Chapter 2 Notes I

Describing Motion Motion, Speed and Velocity

Motion -What does it mean for an object to be in motion?

Motion: an object changes position

relative to a reference point.

Frame of reference

Motion is the changing of distance

between two objects.

Is the brick wall moving?

Not from where she’s sitting, but…

Motion What does it mean for

an object to be in motion?

…from space, the earth rotates and the wall with it.

So, whether or not something is moving

depends on your frame of reference.

SCALAR

VECTOR Oh Yeah

A measurement WITHOUT direction

Examples: time, temp, mass, speed

Measurement that has both MAGNITUDE and DIRECTION

Examples: velocity, displacement

Magnitude *** fancy physics word alert***

A measurement or number

Example: an object is moving at 10 m/s, what is the magnitude of the object’s speed? 10

RATE

a change in a given quantity over a specified period of time

Example: velocity and acceleration

Distance (d)

total amount an object travels SCALAR UNIT is meters (m)

What is the distance traveled by Bob the Bear ?

Displacement (Δd)

objects final position HOW FAR IT IS FROM WHERE IT STARTED VECTOR Unit is meters (m)

DISTANCE VS DISPLACEMENT

Started at 2 m NE and ended at 10 m

NE so the final displacement is 10 m NE

20 m

What is the displacement for Frank the Fish?

Example Problem 1:

A man in a car travels east along a straight road for 54 km, then

realizes he is almost out of gas, and turns around and goes 14 km back

west to the nearest gas station. He then drives another 50 km east to

his destination. What is the man’s displacement?

1st: 0m 54 km

? 54km

40 km 90km

Total displacement is 90 km E

Total distance traveled is 54+14+50 = 118 km

A softball player leaves the batter’s box, overruns first base by 3.0 m and then returns to first base. Compared to the total distance traveled by the player, the magnitude of the players total displacement from the batter box is smaller / larger/ the same? The player traveled 30 m past 1st base and then 30 m back to 1st meaning his total distance traveled was 60 m more than his displacement.

Example Problem 2:

1st

Example Problem 3:

Travis needs his physics notes. He walks from his house 8

yards due east towards Shannon’s house to borrow her

notes. At this point Travis realizes he left his phone at home

and runs back to get it. After picking up his phone he

continues the 14 yards east to Shannon’s house. What was

the distance and displacement?

Distance = 8yds + 8yds + 14 yds = 30 yds

Displacement = +8yds + -8yds + +14 yds = +14 yds or 14 yds East

You can describe the motion of an object by its position, speed, direction, and acceleration.

Speed and Velocity

SPEED: SCALAR Distance/time Units: m/s

VELOCITY VECTOR displacement/time Units: m/s

Both are the rate at which an object moves

FORMULA

COMMON UNITS miles per hour (mi/h) kilometers per hour (km/h) centimeters per day

Dd Dt

vavg =

Change in position Change in time

The car below is traveling at a

constant speed of 100 mph. Is the

car’s velocity the same?

The speedometer of a car moving northward reads 60 km/h. It passes another car that travels southward at 60 km/h. Do both cars have the same speed? Do they have the same velocity?

Answer: Both cars have the same speed, but they have opposite velocities because they are moving in opposite directions.

A racing car driven by Speed E. Demon travels 480

kilometers in 2.0 hours. Calculate the average speed in

km/hr and convert to m/s.

A racing car driven by Speed E. Demon travels 480 kilometers in 2.0 hours. Calculate the average speed in km/hr and convert to m/s.

Step 3

Show substitution (with units) and answer

v = 480 km= 240 km/hr

2 hrs

Step 1 List Variables d= 480 km v = ? t = 2 hrs

Step 2 Show Formula (arranged to solve for unknown) v = d

t Step 4 This problem requires dimensional analysis 240km 1000m 1hr = 33.33 m/s 2 hr 1 km 3600s

FIRST STEP IN SOLVING PHYSICS PROBLEMS… MAKE SURE ALL OF YOUR UNITS MATCH.

A dolphin swims 15 km/hr. How far will it travel

in 5 minutes? 25 seconds?

Sunday Driver takes her Cadillac for a spin and travels 50.0 km at an average speed of 35.0 m/s. How long (in seconds) was she driving her car? 1428.57 sec Remember to change km to m! t = d/v NOT dv

Distance is a scalar quantity, which means that is only a magnitude. It is how far you are from a reference point.

Displacement is a vector quantity, which means it needs both magnitude and direction. It is how far you ended up from where you started and in what direction.

EX

For instance, if you leave home and drive from Houston to Dallas, your displacement is approximately 200 mi, North. When you return to Houston at the end of the trip, your displacement is now zero because you are now back at the place you started.

Summary: Distance vs. Displacement

Summary: Velocity vs. Speed Speed is a scalar quantity. It is how fast you are

traveling. When you look at the speedometer on your car, it tells you the speed you are traveling at that moment.

Velocity is a vector quantity. It is how fast you are going and in what direction. If you are traveling to Dallas, you may be traveling at 70 mi/hr, North.

http://www.thinglink.com/scene/385974212231168000

distance

How fast

an object

is moving

40 mph

direction

scalar

Distance/time

speedometer

vector

40 mph north

displacement

displacement

/time

Tutorial Links http://www.problemsphysics.com/mechanics/motion/velo

_spe_tutorials.html

Tutorial for help with velocity and speed

http://www.problemsphysics.com/mechanics/motion/disp_dist_tutorials.html

Tutorial for help on distance vs. displacement