advertisement

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 time it takes Or, average speed = displacement time In symbols, v = d or ∆x Units of speed: m/s, km/h, mi/h, etc. 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 It velocity of an object changes if: 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 is the rate velocity (not speed) changes. 2 kinds: Acceleration Average acceleration Instantaneous acceleration Average Acceleration Ave. Accel. = change in velocity time it takes in symbols, a = ∆v t Accelerations are not relative quantities. 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/s2 km/hr/s = km/hr.s m/s/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. The End