7:19
• I don’t think the user means you would begin slide but rather that with a slippery surface you can’t push off anything. So zero horizontal acceleration Commented Jul 17, 2021 at 7:24
• Oh. I get it now. I misunderstood. Thank you
– Ali
Commented Jul 17, 2021 at 7:26
• Can you tell me what happens if I put a force greater than the limiting value of friction force ? ( Now the ground has friction and I am walking on it )
– Ali
Commented Jul 17, 2021 at 7:27
• No problem. So now things get a bit complicated and we have to be careful. The simple answer is that you go from static to kinetic friction. Kinetic friction is less than static friction.what this means is that if you start going over static friction you will keep slipping, Because now there is even less friction. However that is just a simple model. I won’t pretend to be an expert in this field, things can get very complicated depending on the circumstances! Commented Jul 17, 2021 at 7:31

I wanted to ask why doesn't the car move on a frictionless surface. Suppose you are on a frictionless surface. When you press the accelerator the wheels start rotating and apply a force to the ground.

Which force? No friction = no force applied to ground

Then the ground should also apply force in the opposite direction, regardless of presence or absence of friction.(Third law)

"Opposite" to what? No force = no opposite force

Or the rotating wheels don't apply any force to the ground ?

Exactly. The friction is what turns wheels torque ("circular force") into the linear force which pushes the car. No friction = no force acting on car.

But the force by the wheels on the ground should not depend on the friction of the road right ?

The friction on the road is the ONLY thing which allows wheels to act on the road and to cause the force which phushes the car.

The same thing I can ask for moving on ice. Suppose the ice offers no friction. You are standing on it. You apply some force. The ice applies the same force to you( Third law ) and you move on a frictionless ice. Where is the flaw in my logic ?

On ice, friction is small but not 0; hence a small force opposite to the force of your feet exists and pushes you.

Edit -1: Many users are pointing out that I won't be simply able to apply a horizontal force to the ground.(because there is no friction). Now suppose there is some friction and I am walking on the ground. Can I apply some force greater than the limiting value of the friction of the ground ?

No. This is called "skidding".

What will happen then ?

Once the force of the wheels overcomes the friction force, wheels start skidding. Electronic devices mounted on modern cars prevent this from happening.

Please explain the physical situations and also how third law holds in each case.

Without friction, only throwing away a mass would result in your movement. The mass can be a bullet, or a rock, or hot gas ejected from an engine.