Physics                Unit A                 Day 3 (Aug 28, 2008)

 

Objectives         

¨      Draw and use motion diagrams to describe motion

¨      Use particle model to represent moving objects

¨      Differentiate between vector and scalar quantities

¨      Define displacement and distance and compare and contrast them

 

1.              Homework check and discuss.

 

2.              Motion Diagrams

Use lab activity A1 to introduce the concept of motion diagrams.

Students will draw motion diagrams for a lab cart under the following conditions:

i.               traveling uniformly to the right

ii.              traveling uniformly to the left

iii.            traveling uphill

iv.             traveling downhill

-arbitrarily chose a time interval (i.e. 1/10 s)

 

Discuss the shape of the motion diagrams for each condition.

Easier to represent the motion with “dots” (particle model)

 

Ex.                                           represents an object slowing down over time

 

3.              Position of an object

Like other measurements, position is measured relative to a starting point or reference.

The origin is the point at which the variables have the value of zero.

 

Place a mark of “0” on the board and label it origin.

Mark a position (A) 1.0 m to the left and another (B)1.0 m to the right of the origin.

 


           

                      A                                             0                                  B

How are these positions the same?  (same distance from 0)

How are they different?                 (different direction)

How can we distinguish?               (A –1.0 m, B +1.0 m)

How far are A and B apart?            (2.0 m)

Does this require any further information (i.e. direction)?         (No)

 

Distance is the measured separation between two positions.

Ex. the distance between Doha and Dukhan is 80 km

 

Let’s move from A to B. Is this the same as moving from B to A? (No)

The change in position from A to B is  2.0 m to the right or +2.0 m

The change in position from B to A is 2.0 m to the left or –2.0 m

 

Displacement defines the distance and direction between two positions.

Ex. The displacement of Dukhan is 80 km west of Doha

 

This can calculated using delta notation (D– change in)

            Dd = df - dI                      Dd = dBdA =  +1.0 m – (-1.0 m) = +2.0 m

 

Some descriptions require a direction!

 

4.              Vector and Scalar Quantities

A vector quantity is represented by both a magnitude (amount) and a direction.

Ex. displacement 80 km [E], 12 cm up; velocity  -14.5 m/s, force pull down with 15 Newtons

 

A scalar quantity is represented by a magnitude only. Ex. distance, speed, mass, time, temp.

 

5.         Assign Exercise A1 (handout) Ch 2: #5 – 7, 35, 37, 38, 49, 51, 53