MotionIntro with Tim and Moby

Describing Motion

•Whether or not an object is in motion depends on the reference point you choose.

Reference point (Frame of reference): background used to tell if an object is moving

Average Speed

Speed is the rate at which an object moves.

•To find speed you need two bits of information Total Distance the object moves Total Time it takes to move that

distance

Calculating average speed

• Formula for speed

Key:

s = speed

d = distance

t = timet

ds

Calculating Speed(a.k.a Average speed)

• A car travels 150 kilometers in 3 hours. What is the average* speed of the car?

• Formula:

• Substitutions:

• Answer (with units!):

– *average means the car goes slower sometimes, faster sometimes, and may even stop during the trip

Velocity

• Speed in a given direction– Mostly used for airplanes to include direction

and speed– “450 km/hr NORTH”– Example: Storm movements to know how fast

and what direction they travel• “storm moving at 10 km per hour from the west”

• Velocity video clip

Acceleration

• The rate at which speed changes– “How fast is the speed

changing?”

• Acceleration is a change in speed– Positive acceleration is an

increase in speed– Negative acceleration is a

decrease in speed (deceleration)

Calculating Acceleration• To determine the acceleration of an

object moving in a straight line, you must calculate the change in speed per unit of time.

Starting velocity

Vs

Final velocity

Vf

Calculating Acceleration

a = vf – vs

t

a = acceleration

vf = final or ending speed

vs = starting speed

t = time

Acceleration = Final Velocity – Starting velocity

time

Acceleration is always expressed in units with velocity (ex. Meters per second, km/hr, etc.) over time (seconds, minutes,etc).

t

VonAccelerati

Acceleration is the change in

velocity over a period of time

An airplane takes off down the runway. It started at 0 m/s and accelerated to 24 m/s in 3.0 seconds time. How fast did it accelerate?

What information is given?

•Starting (initial) velocity of 0 m/s

•Final velocity of 24 m/s

•Time of 3.0 seconds

What is being asked?

•Find the acceleration.

What formula will solve this?•a = Final – starting

timea = 24 m/s – 0 m/s

3.0 sa = 24 m/s 3.0 sa = 8 m/s or 8 m/s/s or 8 m/s2

s

• What is being asked?– Find the acceleration.

• What formula will solve this?

• a = vfinal – vstarting

t• a = 18m/s – 4 m/s

2.0 s• a = 14 m/s

2.0 s• a = 7 m/s or 7 m/s/s or 7m/s2

s

What information is given?

•Starting (initial) velocity of 4 m/s

•Final velocity of 18m/s

•Time of 2.0 seconds

A roller coaster accelerates from 4 m/s to 18 m/s in 2 seconds. How fast did it accelerate?

Negative Acceleration(Deceleration)

• The same roller coaster comes back into the start house and stops. If it started at 18 kilometers per hour and took 6 seconds to come to a complete stop, what is its acceleration?

• Formula:

• Substitutions:

• Answer (with units!):

•Gravity accelerates all objects towards the Earth at the same rate

(9.8 meters per second per second)

(that’s 32 feet per second per second)

•Free Fall: the only force acting on an object is gravity as it accelerates towards earth

Gravity and acceleration

Air resistance • Opposing force of air on an object

– A type of friction

• Also known as “drag” or air friction• Terminal Velocity: the greatest

velocity a falling object can reach due to drag

• free fall down = air resistance upward• Mythbusters:Feather and Hammer video

An elephant and feather fall at the same rate (9.8 m/s/s) with no air friction

With air resistance:

Summary

•How fast = velocity.•How far = distance.•How quickly “how fast” changes = acceleration