FORCES AND MOTIONIntro #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 (theamount 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. Blue – Forceb. Green – Positionc. Red – Velocityd. 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 contactB. 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 forceb. 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 Direction

1.Gravity (weight)

F(G)  = mobject*g        (9.8 m/s2) Downward

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

4.Friction F(fr)

not slipping:  between zero and s*F(N) slipping: = k*F(N)

Direction opposing motion.

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.

Websitesa. Free-body diagramsb. Newton’s Lawsc. More Of Newtond. More free Body diagrams