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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