Physics 105, Fall Term, 2009

Reading: Chapter 2.1-2.4

Did you complete the reading assignment? Yes No

1) Speed is a scalar quantity for total distance traveled in a specified amount of time.

2) Velocity is a vector quantity for the total displacement of an object in a specified amount of time.

3) Acceleration is the change in velocity an object experiences in a specified time.

Most people are fairly familiar with terms like acceleration and velocity. Mastering what these terms actually are will be essential for you throughout the semester.

Here's a few sample questions of how concepts can be confused with these terms:

A racecar begins moving north around an oval racetrack at a constant rate of 150 mph. What is the car's speed after exactly one lap around the track?

0 mph 150 mph 150 mph north Not enough information (need distance of track and time elapsed)

What is the car's average velocity after exactly one lap around the track?

What is the car's instantaneous velocity after exactly one lap around the track?

What is the car's average velocity after exactly half of a lap around the track?

The driver applies the brakes after exactly one lap of the racetrack. What is the magnitude (don't worry about direction) of the acceleration of the car just after the brakes are applied?

negative, less than 0 positive, between 0 and 1 positive, more than 1 Not enough information (need distance of braking and time elapsed)

Now go to the following applet to see how we graphically view these concepts: http://phet.colorado.edu/sims/moving-man/moving-man_en.jnlp

You can control the motion of the man by clicking and dragging him or by putting numbers into the boxes at the left side of the graphs. You can increase the values of the y-axis of the graphs by clicking the plus sign on the right side of the graph. To restart the graphs, just press the 'clear' button on the left side of any of the graphs. Also a note so that you do not get confused; the velocity it refers to is instantaneous velocity not average velocity.

Move the moving man back and forth at various speeds using the mouse while watching the graphs. Does time have an affect on position?

Yes No

Does the moving man need to have a negative position to have a negative velocity?

Does the moving man need to have a negative velocity to have a negative acceleration?

Now clear the graphs and insert a velocity of 2 for the moving man while keeping the position and acceleration settings at 0. Press the 'Go' button. What best describes the position graph while the man is moving?

Linear, slope of 0 Linear, positive slope Linear, negative slope Exponential, positive Exponential, negative

What best describes the velocity graph while the man is moving?

What best describes the acceleration graph while the man is moving?

Do it again, but this time with a velocity of -2. What changes about the graphs?

They are the same, but all negative.

Now set position and velocity to zero with an acceleration of 2 and press the 'Go' button.

What best describes the position graph while the man is moving?

Now give position, velocity, and acceleration initial values. What changes about the graphs?

The y-intercept changes, otherwise they are the same.

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Was there anything that you didn't understand in the reading assignment? What was confusing to you?

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