CHAPTER 5

This chapter is about the study of kinematics and introduces to you that subject of dynamics.

NEWTON'S LAWS OF MOTION

Force can be positive or negative depending on the direction of the force. There are four forces:

  • gravitational force
  • electromagnetic force
  • strong nuclear force
  • weak nuclear force
The forces are good to know about, but they are relatively unimportant to memorize:

Gravitational force is the weakest of the four forces. It is the pull between objects that have mass. All objects exert this force, even us humans, but it is so weak that we do not realize it.

Electromagnetic force gives materials their strength, their flexibility (ability to bend), squeeze, stretch or shatter. This is so because of the electrons in the objects. Objects at rest exert electric charges on each other, when these particles move, it produces magnetic forces. It is much larger than gravitational forces.

Strong nuclear force holds the particles in a nucleus together. It is the strongest of the four forces, but only acts over a small distance, which is the size of a nucleus. This force is hudreds of times stronger than the electromagnetic force.

Weak nuclear force, is actually a form of electromagnetic force. It is involved in the radioavtive decay of some nuclei.

NEWTON'S FIRST LAW OF MOTION

Newton's first law of motion is quite simple, any object that is moving, tends to keep on moving, unless an outside force is exerted on it, such as friction. Any object that is at rest tends to stay at rest, unless a force is exerted upon it.

EXAMPLES:


This is an example of Newton's first law (an object at rest). There is no force exerted upon it. The result is equilibrium.



This is
NEWTON'S SECOND LAW OF MOTION

Newton also theorized that if any object has a net force exerted upon it, then it accelerates. It is summed up in the formula:

F = ma

IMPORTANT NOTE: the units for force (F) is newtons (N), which is the equivalent to (kg * (m/s²))

NEWTON'S THIRD LAW OF MOTION

When an object exerts a force on a second object, the second object exerts a force on the first object that is equal in magnitude, but in the opposite direction. For example, if you punch the wall, the wall will exert all of that force that you used to punch it back at your hand. The force from the first object is called the action force, and the force from the second object is called the reaction force. The action froce is always equal to the reaction force (action-reaction forces).

MASS AND WEIGHT

I remember when i first learned physics, I found out that mass and weight were 2 totally different things. Mass is a scalar quantity (with no direction), and weight is a vector quantity. Weight is found by multiplying the mass of the object by the gravity (Earth's gravity is a constant 9.8 m/s² down).

This section also covers:

m = F / a

It can be used to find the mass of an object. For example, if the same force was exerted on two different object, and the first object accelerated at twice the speed of the second one, we can conclude that the mass of the first object is ½ that of the second object.

FRICTION

Friction is that force that opposes movement between two surfaces that are in contact. This force is parallel to the surface, and the direction is opposed to the force acting upon it, for example, I pull an object toward me, friction is in the opposite direction of my pulling.

There are two different type of friction:

  • static friction
  • kinetic friction (described as being sliding friction in the book)

Static friction is the force that opposes the start of motion, whereas kinetic friction is the force between surfaces in relative motion (meaning, while it's moving). There are new terms that are described in the book, and pictures that go with them, you should look at the book to see the diagrams, but I will explain briefly what each new term is:

  • FA is the force that is applied onto the object (pushing/pulling on the object)
  • FF is the force of friction it is µ * FN
  • FN is the normal force, it is the force that is holding the book up (the surface exerts it), it is always perpendicular to the surface (if the surface is horizontal, then the FN is mg
µ (mu) is the coefficient of friction, it is a constant that depends on the two surfaces in contact. It is perpendicular to the surface, so if the surface was on an angle, the FN (normal force) is not straight up. In these basic problems though, FN is equal to m*g (mass times gravity (9.8 m/s²)) since mg is perpendicular to the surface.

If there is a force on an object, the acceleration is found using the formula:

a = F / m

The force is divided by the mass, quite simple. This formula is also the re-arranged form of F = ma, so it shouldn't be too hard to remember.

In an ideal world (where there is no air resistance) all objects would fall at the same speed. A hammer would fall just as fast as a feather. Terminal velocity is explained as well, it is the velocity of a falling object when the force of air resistance is equal to the weight. It is best to read the book for furthur explanation.