Motionturn to 2.1 in your physics booklet

January 22nd 2009

Rate

Rate - a change in some quantity divided by time

•For example:

• heart rate

• birth rate = babies per year

• speed = miles per hour

Speed

• Speed is the rate at which distance

changes per unit time.

• speed = Δ distance / Δ time

• speed = Δd/Δt (Δ=change in)

Speed

Average speed = total distance / total time

v=Δd/Δt

Instantaneous speed is the speed at a given instant (a small amount of time).

Δt is close to zero

Velocity = speed with a direction.

If either the speed or direction of a object changes, then its velocity changes.

Velocity

Average speed, Instantaneous

speed or Velocity?

Paul got pulled over for going 75mph.

A car is moving 30mph north on Hwy N.

He ran the mile in 5:33 minutes.

distance vs. displacement

Displacement - length from the starting point to the ending point

Distance - length of the path traveled

http://www.upscale.utoronto.ca/GeneralInterest/Harrison/Flash/ClassMechanics/DisplaceDistance/DisplaceDistance.html

Acceleration

Acceleration = rate at which your velocity changes per unit time

a = Δv/Δt

Example Problem

A bike goes from 0-10 m/s in 5 second.

What is its acceleration?

a=? Δv= 10 m/s Δt= 5 s

a = Δv/Δt

a = 10 m/s / 5 s m/s/s=m/s2

a = 2 m/s2

DecelerationWhat is the acceleration of a pitch that slows

from 95-87 m/s in going from the mound to home plate in .41 seconds?

- 19.5 m/s2

Deceleration = negative acceleration

Free Fall Ignore air resistance Affected only by gravity Gravity is symbolized by “g” g=9.8 m/s2

An object in free fall has a constant acceleration of 9.8 m/s2

time acceleration

1 s 9.8 m/s2

2 s 9.8 m/s2

3 s 9.8 m/s2

Free Fall

If you drop a ball off a cliff, how fast would it be going after 1 sec?

v=at

time velocity

0 s 0 m/s

1 s 9.8 m/s

2 s 19.6 m/s

3 s 29.4 m/s

Free Fall If I drop an object, how far has it

fallen after 1 sec? Not 9.8 m 4.9 m

d=½at2

http://www.teachersdomain.org/resource/phy03.sci.phys.mfw.galileoplane/

Equations

Velocity v=Δd/Δt or d=vt

Acceleration a = Δv/Δt

Free Fall (starting from rest) v=at d=½at2

Review

Does the speedometer of a car read instantaneous or average speed?

What is the difference between speed and velocity?

What 3 controls on a car causes a change in velocity?

What is the acceleration of a car that travels at a constant speed of 100km/hr?

Distance vs. Time Graph

slope = velocity neg. slope = change in direction steeper slope= faster

Distance

(m)

Time (s)

Motion Graphs

What does this graph represent?

Motion Graphs

Motion Graphs

Stage 1: 100 m in 10 s. Stage 2: 50 m in 10 s. Stage 3: 150 m in 20 s.

Velocity vs. Time Graph

Slope = Acceleration Neg. Slope = Deceleration Neg. Velocity = change in direction Steeper Slope = > Acceleration

Velocity

(m/s)

Time (s)

Motion Graphs

What is happening here?

..\Animations\motion graphs.avi