Work & SpeedMarch 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 = F x D

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 changes2. 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