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Motion. 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.
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Motion 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 xo to x, displacement, d = ∆x = x - xo
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 t t
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/s2 • 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/s2, or 9.8 m/s2 if you have a calculator (or 32 ft/s2 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.