Introduction to Mechanics

Mechanics It has nothing to do with the people you call when your car needs to be repaired. It is the study of motion.

Historical Development of Mechanics Aristotle vs. Galileo

He said that we must first understand why objects move. Aristotle He said that we must first understand why objects move.

Aristotle Things move because they “desire” to do so. Light things “desire” to rise to the heavens. Heavy things “desire” to sink to earth. In short, objects have a natural tendency.

Early scientists like Aristotle were called natural philosophers.

Galileo Galileo said that we should first study how things move, and then we should describe why they move.

Stationary things react to pushes and pulls. Mechanics the study of motion Dynamics Kinematics Statics Why? cause How? Stationary things react to pushes and pulls.

Mechanics is the study of life. motion. work. systems. Question

T/F Aristotle believed that we should first determine why things move. Question

an artificial boundary used to isolate an object or objects System an artificial boundary used to isolate an object or objects

everything outside of the system Surroundings everything outside of the system

Systems Scientists are free to select any system as they study the motion of objects. Examples: you, your desk, the floor you and your desk

Frame of Reference When a car zooms by you, it is moving.

Frame of Reference But if you are in the car, it seems that the car is standing still and everything else is speeding past the windows.

your frame of reference What’s the difference? your frame of reference

you (How self-centered!) Frame of Reference What is THE frame of reference? you (How self-centered!) the earth the sun the galaxy

Frame of Reference There is no “THE frame of reference.” Choose the best frame of reference for the problem being solved.

Frame of Reference The frame of reference you choose Sun Earth North Pole The frame of reference you choose determines how the motion will be described.

Kinds of Reference Frames Fixed—the reference frame is stationary, but the system moves. Accelerated—the reference frame accelerates with the system. Rotational—the reference frame accelerates, but the system is stationary.

Coordinate Axis Number Line Zero is the origin. Negative numbers are to the left of the origin. Positive numbers are to the right of the origin.

non-physical continuum that orders the sequence of events Time non-physical continuum that orders the sequence of events

Time sometimes called the space-time continuum created by God Before time was, God is. “I AM.”

Time Any event that happens must occur within a span of time. The start of that time span is called the initial time (ti). The end of that time span is called the final time (tf).

Time The difference between the initial and final time is the time interval. It is called Δt (“delta tee”) and is found by subtracting the initial time from the final time.

Question What is another name for a coordinate axis? fulcrum space-time continuum number line reference frame Question

measurement that has a magnitude (amount) with no direction indicated Scalar measurement that has a magnitude (amount) with no direction indicated Examples: 13 m 47 km/h

This paper has a measurement of 215.7 mm. Scalar Since the smallest measurement is zero, scalars never have a negative magnitude. This paper has a measurement of 215.7 mm.

measurement that has both a magnitude and a direction Vectors measurement that has both a magnitude and a direction Examples: 13 m forward 47 km/h ENE

The magnitude part of a vector is considered to be a scalar. Vectors The magnitude part of a vector is considered to be a scalar.

Vectors Vectors are shown on the coordinate axis by an arrow. The length indicates the magnitude. The arrowhead indicates the direction. force (F) weight (w)

T/F Scalar measurements have magnitude and direction. Question