Friday, Feb 13, 2009 - Block 2

Gave students average speed problem to work on.
A plane flies 200 miles at a speed 80 miles/hr. It then immediately turns around and flies back at a speed of 120 miles/hr.
What is the total distance traveled?
What is the displacement?
What is the average speed?
What is the average velocity?

Most students gave the average speed as 100 miles/hr but it should be 96 miles/hr since average speed = total distance/total time.

Gave students the bike and bee problem. There are often many ways of doing a problem, some easy, some hard. Think about the problem and choose an easy way - life is too short.

Went over Seattle example.
Showed that we get the equations d = v * t, v = d/t, t = d/v
Mentioned that this is the average speed since you probably were not traveling at the same constant speed for the entire trip.

A rate is some quantity divided by time. V is a rate, the rate at which you cover distance.

Went around the room and asked students to give additional examples of rates, something that changes with time.

Hourly wage is the rate at which you make money. The amount you make is the rate * time. Amount you have = amount you started with + rate * time.

Suppose you have a job that pays $10/hr. This is the rate at which you make money.
Suppose the next year, they pay you $15/hr, and the year after that, $20/hr. Note that your hourly wage is increasing by $5/hr/year. This is the rate of a rate.

Did examples of driving a car, speeding up from 45 mph to 55 mph in 2 seconds. The rate at which you increase speed is (vf-vi)/t = 5 miles/hr/sec.

We call this change in velocity/time (the rate at which you change velocity) the acceleration. Acceleration is any change in VELOCITY/time. You are accelerating if you speed up, slow down, or change direction.

If the acceleration is in the same direction as the velocity, you speed up.
If the acceleration is opposite in direction to the acceleration, you slow down.
If the acceleration is perpendicular to the velocity, you don't change your speed, but you do change direction.

Driving the car example. You FEEL acceleration but you do not FEEL constant velocity.

Near the surface of the Earth, the acceleration of gravity is about 10 m/s/s down. This is the rate at which a dropped object picks up speed.

Did picket fence demo to show not only how to use the computer and how a photogate works but also how to measure the acceleration of gravity. We measured about 9.76 m/s/s.

Went around the room and students answers how fast an object would be traveling after falling for a given time.

vf = vi + a * t which we got from the definition of acceleration.

Did several examples of dropping an object, throwing an object up, throwing an object down.

Referred back to the Seattle Example: d = v* t
This gives the displacement, but what do we use for v since it is not constant?

If the acceleration is constant, as it is near the surface of the Earth, then we can use vavg = (vi+vf)/2

d = vavg*t = (vi+vf)/2 * t

Did several examples of finding displacement for dropping, throwing up, or throwing down.

Handed out RA 2.2, 2.3, 2.4
RA 2.2 is due on Tuesday, RA 2.3, 2.4 are due on Wednesday