Chapter C2 Vectors Friday, August 24, 27

Scalars Most physical quantities can be completely described by a single number. Some examples are: Most physical quantities can be completely described by a single number. Some examples are: Mass Mass Volume Volume Time Time Charge Charge Height Height Score on test Score on test Physical quantities that can be described by a single number are called scalars. Physical quantities that can be described by a single number are called scalars.

Vectors Certain physical quantities need a direction as well as a size (magnitude) to completely describe them. Some examples are: Certain physical quantities need a direction as well as a size (magnitude) to completely describe them. Some examples are: Velocity Velocity Force Force Displacement Displacement Momentum Momentum Acceleration Acceleration These are called vectors. These are called vectors.

Vectors

adding Vectors You may move a vector anyplace as long as the magnitude and the direction remain unchanged You may move a vector anyplace as long as the magnitude and the direction remain unchanged When adding vectors, 1) move the second vector, putting the tail of the second vector on the head of the first vector. When adding vectors, 1) move the second vector, putting the tail of the second vector on the head of the first vector. 2) Begin the tail of the resultant vector on the tail of the first vector and put the head of the resultant on the head of the last vector. 2) Begin the tail of the resultant vector on the tail of the first vector and put the head of the resultant on the head of the last vector. 3) Be certain to draw the head on the resultant vector! 3) Be certain to draw the head on the resultant vector! To subtract vectors, change the direction of the vector with the minus sign and add. To subtract vectors, change the direction of the vector with the minus sign and add.

adding Vectors

Chapter C2 problems are due today Chapter C2 problems are due today Test will be Friday. Ask any questions about the practice test today or Wednesday. Test will be Friday. Ask any questions about the practice test today or Wednesday. Tuesday and Thursday labs will be on vectors. Tuesday and Thursday labs will be on vectors.

Uncertainty revisited What you are expected to know. Be able to estimate the uncertainty in a particular measurement. Be able to estimate the uncertainty in a particular measurement. Be able to estimate the uncertainty of the result when quantities containing uncertainties are used in calculations. Be able to estimate the uncertainty of the result when quantities containing uncertainties are used in calculations.

Example What is the uncertainty in the area of circle whose radius is 20 paces as determined by my “pacing it off” if I know my normal pace is.8 m? What is the uncertainty in the area of circle whose radius is 20 paces as determined by my “pacing it off” if I know my normal pace is.8 m? Step 1 – What is the uncertainty in my pace? Step 1 – What is the uncertainty in my pace? There is no single correct answer to this, give an answer you can defend) There is no single correct answer to this, give an answer you can defend) I will guess the uncertainty in my pace is 5 cm/pace. I will guess the uncertainty in my pace is 5 cm/pace.

The 5 cm represents 5/80 = 6% The 5 cm represents 5/80 = 6% The radius of the circle is.8 x 20 = 16 m The radius of the circle is.8 x 20 = 16 m The area of the circle is πr 2 = π16 2 =804 m 2 The area of the circle is πr 2 = π16 2 =804 m 2 Uncertainty is ( ) ½ = 8% Uncertainty is ( ) ½ = 8% 8% of 804 is 64 m 2 8% of 804 is 64 m 2 Final answer 804±64 m 2 Final answer 804±64 m 2 It would probably be better to write it as 800±60 m 2 It would probably be better to write it as 800±60 m 2

Vector Components θ This is the length or the magnitude of the vector. The choice of a coordinate system is arbitrary. In physical problems there is often a particular choice that makes the problem easier.

Vector Components θ u y =u sinθ u x =u cosθ tanθ=u y /u x

Unit vectors are unit vectors Note that u x,u y and u z are not vectors

In class exercises

On your paper, draw the following vectors.

In class exercises Find the components of the vector a below if its magnitude is 30 and the angle with respect to the x axis is 35º. Find the components of the vector a below if its magnitude is 30 and the angle with respect to the x axis is 35º. Find the components of the vector B below if its magnitude is 20 and the angle with respect to the y axis is 75º. Find the components of the vector B below if its magnitude is 20 and the angle with respect to the y axis is 75º. A ayay axax θ 75º Sketch the resultant and calculate the magnitude and direction of the vector B-2A. B

Same problem, continued add the two vectors add the two vectors Give the components of the resultant Give the components of the resultant Give the magnitude of the resultant Give the magnitude of the resultant Give the angle of the result with the x axis Give the angle of the result with the x axis Draw a figure showing your results. Draw a figure showing your results.

C θ C = 29.9 Θ =35.9° Φ=54.1°

Use of the unit operator Given that there are 2.21 lbs/kg, calculate the number of nanograms in a ton (2000 lb). Given that there are 2.21 lbs/kg, calculate the number of nanograms in a ton (2000 lb). lbs  ng lbs  ng

On Wednesday we will review for the test over chapters 1 and 2. On Wednesday we will review for the test over chapters 1 and 2. Ask any question – ask to see any type of example worked. Ask any question – ask to see any type of example worked. Hint for test: You not only need to know how to do the problem, you must clearly show how you arrived at your answer. Hint for test: You not only need to know how to do the problem, you must clearly show how you arrived at your answer. Thursday – Finish vectors lab Thursday – Finish vectors lab Wednesday after Labor Day begin C3 discussion Wednesday after Labor Day begin C3 discussion