What are the definitons of force and the definitions of Dynamic and static friction?

Question

I need to know what is the meaning of force how it works and what it measures in. Also what are the definitions of dynamic and static friction and how is this realted to force. e.g how much force do u need to put in something with X amount of friction. The teacher said something about dynamic friction minus static friction but i did not underdstand? plzz helpp

Answer 1

Conceptually, force is defined by Newton's first law. An external net force will change the direction of motion of an object. An object will tend to move at a constant velocity in the same direction when there is no net force acting on the object. Force changes the motion of an object.

Friction is basically a force that acts on an object opposite the direction of an applied force or the direction of motion depending on the situation.

Static friction is the friction caused by a stationary object and is related to the amount of force you need to put on an object at rest to start motion. If I want to move my desk, there is a certain amount of force I must put on the desk before the desk moves.

Dynamic (or kinetic) friction is the friction caused by a moving object. It is related to the amount of force needed to keep an object moving at a constant velocity. If I am driving down the highway and set my cruise to 65 mph, my tires must put some amount of force on the highway to keep me moving at a constant velocity.

I am not sure what your teacher means by dynamic friction minus static friction as this is not a useful physical quantity.

Answer 2

I would like to give the following definition of force:

Force is what Newton's laws tell us about it. Nothing more and nothing less. Nothing less means if we do not understand all the laws correctly then we may have certain incorrect ideas about force. Nothing more means what we think about force intuitively without referring those ideas to Newton's laws at all.

There is dichotomy between first and second law on one hand and the third law on other. Where as the first and second law in a way defines the net force acting on any object and this net force can be calculated on the basis of kinematics of the object. the laws are completely silent about the single or multiple causes of force/forces. Whereas the third law tells us that any force that occurs in a situation is a result of interaction between two objects. And this interaction consists of two forces equal and opposite of one acting on the other and vice-a-verse.