Friction Questions: Explaining Static Friction & How it Relates to Walking

[indoguy427]

Homework Statement

I am trying to understand a concept that pops up in my hwk problems about friction, specifically static friction. In an example in my book, it shows that when you turn a car around a corner, you have 3 forces: normal, gravity (these cancel and are in the vertical direction) and static friction (provides the inward centripetal force to allow you to turn in a circle). However, I don't understand how static friction can exist alone as a horizontal force. I thought that static friction results in response to another force. For example, if i try to push a block, static friction force keeps it from moving. However, if the block is just sitting there, there won't be a static friction force. Can someone please explain this to me?


Also, when we walk, I understand that when I push the ground, the ground pushes back on me and that allows me to walk forward. I don't understand how friction plays into the FBD of this situation. Is friction the force of the 'ground pushing back on me'? How does friction play into Newton's third laws?

Homework Equations

The Attempt at a Solution

 

[dynamicsolo]

This is the problem with teaching about some forces without explaining to students how those forces come about.

If you think about the surfaces of two objects in contact at the molecular scale, you have irregular-looking arrangements of atoms at the surfaces of each object. In many places, the atoms of each object will be very close to each other, close enough that the valence electrons (the electrons in the outermost orbitals) will not be able to distinguish which atom they "belong to". So molecular bonding takes place at those points of "contact" between the two objects.

When the objects are just sitting there, you won't notice this taking place. But when you go to move one object past the other, say, by sliding, you are putting energy into breaking those bonds between the "contacting" atoms. So your applied force will be resisted by those bonds, up to a limiting point. At the large scale, you experience this as a resisting "static frictional force" which has a maximum magnitude, generally expressed as (mu_s)·(normal force), where mu_s is the proportionality constant we call the "coefficient of static friction". This is why "static friction" can vary from zero (no applied force, so no resistance experienced) up to f_s_max .

As for your second question, this static friction must act equally on both objects in contact, and in opposite directions on each object (Newton's Third Law).

Think about what happens when you try to walk on dry sand, wet ice, wet linoleum, etc. (something with very little friction). When you push off with your foot, your shoe sole or skin slides easily along the surface with little resistance, giving you little traction. (This is why fiendish exercise programs make you run on dry sand.) In the limit of frictionlessness, your foot would simply slip along the surface and your body's center-of-mass would go nowhere.

So the static friction is resisting the motion of your foot relative to the surface you are trying to move on. In a free-body diagram, you would show the static friction, f_s, acting on the ground pointing behind you and acting on your foot pointing ahead of you. So the static friction acts to push you forward, to the extent that it is present. The static friction is also acting to push the ground backwards from you; think about which way loose sand or dirt flies underfoot as you are running on it.

 

[Moetasim]

Hello,

I can certainly help explain the concept of static friction and how it relates to walking. First, let's define static friction. Static friction is the force that prevents two surfaces from sliding against each other when there is no relative motion between them. In other words, it is the force that keeps an object at rest from moving when a force is applied to it.

In the example of turning a car around a corner, the static friction force is necessary to provide the centripetal force that allows the car to turn in a circle. This force is provided by the tires of the car gripping onto the road surface. The force of static friction acts in the opposite direction of the car's motion, towards the center of the circle, and is equal in magnitude to the centripetal force needed for the car to turn.

In the case of pushing a block, the static friction force is necessary to prevent the block from sliding. The force of static friction is always equal and opposite to the force applied to the block, up until the maximum static friction force is reached. This maximum force is typically greater than the force applied, which is why the block does not move. Once the maximum static friction force is reached, the block will start to move and the force of kinetic friction will take over.

Now, let's discuss how friction plays into walking. When we walk, we push off the ground with our feet, and the ground pushes back on us with an equal and opposite force. This is known as Newton's third law of motion. Friction comes into play because the ground is not a perfectly smooth surface, and there is a slight resistance to our foot pushing off. This resistance is provided by the force of static friction between our foot and the ground. Without this friction, we would not be able to push off and walk forward.

I hope this explanation helps clarify the concept of static friction and its role in both turning a car and walking. It is important to remember that friction is always present in situations where there is contact between two surfaces, and it plays a crucial role in allowing us to move and interact with our environment. Let me know if you have any further questions.