Chapter 3 notes

Chapter 3: Describing motion

Position and Distance

1) Position is defined as the separation between an object and a reference point; distance is the measured value of this separation. Displacement is the distance and direction between two positions, or the distance between the final position and the initial position. In the MKS and SI systems, the unit of both distance and displacement is the meter (m). Here's an online tutorial about distance and displacement,

2) A scalar quantity is physical quantity that can be described with only one value- the magnitude; a direction is not required. Examples are distance, speed, time, and temperature. Scalar is another name for a number.

3) A vector quantity is physical quantity that requires both a magnitude and a direction. Examples are displacement, velocity, acceleration, and force.

Study this page at on describing motion with words.  

Average and Instantaneous Speed

1) speed = distance/ time or v = d/t ; measured in m/s

2) average speed: , where d is the total distance and t is the total time elapsed. *Note, place a bar above the v.

Work practice problems 1-4, page 45-46. (refer to the top of page 45 for review of dimensional analysis techniques required to convert between units of m/s and km/hr.

3) Instantaneous speed is the speed at any instant, this quantity is what a speedometer measures. On a position vs time graph, find the slope of the line at any particular point in time to get instantaneous velocity. This may require constructing a line tangent to a point on a curve, and then determining the slope of the tangent. Do you know another analytical method of finding the slope of a curve at a particular point using the calculus?

Velocity and Speed

1) velocity = displacement/time, plus the direction. , (north, south ... etc.)

2) Displacement is the distance between the final position and the initial position or
displacement plus a direction. Remember that distance is a scalar quantity and displacement is a vector quantity.

Example: You walk from A to B, then you turn around and walk back to A.

distance compared to displacement

The distance you traveled was 2AB; but your displacement was zero.

3) velocity

The above formula only provides average velocity. You only deal with two known data points. Graphing this data would, of course, produce a straight line on a position vs. time graph. This would indicate a constant velocity, but the graph would not be correct. Using only two data pieces, the initial and final values, doesn't really tell us anything about what happened with regard to the motion of the object between these two points. Average velocity has very little meaning in a situation such as moving an object at rest to a new position, a short distance away in a short amount of time.

Relativity of Velocity

Velocity depends on your frame of reference. For example, throwing a
baseball (10 m/s) from the back of an airplane to the front of the plane
(500 m/s).

Homework: Set #1: review problems 1,3,6,7,14,15,16

Set #2: review problems 17-23 (note: you only make motion diagrams for these problems!)

Activity: Page 58. Devise an experiment to collect motion data, and make pictorial and physical motion diagrams for what happened.

Video: Frames of Reference