Transcript Work & Speed

Work & Speed
March 1, 2010
Work or Not Work?
• Problem #1:
–D.J. carries a backpack from the
cafeteria to the 8th grade hallway.
• Problem #2:
–Austin applies force to (pushes) a
brick wall.
Work or Not Work?
• Problem #3:
–Mykia pushes a stack of books
across the floor.
• Problem #4:
–Alexis wears a backpack and climbs
up the side of a mountain.
• We have learned that
forces can change both
the direction and speed
of an object.
–For example…
To determine speed:
Average speed = distance
time
Practice Problem #1:
--Daniela drives a car 100 miles (m) in 2
hours (h). Using the formula for speed,
determine Daniela’s average speed
during that time.
Practice Problem #2:
--Alonzo jogs to his house 72 meters (m)
away in a time of 36 seconds (s). What
is Alonzo’s average speed?
Practice Problem #3:
--If you travel 7.5 kilometers (km) and
walk for 1.5 hours (h), what is your
average speed?
Practice Problem #4:
--An airplane traveling from Atlanta to
Las Vegas travels 1,260 km in 3.5 h.
What is the airplane’s average speed?
To leave class…
•
Which of these statements is true
about a moving object that increases
its speed?
A. The object’s mass increased.
B. The object is not accelerating.
C. A force is acting on the object.
D. No forces are acting on the object.
To review from yesterday…
• Practice Problem Z:
• If you rode your bike 2 hours and traveled
24 miles, what is your rate of speed?
Now let’s find time and
distance…
Example #1:
--On vacation to Florida, Alexia’s family
traveled 300 miles at 30 mph. How
long did the trip take?
Example #2:
--Cristian drove his truck from Nashville
to Chicago at a rate of 30 miles per
hour. How far could Cristian drive in 3
hours?
Example #3:
--Daniel drove his red Dodge Charger at
an average speed of 100 kilometers
per hour for 10 hours on a country
road. How far did Daniel travel?
Units of Measurement
• Speed=miles/hour or meters/second
• Distance=meters, miles, kilometers…
• Work=joules or newton-meters
• Force=newtons
Wednesday Example #1:
• A snail moves a distance of 8 m in 2
hours. Calculate the average speed of the
snail.
Wednesday Example #2:
• Tammie cycles 20 miles per hour on her
bike for 4 hours. How far does Tammie
cycle in this time?
•
How long does it take to travel:
a. 120 miles at 40 mph
b. 60 miles at 40 mph
c. 390 miles at 60 mph
•
Calculate the distance that you would travel
if you drove for:
a. 3 hours at 20 mph
b. 8 hours at 60 mph
c. 1.5 hours at 45 mph
The equation for WORK is…
work= force x distance
or
W=FxD
Work Problem #1:
--Kammi uses a lever to lift a large stone
from the ground. Kammi applies a force of
50 newtons and moves the stone 5
meters. How much work is being done by
Kammi?
Work Problem #2:
--Mariah wants to move a stack of
magazines from one room of her house to
another. Using a wheel and axle, Mariah
applies a force of 100 newtons and moves
the stack 10 meters. How much work is
being done by Mariah?
The equation for FORCE is…
Force = work
_________
distance
or
F= W
______
D
Force problem #1:
--Tabetha performed 100 joules of work
by moving a pile of rocks 10 meters
with a lever. What was the force on the
rocks?
Force Problem #2:
-Cameron performed 500 joules of work
by lifting a car 50 meters with a pulley.
What was the force on the car?
VELOCITY
• The measure of an object’s
SPEED and DIRECTION
For example: a person walks 5 m/h
east or a bird flies 10 km/s north
Think…
• Imagine that two birds leave the
same tree at the same time.
They both fly at 10 km/h for 5
min, 12 km/h for 8 min, and 5
km/h for 10 min. Why don’t they
end up at the same place?
Velocity changes if:
1. Speed changes
2. Direction changes
For example, as a bus traveling at
15 m/s south speeds up to 20
m/s south, its velocity changes.
Acceleration
• The rate at which velocity
changes (either speed or
direction)
Which of the following is NOT
an example of acceleration?
A. a person jogging at 3 m/s along a
winding path
B. a car stopping at a stop sign
C. a cheetah running 27 m/s east
D. a plane taking off