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Lecture 3: Introduction to Physics 101

Lecture 3: Introduction to Physics 101. Chapter 1 : Scalars and Vectors (1.5) Chapter 2: Distance and Displacement, Speed and Velocity (2.1,2.2). correct. Conversion of Units – Example. A Cheetah’s acceleration when hunting can be a=6.1 m/s 2

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Lecture 3: Introduction to Physics 101

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  1. Lecture 3: Introduction to Physics 101 Chapter 1 : • Scalars and Vectors (1.5) Chapter 2: • Distance and Displacement, Speed and Velocity (2.1,2.2)

  2. correct Conversion of Units – Example A Cheetah’s acceleration when hunting can be a=6.1 m/s2 What is its acceleration in ft/min2 ? Conversion factor: 1 ft = 0.3048 m 1 - 72 ft/min2 2 - 0.072 ft/min2 3 - 72 x 103 ft/min2

  3. correct Dimensional Analysis - Example Determine how the time t of a free fall of an object depends on its mass m, the height h from which it is dropped and the acceleration g due to gravity. 1 - V h/g 2 - m h/g 3 - h/g

  4. correct Trigonometry - Example Which one of the following expressions may be used to correctly find the angle q ? q 2 cm 90 5 cm 1 - q = cos-1 (5/2) 2 - q = tan-1 (2/5) 3 - q =tan-1 (5/2)

  5. Trigonometry Right triangle: Definition of sine, cosine and tangent: hypotenuse h q adjacent to angle ha Sin q = ho/h Cos q = ha/h Tan q = ho/ha 90 ho opposite to angle Pythagorean Theorem: h2 = ho2 + ha2 Any Triangle: Laws of Cosines, Sines (see E.2)

  6. Trigonometry - Example The silhouette of a Christmas tree is an isosceles triangle. The angle at the top of the triangle is 30 degrees, and the base measures 2 meters across. How tall is the tree (in meters) ?

  7. Scalars and Vectors Scalars are quantities which are completely specified by their magnitude (single number+unit). There are physical quantities which are not completely specified by their magnitude. Example: Displacement

  8. Scalars and Vectors Example: Displacement Displacement is the difference between final and initial position of a body. Assume a person is at an initial position A. What do you have to specify to completely pin down the person’s final position with respect to A ? To be able to describe both magnitude and direction of a physical quantity we use vectors.

  9. Vectors Vectors are graphically represented by arrows: • The direction of the physical quantity is given by the • direction of the arrow. • The magnitude of the quantity is given by the • length of the arrow.

  10. Addition and Subtraction of Vectors • Graphical: Tail-to-head method • Using vector components (e.g. 2 dimensional space) : R = A+B = (Ax+Bx) x + (Ay+By) y

  11. Lecture 3: • Scalars and Vectors • Distance and Displacement I strongly suggest that you try the example problems in the textbook. If you have trouble with any of them, please go to office hours for help!

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