# Conceptual Physics Alive: Linear Motion, Vectors & Projectiles

**Item**# PX-9101

Master teacher Paul Hewitt discusses the reasons to learn physics, and teaches Linear Motion, and Vectors & Projectiles

Master teacher Paul Hewitt teaches non-computational Conceptual Physics. Observe Hewitt teach in a classroom with real students, using engaging demonstrations and artwork.

DVD Includes 3 Episodes:

**Episode 1: Introduction:**

Paul Hewitt talks about teaching physics conceptually and his earlier efforts at videotaping. Short segments of demonstrations done in 1974 and 1984 are shown. We see Paul's opening day lecture at his fall 1989 conceptual physics class at the University of Hawaii, during which Paul poses some thought-provoking questions about the physical world. The introduction is designed to give a flavor of what the Conceptual Physics course is all about.

Segment length: 37 minutes

**Episode 2: Linear Motion:**

Concepts of speed, velocity, and acceleration are introduced and supported with a variety of examples. The effect of air resistance on falling objects is also discussed.

Segment length: 33 minutes

**Episode 3: Vectors and Projectiles:**

Vector addition and vector resolution are explained, using examples such as airplanes flying in the wind and projectile motion. The idea of the independence of the horizontal and vertical components of motion for projectiles is introduced.

Segment length: 47 minutes

**Intro includes:**

- Begins with Paul Hewitt speaking from his San Francisco apartment. Hewitt explains that physics is a study of the rules of nature.
- Conceptual physics is the description of physics in the English language-equations are seen as guides to thinking, rather than as formulas for plugging numerical data.
- An excerpt from a black-and-white video is shown of Hewitt's class in 1974 -the classic sledgehammer and anvil-on-the-stomach demo.
- Excepts from Hewitt's class in 1984 are shown (from the video series Conceptual Physics with Hewitt, distributed by Addison Wesley as an ancillary to the first edition of his high school text, Conceptual Physics). Van de Graaff generator demo; inertia demo with hoop, chalk, and bottle; bed of nails and sledgehammer demo with Paul Robinson.
- Hewitt introduces the viewer to his opening day class at the University of Hawaii.
- Hewitt poses a few questions, interspersed with his teaching philosophy, that reveal the scope of the course and the diversity of physics related subjects:
- Why do heavy and light objects pick up the same speed when dropped from rest? [ratios of weight/mass = g; the same.]
- What do rockets push against in outer space for propulsion? [They push against their exhaust gases, which In turn push and propel the rocket]
- Why don't satellites fall? [They do fall, but their tangential speed is great enough so they fall around the world rather than into it.]
- Which falls to the ground first, a bullet fired from a horizontally held rifle, or a bullet dropped from the same height when firing? [Both hit the ground at the same time because both undergo the same acceleration, g.]
- Which will skid to a stop quicker; a light truck or a heavily loaded truck traveling at the same speed? [Both skid the same distance; the heavier truck encounters correspondingly more friction.]
- Why is the force of impact less when one fails on a floor with 'give'? [More give means more time during stopping, which results in less force; in accord with Impulse= change in momentum.]
- Why will you be safe when touching a high voltage van de Graaff generator, but harmed when touching a faulty 110 volt household circuit? [The high voltage generator transfers low energy briefly; the 100 volt circuit can transfer a sustained high energy.]
- Why is your hand burned when touching the inner surface of a hot pizza oven, but not when touching the hot air Inside? [Air has low conductivity.]
- Why can one walk barefoot on red hot coals without harm? [Low conductivity.]
- Why does warm air rise? [It is buoyed upward by denser air below.]
- Why does expanding air cool? [Molecular collisions during expansion are on the average between receding neighboring molecules, so rebound speeds are lower (cooler).]
- How old are the atoms in your body compared to the age of the sun? [Older].
- Why are objects colored? [Objects absorb some light and reflect the rest; the apparent color is the 'opposite' of the color absorbed.]
- Why is the sky blue? [Tiny particles scatter light of high frequencies, larger particles scatter lower-frequency light Since the atmosphere is made of tiny particles, light of high frequency is scattered -blue.]
- What keeps the interior of the earth hot? [Radioactive decay; nuclear power.]
- Will time run differently at high speeds? [Yes, this is relativistic time dilation.]
- Hewitt closes with comments about learning readiness, and requests students to read about motion before the next class.

**Linear Motion includes:**

- Hewitt begins with the idea of learning joke telling not by hearing jokes, but by telling them. In the same way students will learn more physics by talking to their neighbors about the ideas of physics throughout the course; hence the “check-your-neighbor” routine that is a very important part of this course.
- Speed is defined as the ratio distance/time.
- Numerical examples of speed are given.
- Speed Is distinguished from velocity.
- Average speed is defined.
- Speed equation is rearranged to read d = vt.
- Numerical examples of distance traveled are given.
- Acceleration is introduced and defined.
- Hewitt shows a massive cylinder rolling at constant speed across a table.
- Acceleration In given in equation form; a= chg. in v/t.
- Numerical examples of acceleration are discussed.
- Hewitt drops a piece of clay and asks if it accelerates, first at floor level, then from the top of the lecture table.
- Acceleration of a freely falling object is 10 m/ s2.
- When acceleration Is due to gravity, we say g = 10 m/s2.
- Hewitt drops the clay from atop the lecture table and asks for the velocity at different times. This leads to the equation v = gt.
- Hewitt drops a book and sheet of paper, then a book and the same paper crumpled up to show the effects of air drag.
- Review time: Questions about speed, distance, and acceleration are posed, then explained.

*Next-Time Question:* For an object thrown straight upward, what is the acceleration at the top of its path? (10 m/s2.) When a piece of paper is placed atop a book that is dropped, which has the greater acceleration, the book or the paper? [Both accelerate at 10 m/s2, for the paper, quite surprisingly, falls as fast as the book. Why? Because the book 'breaks' the air and provides a path of no air resistance for the paper!]

**Vectors and Projectiles includes:**

- Hewitt begins with a chalkboard discussion of what a vector is.
- Vectors are drawn to represent the airplane speed- against the wind, with the wind, then cross wind.
- Hewitt rolls bowling ball across a table, then represents its velocity on chalkboard with vectors. Questions to clarify the concept of the independence of horizontal and vertical motion are posed, and explained.
- Hewitt writes the formulas for acceleration and velocity on the board, and discusses numerical examples.
- Hewitt stresses that the equations of physics are to physics types what sheet music is to musicians.
- Hewitt draws a vector diagram of the ball rolling off the edge of the table. Only the vertical component of motion changes; the sideways (horizontal) part stays the same.
- Hewitt demonstrates the independence of horizontal and vertical motion with a device that shoots a ball horizontally while simultaneously dropping another vertically. Then he humorously repeats the demo in "slow motion."
- Hewitt tells the humorous story of the lonesome person who joins a church group picnic at the cliffs over the ocean, and who tries to answer the question of a delightful person who asks how high the cliff is above the water. The concept that the time it takes for a rock to drop straight down is the same time it takes to drop if thrown horizontally is illustrated.
- Hewitt draws a sketch of a baseball pitcher who throws a ball horizontally from a tower 5 m tall. The ball lands 25 m downrange. What is the speed of the ball thrown by the pitcher? Hint: Speed= distance/time. [25 m/s.]
- Would the ball be in the air if the earth's curvature were a factor? [Yes, but for a longer time.]
- The concept of orbital motion is introduced via the idea of firing a cannonball faster and faster from a mountain top.
- Hewitt draws an analogy between an orbiting cannonball and the space shuttle.
- Hewitt guides the class to deriving the speed needed for a cannonball to orbit the earth. This is done by considering the falling of a cannonball fired across a desert floor. The speed is 8 km/s.
- Hewitt cites the case of a kid who wants to know why satellites don't fall to earth, and guides the class to formulating an answer for the kid.