Physics Chapter 4 Forces and Motion
Tom Adams will teach the following concepts:
The Concepts of Force and Net Force:
Inertia and Newton’s First Law of Motion
Newton’s Second Law of Motion
Newton’s Third Law of Motion
More on Newton’s Laws: Free-Body Diagrams and Translational Equilibrium Friction
A force is something that is capable of changing an object’s state of motion, that is, changing its velocity.
Any particular force may not actually change an object’s state of motion, as there may be other forces that prevent it from doing so.
However, if the net force—the vector sum of all forces acting on the object—is not zero, the velocity will indeed change.
We distinguish two types of forces:
A contact force, such as a push or pull, friction, tension from a rope or string, and so on.
A force that acts at a distance, such as gravity, the magnetic force, or the electric force.
According to Aristotle, the natural state of objects was to be at rest, and if you got them moving, eventually they would come to rest again.
Galileo did experiments rolling balls down and up inclined planes, and realized that, in the absence of some kind of force, an object would keep moving forever once it got started.
Galileo called this inertia:
Inertia is the natural tendency of an object to maintain a state of rest or to remain in uniform motion in a straight line (constant velocity).
Later, Newton realized that mass is a measure of inertia.
Newton’s first law is sometimes called the law of inertia:
In the absence of an unbalanced applied force (Fnet = 0), a body at rest remains at rest, and a body already in motion remains in motion with a constant velocity (constant speed and direction).
Experiments show that the acceleration of an object is proportional to the force exerted on it and inversely proportional to its mass.
The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass. The direction of the acceleration is in the direction of the applied net force.
The units of force are called newtons.
1 N = 1 kg . m/s2.
An object’s weight is the force exerted on it by gravity.
Here, g is the acceleration of gravity:
g = 9.81 m/s2
Weight therefore has the same units as force—newtons.
Newton’s second law may be applied to a system as a whole, or to any part of a system. It is important to be clear about what system or part you are considering!
Newton’s second law applies separately to each component of the force.
For every force (action), there is an equal and opposite force (reaction).
Note that the action and reaction forces act on different objects.
This image shows how a block exerts a downward force on a table; the table exerts an equal and opposite force on the block, called the normal force N.
A free-body diagram draws the forces on an object as though they all act at a given point. You should draw such a diagram whenever you are solving second-law problems.
If an object is to be in translational equilibrium, there must be no net force on it. This translates into three separate requirements—that there be no force in the x-direction, the y-direction, or the z-direction.
The force of friction always opposes the direction of motion (or of the direction the motion would be in the absence of friction).
Depending on the circumstances, friction may be desirable or undesirable.
Types of friction:
Static friction: when the frictional force is large enough to prevent motion
Kinetic friction: when two surfaces are sliding along each other
Rolling friction: when an object is rolling without slipping
We observe that the frictional force is proportional to the normal force. For static friction:
The constant μs is called the coefficient of static friction.
The static frictional force may not have its maximum value; its value is such that the object does not move, and depends on the physical circumstances.
For kinetic friction:
The constant μk is called the coefficient of kinetic friction, and is usually smaller than μs.
The coefficients of friction depend on both materials involved.
This form for the frictional force is an approximation; the actual phenomenon is very complicated. The coefficient of friction may vary somewhat with speed; there may be some dependence on the surface area of the objects.
Also, remember that these equations are for the magnitude of the frictional force—it is always perpendicular to the normal force. (Why?)
Air resistance is another form of friction. It depends on an object’s shape and size, as well as its speed. Purchase a TI-84 CE Calculator:
https://amzn.to/3i2EgDm
In this tutorial, Tom Adams will show you how to use your TI-84 Plus CE calculator.