Lecture 3: Introduction to Physics 101

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# Lecture 3: Introduction to Physics 101 - PowerPoint PPT Presentation

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

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/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

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

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)

Trigonometry

Right triangle:

Definition of sine, cosine and tangent:

hypotenuse

h

q

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)

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) ?

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

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.

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.
• Using vector components (e.g. 2 dimensional space) :

R = A+B = (Ax+Bx) x + (Ay+By) y

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!