Monday, November 24, 2008 - Block 4A

Handed back ball and ramp labs.

We found that when the ball rolled down the ramp, it picked up speed.
Today we are going to investigate what happens when an object picks up speed.

Content Objectives:
Student will be able to
. Define velocity
. Define and calculate acceleration
. Measure the acceleration of gravity in this classroom
. Calculate how fast an object is falling after a given time when dropped from rest
. Calculate how far a dropped object has fallen in a given time

Speed - how fast
If you are driving to Seattle at 60 mph, does it matter if you head north or south?
Yes, direction matters.
If you combine speed with direction you have velocity.
Read speed from speedometer of car. Some cars have compasses built into their rear-view mirrors. Velocity is how fast and in what direction.

What do we mean by acceleration? Student answers - speeding up.
Acceleration is the rate at which your speed or velocity changes. You can be speeding up, slowing down, or changing direction. It is a difficult concept because it is the rate of a rate.

Example of making money and getting yearly raises.
Your hourly wage is the rate at which you make money.
Your yearly raise is the rate at which your hourly wage increases.

Example of car. Starts at 45 mph and 5 seconds later is going 55 mph. Has it's speed increased? Yes.
acceleration = change in speed/time interval = 10 mph/5 sec = 2 miles/hr/sec

If you continued accelerating, in 5 more seconds you would be going 65 mph.

In your ball and ramp lab the ball started from rest and increased its speed. Show shape of distance vs time graph. Suppose the acceleration was 1 m/s/s meaning that every second it increased its speed by 1 m/s. After 1 second it was going 1 m/s, after 2 seconds, 2 m/s. Show graph of speed vs time - straight line. The slope of this line is the acceleration which shows the rate at which you gain speed.

What if you drop an object? Would you expect the rate at which you pick up speed to be greater or less than on a ramp? greater
What would you predict for the rate at which the speed increases? Students give guesses.
Today, you are going to measure this rate in the classroom.

Have instructions for using computer on board.

Students have 1 minutes to get into groups of 3 by computer.
Explain to students to plug in both plugs, make sure green light is on for ULI, turn on computer. While the computer is starting up, get out photogate and attach to ring stand with clamp so that the opening of the photogate allows you to drop something through the photogate into the padded box on floor. In 4A I had students make landing pads by putting crumpled up newspaper into a bag and then the bad into a box.

Plug photogate into DG 1 of ULI.

Cancel out of log in window, and select logger pro 2.1 from startup. Select file, open, physics with computers, experiment 5- picket fence. Get picket fence from box. Explain that when the dark area is between the photogate beam, the beam is blocked. When the clear area is between the beam, the gate is unblocked. Students tested this. If the fence fell at constant speed, you would have equal times for blocked and unblocked as the fence fell. But the fence speeds up so the times get shorter and shorter. The computer graphs this motion in both distance vs time and velocity vs time.

To find the rate at which the speed increases, you need to highlight a portion of the velocity vs time graph and choose the linear fit icon.
From math classes, what is the equation for a straight line? y=mx +b where m is the slope. The slope is the acceleration. Students run lab and record slopes.

Slopes are between 9.6 and 9.6 m/s/s. This is the acceleration of gravity in the classroom. It is the rate at which a falling object picks up speed. The accepted number is about 9.8 m/s/s but we will round this to 10 m/s/s. What this means is that a falling object will pick up speed at a rate of 10 m/s for every second it falls.

Go around classroom and ask for speeds at different times. Have partners quiz each other.

We know the equation for distance is d = speed * time. If you drop an object and it falls for 1 second it is going 10 m/s. If we wanted to find how far, what speed should we use? We use the average. Explain averages. Show how to calculate distance. Partners quiz each other.

Hand out graph paper. Students plot distance and time data.
Title: Distance vs time for reaction time.

Students drop meter sticks and find reaction time.

Show how students can test this on parents and get rich.

Review content objectives.