 # Concept Summary Batesville High School Physics. Motion of What?  To simplify things as much as possible, we will first consider one-dimensional motion.

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Concept Summary Batesville High School Physics

Motion of What?  To simplify things as much as possible, we will first consider one-dimensional motion (motion along a straight line) of particles (points that can’t spin, rotate, flip, flop, or wiggle around).

Describing Motions  We will concern ourselves (for now) with describing motion - kinematics.  We will worry about explaining motion (dynamics) later.

Two Simple Motions  In our course, we will be primarily concerned describing with 2 simple motions:  Motion with constant velocity  Motion with constant acceleration

Position  Mark a zero point on the line, pick a direction to be positive, and measure from there.  Positions can be positive or negative.  Units of position: centimeters, meters, kilometers, inches, feet, miles, etc.  Common symbol: x

Operational Definition  Position, like other physical quantities, is defined by telling how you go about measuring it - not by giving synonyms or descriptive phrases. This is called an operational definition.

Positions are Relative  Different people can mark the line differently, so they can get different numbers for position.  The position number (and unit) really don’t mean anything until you specify where you marked “0”, and which way you made positive - your frame of reference.

Displacement  Displacement = net distance moved or net change in position  Common symbol, d or ∆x  If you move from x o to x, displacement, d = ∆x = x - x o

Rates  A rate measures how fast something changes.  In physics, a rate is almost always calculated as a quantity divided by time.  Rate Q changes = change in Q time for Q to change

Speed  Speed is the rate position changes, or the rate distance is covered.  There are two kinds of speed:  Average speed  Instantaneous speed

Average Speed  Average speed = distance traveled  Or, average speed = displacement  In symbols, v = d or ∆x  Units of speed: m/s, km/h, mi/h, etc. time it takes time tt

Instantaneous Speed  Instantaneous speed is what the speedometer says.  It is not measured over a time interval, like average speed.

Constant Speed  If an object’s instantaneous speed is always the same value, the object has a constant speed.  In this case, average speed = instantaneous speed

Velocity  Velocity = speed + direction  2 kinds of velocity  Average velocity = average speed + direction  Instantaneous velocity = instantaneous speed + direction

How Velocity Changes  The velocity of an object changes if:  It speeds up, or  It slows down, or  It changes direction.

What Velocity Means  An object’s velocity tells you how fast its position is changing.  5 m/s means the object’s position changes by 5 meters each second.  60 mi/hr means that the object’s position changes by 60 mi each hour.

Velocities are Relative  Speed and velocity are relative quantities. Different observers, in different frames of reference, can measure different velocities.  You measure speed and velocity by comparing two motions.

Acceleration  Acceleration is the rate velocity ( not speed) changes.  2 kinds:  Average acceleration  Instantaneous acceleration

Average Acceleration  Ave. Accel. = change in velocity in symbols, a = ∆v Accelerations are not relative quantities. time it takes t

Units of Acceleration  Since acceleration is a velocity divided by a time, its units are a distance unit divided by 2 time units.  This is commonly written 2 ways:  m/s/s = m/s 2  km/hr/s = km/hr. s

Constant Acceleration  In many common situations, an object’s acceleration is constant, or at least approximately constant.  In this case:  Average accel. = instantaneous accel.

Free Fall  Free fall is motion under the influence of gravity only - no friction or air resistance.

Acceleration in Free Fall  The acceleration of an object in free fall is constant.  At the surface of Earth, the free-fall acceleration is about 10 m/s 2, or 9.8 m/s 2 if you have a calculator (or 32 ft/s 2 or 22 mi/hr/s in “English” units).

Air Resistance  The effect of air resistance is to slow an object down and/or decrease its acceleration.

The End 

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