Transcript Force Diagrams

FORCES AND MOTION
Intro #1
The Accomplishments of Newton
(1642-1727)
We shall concentrate on three developments
1) Newton's Three Laws of Motion
2) The Theory of Universal Gravitation
Newton's First Law of Motion:
I. Every object in a state of uniform motion
tends to remain in that state of motion unless an
external force is applied to it.
This we recognize as essentially Galileo's concept of
inertia, and this is often termed simply the "Law of
Inertia".
Newton's Second Law of Motion:
II. The relationship between an object's mass m, its
acceleration a, and the applied force F is F = ma.
Acceleration and force are vectors (as indicated by their
symbols being displayed in slant bold font); in this law the
direction of the force vector is the same as the direction
of the acceleration vector.
Newton's Third Law of Motion:
III. For every action there is an equal and opposite reaction.
FORCE, SYSTEM, ENVIRONMENT
-Force: a push or pull exerted on an object
-System: The object being acted on
-Environment: The area around the object from which the
force is exerted
FORCE: is a vector quantity because it has magnitude (the
amount of push or pull) and direction ( of the push or pull)
Note: the magnitude and direction can be a sum of
several vectors.
Color Conventions for vector arrows:
a.
b.
c.
d.
Blue – Force
Green – Position
Red – Velocity
Violet - Acceleration
Force is a quantity which is measured using a standard
metric unit known as the Newton. One Newton is the
amount of force required to give a 1-kg mass an
acceleration of 1 m/s2. A Newton is abbreviated by an
"N." If you say "10.0 N," you mean 10.0 Newtons of
force. Thus, the following unit equivalency can be
stated:
Two types of forces exerted by the environment:
A. Contact Force – A force exerted on an object by direct
contact
B. Long-range Force – A force exerted without contact
Contact Forces
Action-at-a-Distance Forces
Frictional Force
Gravitational Force
Tensional Force
Electrical Force
Normal Force
Magnetic Force
Air Resistance Force
Applied Force
Spring Force
Agents of Force:
a. Agent: The specific identifiable source of a force
b. Pictorial Diagrams – are good for identifying agents
acting on objects.
Force Diagrams (Free-body Diagrams)
A force diagram is simply a
diagram showing all the
forces acting on an object,
the force's direction and
its magnitude
It is a simplification of the
picture that shows just the
forces.
Steps for drawing a force diagram:
1. Identify the object you will draw a diagram for.
2. Identify all the forces acting directly on the object
and the object exerting them
3. Draw a dot to represent the object of interest.
4. Draw a vector to represent each force. Draw it in the
direction the force is being exerted, and label it by (a) the type
of force, (b) the object exerting the force, and (c) the object
receiving the force (which will be you object of interest).
5. If the object is stationary or is moving at a constant
velocity, the vectors should graphically add up to zero. If
the object is accelerating, the sum of the vectors should
produce a vector in the same direction as the acceleration.
Types of forces typically used
Force
Symbol
Magnitude
1.Gravity
(weight)
F(G)
= mobject*g
2.Normal
(surface)
F(N)
any (up to breaking load)
perpendicular to
surface
3.Tension
F(T)
any (up to breaking load)
along
string/rope/chain
F(fr)
not slipping: between zero
and ms*F(N)
slipping: = mk*F(N)
Direction opposing
motion.
4.Friction
Direction
(9.8 m/s2)
Downward
Writing down the sum of the forces
1. Identify direction of every force and of acceleration.
2. Pick a coordinate system to minimize the number of things
(forces and acceleration) that must be broken into components,
especially unknown values
3. Draw the components for any forces or acceleration that does
not lie along the X or Y axis, and identify the angle that is given (or
being looked for).
4. Pick one direction and write down all the forces or components
of forces in that direction, using positive and negative signs to
identify those in the positive and negative directions.
5. Set the sum of the forces in that direction as equal to the mass
multiplied by the acceleration in that direction. (If not moving or
moving at a constant velocity in that direction, acceleration will be
zero.)
6. Repeat for the other direction.
Websites
a. Free-body diagrams
b. Newton’s Laws
c. More Of Newton
d. More free Body diagrams