Concept Summary Batesville High School Physics

1. Motion Concept Summary Batesville High School Physics

2. 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).

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

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

5. 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

6. 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.

7. 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.

8. 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

9. 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

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

11. 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

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

13. 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

14. Velocity • Velocity = speed + direction • 2 kinds of velocity • Average velocity = average speed + direction • Instantaneous velocity = instantaneous speed + direction

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

16. 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.

17. 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.

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

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

20. 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

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

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

23. 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).

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

25. The End