Unit 2: Motion-- Speed and Velocity and Acceleration

What’s the difference between distance and displacement?

Distance: the total magnitude traveled (length without direction)

Displacement: the shortest distance from the starting point to the ending point

Position: where an object is, including direction, compared to where it was

Ladybug Example

What is speed?Speed–The rate of change in distance with respect

to time.Non moving objects: speed = 0 m/sUnits: m/s, cm/s, mphSince speed is built from distance, a scalar

quantity, then speed is also a scalar quantity.  Scalar Quantities carry no direction information

with them.

What is Velocity?The rate of change in displacement with respect

to time.  Since displacement is a vector quantity, then

velocity is also a vector quantity.  v=d/t

v= velocity (m/s)d= distance (m)t= time (s) D

V T

Speed & Velocity ExamplesSpeed is distance divided by time

2 cm/s

Velocity includes the direction2 cm/s west

West

What is the difference between average velocity and instantaneous velocity?

Average Velocity: The longer the time period measured, the more it leads to calculating an average velocity.

Instantaneous Velocity: The shorter the time period measured the closer it brings you to calculating an "instantaneous velocity".  Only if the time period becomes zero would we truly have an instantaneous velocity. 

Acceleration“Acceleration” occurs

when your speed changes up or down (or when you change direction).

It is the rate speed changesLevel ground- no

acceleration due to gravity

Steep down hill- acceleration due to gravity

Steeper hill= more acceleration

Deceleration is slowing down

Zero acceleration is at a constant speed AND not changing direction

Acceleration

Can use a graph to show acceleration+ slope = +acceleration (speed up)- slope = - acceleration (slow down)0 slope = no acceleration

Acceleration= change in speed (cm/sec)time (sec)

a = vf-vi

tUnits: cm/sec*sec or cm/sec2

Free Fall- Drop straight downAcceleration due to

gravity 9.8 m/sec2

Terminal Velocity- Highest velocity reached by a

falling objectWhen an object stops

accelerating, but continues to fall

When air resistance balances the pull of gravity

Projectiles

An object that is sent through the air is a projectile

The path of a projectile is called a trajectory

In the absence of air resistance, all objects fall with the same uniform acceleration = 10 m/s2

Two objects of different sizes and weights, dropped from the same height, will hit the ground at the same time.

Resolving VectorsTo add vectors (going in same direction): line up tail to tip

and add the numbers & keep the direction the same. Ex: 4 m N + 3 m N = 7 m N

To subtract vectors (going in opposite directions): Subtract smaller # from larger # and keep direction of the larger #. Ex: 15 m S - 3 m N = 12 m S

If the vectors are at right angles to each other, line up tail to tip and find the hypotenuse (Remember a2 + b2 = c2 and then solve for c) and combine the directionsEx: 3 m N + 4 m E = 32 + 42 = c2 25 = c2 5 m

NE = c