PHYS 100 Chapter Notes - Chapter 4: Electric Field, Free Body Diagram, International System Of Units
10 May 2018
School
Department
Course
Professor
Chapter 4
Development of Force Concept
- Dynamics is the study of the forces that cause objects and systems to move
o Force is defined as a push or pull
- Free-body diagrams are used to illustrate all the external forces that act on a body
o THe body is represented by a single isolated point and only those forces acting on the
body are shown
Newtons First Law of Motion: Intertia
- A body at rest remains at rest, or if in motion, remains in motion at a constant velocity unless
acted on by a net force
o There must be a cause for there to be any change
o Cause and effect is crucially accurate in describing what happens in various situations
- Mass
o The property of a body to remain at rest or to remain in motion with constant velocity is
inertia
āŖ Some objects have more inertia than others
āŖ The inertia of an object is measured by its mass
ā¢ Mass is a measure of the amount of āmatterā in something
ā¢ It is determined by the number of atoms and molecules of various types
it contains
o Mass does not vary with location
Newtons Second Law of Motion: Concept of a System
- Used extensively to calculate what happens in situations involving a force
- Acceleration is defined as a change in velocity
o Net external force causes a change in motion
āŖ Net external force causes acceleration
ā¢ Exgernal force acts from outside the system of interest
- You must define the boundaries of the system before you can determine which forces are external
- The vertical forces are the weight (w), and the supporst of the ground N, and the horizontal force
Friction (f)
o Friction is defined as the force that opposes the motion past each other of obhects that are
touching
āŖ Acceleration is directly proportional to the net external force.
ā¢ A ā Fnet
āŖ It is important to identify the external forces and ignore the internal ones
o Acceleration should be inversely proportional to the mass of the system
āŖ Large the mass, the smaller the acceleration produced by a given force
āŖ Acceleration is exactly inversely proportional to mass, just as it is linearly
proportional to external force.
āŖ Fnet = ma
ā¢ (Mass)(Acceleration)
- Units of Force
o FNet = ma is used to define the units of force in terms of three basic units for mass, length
and time
āŖ SI Units for force is newton (N)
ā¢ 1 N = 1kg . m/s2
- Weight and Gravitational Force
find more resources at oneclass.com
find more resources at oneclass.com
o When an object is dropped, it accelerates toward the center of Earth
o Net force of an object is responsible for its acceleration
o Net force is commonly called its weight (w)
āŖ It is denoted as a vector (w) because it has a direction: Down is the direction of
gravity, and hence weight is a downward force
ā¢ Consider an object with mass (m) falling downward toward Earth. It has
only the downward force of gravity which has magnitude w.
o Since the object experience only downward force of gravity, Fnet
= w.
o We know
object is
due to
gravity is
(g) or a =
g
ā¢ When net external
force is weight, we say it is in free-fall
āŖ When objects fall downward to Earth, they are never truly free-fall because there
is some upward force from the air acting on the object
o Example: Suppose that net external force extered on a lawn mower is 51N parallel to the
ground, the mass of the mower is 24kg, what is the acceleration?
āŖ Fnet = ma
ā¢ Given: M = 24kg
o Fnet = 51N
o A = Fnet/M
āŖ 51/24 = 2.1m/s2
o Example: The sledās intitial acceleration is 49m/s2, the mass is 2100kg, and the force is
650N
āŖ Fnet = ma
āŖ FNet = 4T ā f
āŖ 4T = ma + f
ā¢ Given: Force = 650N
o Mass = 2100kg
o Acceleration: 49 m/s2
o 4T = 2100kg(49m/s2)+650N
āŖ 4T = 1.0x105N
āŖ T = 2.6x104
Newtons Third Law of Motion: Symmetry in Forces
- Whenever one body exerts a force on a second body, the first body experiences a force that is
equal in magnitude and opposite in direction to the force that it exerts
o Forces always occur in pairs and one body cannot exert a force on another without
experiencing a force itself
āŖ Action-reaction
ā¢ The force exerted is the action and the force experimented as a
consequence is the reaction
ā¢ Example: Consider a swimmer pushing off from the side of a pool
o FWall on Feet acts on the wall and not on our system of interest
āŖ The swimmer pushes in the direction opposite to that in
which she wishes to move
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Dynamics is the study of the forces that cause objects and systems to move: force is defined as a push or pull. Free-body diagrams are used to illustrate all the external forces that act on a body: the body is represented by a single isolated point and only those forces acting on the body are shown. It is determined by the number of atoms and molecules of various types it contains. Newtons second law of motion: concept of a system: mass does not vary with location. Used extensively to calculate what happens in situations involving a force. Acceleration is defined as a change in velocity: net external force causes a change in motion, net external force causes acceleration, exgernal force acts from outside the system of interest. You must define the boundaries of the system before you can determine which forces are external.
