# Newton's first law of motion

by hell18
Tags: motion, newton
 P: 21 I've recently brought a physics book, and in it are some questions, which go throughout different chapters. at the moment i am on Newton's first law of motion. I looked at one of the questions on it and it asks " Explain how a car can skid on a icy road?" I remember reading in the book that when force of the tyre is on the ice it has no direction, the car tyre wont stop skidding, but when it comes to a rough surface, the tyre will stop skidding because the surface is giving force back at an equal and oppsoite force from each other, making the car stop skidding. I have a feeling i have included a bit of newton's second law of motion if i am correct that is? thanks
 PF Patron Sci Advisor Emeritus P: 10,400 The fist law: A body will remain in uniform motion unless acted upon by a force. The friction between tire and ice is slight -- so there is little force on the car -- so the car will continue in its original direction of motion, unimpeded, and fly off a cliff. - Warren
 PF Patron Sci Advisor Thanks Emeritus P: 38,413 You "included a bit of Newton's second law of motion" when you included the part about "when it comes to rough surface" which is not relevant to the question: it only asked about ON the ice. By the way, it is not a matter of the force having "no direction". As long as the car is on the ice there is NO force on the tyre horizontally.
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## Newton's first law of motion

 Originally posted by chroot The fist law: A body will remain in uniform motion unless acted upon by a force. The friction between tire and ice is slight -- so there is little force on the car -- so the car will continue in its original direction of motion, unimpeded, and fly off a cliff. - Warren
actually i think it's a case of the car not getting anywhere, certainly not off a cliff, the fist law applies because without friction the car has no force going forward, hence skidding and getting nowhere
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umm...I'm not even sure if I am right since I haven't even got into physics yet, but if the first law was
 A body will remain in uniform motion unless acted upon by a force.
Then how does a car skidding on ice show the first law? unless the car was pushed or something...

Besides i thought things skidded on the ice because ice does not have any friction so it can't stop an object from moving until the object slows down itself right?
 PF Patron Sci Advisor Thanks Emeritus P: 38,413 The car was already moving when it got on the ice! The point is that since there is no friction force, you can't stop! That IS the first law!
 P: n/a To answer the question: "Explain how a car can skid on a icy road" The assumption the question makes is that the car is already in motion. For any object to skid on any surface requires friction. The point of the question is to demonstrate Newton's 1st Law that has been previously stated and to identify that an icy road although approaching a frictionless state still does have friction. Therefore, the car skids on the icy road by applying the brakes as normal or waiting until the slight friction of the icy surface brings the car to a stop. The first law of motion involves the concept of inertia, and since there are still inertial forces acting on the car, there will be a resistance to motion until it results in a constant equilibrium state. Since we are not in space, then in this instance the uniform or constant state is being motionless.
 P: n/a ok everyone is very confused, let's all have a lesson in how friction makes a car move. everyone is assuming the car is a solid object with no moving parts, thus if a force is applied to it on a surface with no friction it will continue in uniform and constant motion as newton described. HOWEVER this is not how cars work, right, if it weren't for friction cars would get nowhere, reason; the wheel of a car is pushing in the direction opposite that of the movement, ie it is pushing backwards (imagine a wheel, imagine the way it rotates TO THE BACK OF THE CAR) it is the friction of the wheel and road that prevent the wheel achieving this backwards motion and the car is propelled forward instead, by the force of friction NOT OF THE WHEEL it just so happens these forces are of equal magnitude. So when we say a car skids (not slides, glides, slips but skids) on an icy road it is implying it doesn't get anywhere! can you imagine trying to get a car to move on an icy surface, the wheels spin but the car doesn't move. this demonstrates newton's first law because the car will remain in equilibrium ie not move until a force ie friction is applied.
 P: n/a I didn't think I made the assumption that the car has no moving parts. A car can skid on any surface. Skidding results from a loss of friction, ie turning sharply on a wet road whilst travelling fast. The car's wheels lose grip on the road and can no longer push off from the ground. If the car was already moving then the car would continue at its initial speed but slow down more slowly than on a normal tarmac road because of decreased friction. Therefore, the skid on a icy road would be prolonged and more likely to occur. Another approach at the original question would be to ask, how would you avoid skidding on an icy road? That is, maintain a level of friction between the road and tyre by travelling at a significantly reduced speed. Braking would need to be more subtle and gradually applied.

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