How to use friction?

Question

I'm having trouble understanding how ot figure out the role of friction in problems. How can I find out what frictional force is acting on an object or the magnitude of a force used to move an object? Which equations can I use to help me with problems involving friction?

Answer 1

friction is used to stop cars ( the breaks causing friction with the wheel)

Answer 2

Friction depends mainly on the nature of the bodies in contact. Whether they are metal, rubber, wood, etc. (they have different coefficients of friction) and also there are different kinds of friction depending whether the objects are sliding, rotating, or one is gaseous, or liquid, etc.

Also, friction at rest is higher than when the objects start moving.

For sliding objects, the easiest way to determine friction is by pulling an object with a dynamometer (spring balance) until it starts moving.

Another way is tilting an inclined plane and measure its angle just when an object on it starts sliding, then calculating the friction force that was holding it from sliding.

Answer 3

Do you know what a Normal force is? Newton's 3rd law = equal but opposite reaction and stuff like that.



Anyway, in your physics problems, any kind of contact force (like a book on a shelf) will have 2 forces constantly working on it, gravity, and the normal. gravity is the force that the book acts on the shelf, and the normal is the reactive force of the shelf acting on the book. Friction in most of your physics problems will be based on this normal force, and it is proportional by some constant, mu.



example: pushing a book on shelf in x dir



net forces in y dir:



-mg + N = 0 (book not moving)



net forces in x dir:



-f = ma (force needed to move book)



f = mu*N



play with those equations to solve for force (ma)



More in-depth problems involve friction in fluid flow, heat transfer, and noise losses. Basically all friction problems involve some transfer of energy from one state (kinetic) to another (heat, sound, work, etc...)