Work & Speed March 1, 2010

Work or Not Work? Problem #1: –D.J. carries a backpack from the cafeteria to the 8 th 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 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