chapter 2

Chapter 2Motion and Speed

Sec. 1 Describing MotionMotion occurs when an object

changes position.You don’t need to see it move to

know that motion has happened.–You just need a reference point (text p 38)–A reference point is a coordinate system

in which the position of the objects is measured.

Frame of reference:

https://www.youtube.com/watch?v=Y75kEf8xLxI

PSCS 8 minutes

Distance and Displacement Distance—how far an object moved

The SI unit for distance is the meter,m Displacement—the distance and

direction of an object’s change in position from the starting point.

Distance and Displacement graphic

Distance vs. Displacement http://

www.bing.com/videos/search?q=distance+vs+displacement+animation&FORM=VIRE1#view=detail&mid=F95E0B662B56B1874E89F95E0B662B56B1874E89

• Distance and Displacement can be the same.

• Distance can be longer than Displacement

• Displacement cannot be longer than Distance

Speed Speed—the distance an object travels per

unit of time.Speed = distance s = d

time t

Ex. You ran 2 km in 10 min. What is your speed?s = 2 km s=0.2 km

10 min min

Speed, Distance examples

https://www.youtube.com/watch?v=5e6QDSo_8W8

Average & Instantaneous Speed

Average speed—total distance traveled divided by total travel time.–Used when speed is changing

Instantaneous speed—the speed at a given point in time.

Which one does a speedometer in a car measure?

Velocity Speed describes only how fast

something is moving. Velocity—includes the speed of an

object AND the direction of its motion– Ex: 40 mi/hr North or 5 m/s up

Change in Velocity Velocity can change in 2 ways

1. Change in speed

2. Change in direction

Or Both Ex: a race car has a constant speed

of 100km/hr around an oval track.– Speed is constant, but velocity

changes

Graphing Motion On a distance vs. time graph, time

goes on the x-axis and distance goes on the y-axis.

The slope (steepness) of the line represents the speed of the object.

Distance

Time↓Speed

Positive SpeedPositive Speed: Moving away

from the origin or reference point–Graph has a positive (upward)

slope

Constant, Positive Speed

•Straight Line•Speed isn’t changing

Fast vs. Slow Positive SpeedFast: has a steep slope Slow: has a shallow,

gradual slope

Changing, Positive Speed

Speeding Up

•Graph has a curve• The slope is changing, so speed is changing

Slowing Down

Negative SpeedNegative Speed: Moving

toward the origin or reference point–Graph has a negative

(downward) slope

Constant, Negative speed

Fast vs. Slow Negative SpeedFast: steep,

downward slopeSlow: shallow, gradual downward slope

Changing, Negative Speed

Not Moving (0 speed)A horizontal line (zero slope) indicates no motion (or no change from the origin)

Time (seconds)

Dis

tan

ce

(m

ete

rs)

Which one is the fastest? Which isn’t moving?

Rearranging the speed equation The original speed equation is s=d/t This can be rearranged to solve for d or t. s = d

t d = s x t (how far?) t = d (how long?)

s

#1 How long will it take a bike rider to travel

450 meters at a constant speed of 2 m/s?

#2 If a car traveled 2500 m in 20 minutes,

what is the average speed of the car?

#3 If a bug travels for 50 seconds at 4 meters

per second, how far does it travel?

Acceleration Acceleration: Change in velocity over

time When something speeds up, it has a

positive acceleration When something slows down, it has a

negative acceleration Units for acceleration have one distance

unit and 2 time units–Example Units: m/s/s or m/s2, mi/hr/min

Changing Direction A change in velocity

can be either a change in speed or a change in direction.

Any time an object changes direction, its velocity changes and it is accelerating.

Calculating Acceleration Acceleration Equation:

acceleration = change in velocity

time

Change in velocity = final velocity (vf) – initial

velocity(vi) We can write the acceleration

equation as a = (vf – vi)

t

(vf -vi)a t

Graphing Acceleration•On a Velocity vs Time Graph, the slope of the line is the acceleration

•Positive acceleration has an upward slope

Positive Acceleration (speeding up)

+ ac

cele

ratio

n

Negative Acceleration(Slowing Down)

•Negative acceleration has a downward slope

- acceleration

Zero Acceleration When an object has a constant velocity, it has 0 acceleration, so the graph is a horizontal line.

time

velo

city

0 acceleration

An Object Changing Acceleration

Acceleration Example #1 An airplane starts at rest and speeds up

to 80m/s at the end of the runway in 20 s. What is its acceleration?– a = (vf – vi) = (80 m/s – 0 m/s)

t 20 s

= 4 m/s2

The airplane is speeding up, so the acceleration is positive.

Acceleration Example #2 A skateboarder is moving at 3 m/s

and comes to a stop in 2 s. What is the acceleration?a = (vf – vi) = (0 m/s – 3 m/s) = -1.5

m/s2

t 2 s

The skateboarder slowed down, so the acceleration is negative.

Note: words like at rest and stop mean velocity = 0.