Download Physical Quantities - Physics - Lecture Slides and more Slides Physics in PDF only on Docsity!
Introduction to Physical
Quantities
A. Scalars vs Vectors
B. Distance & Time
C. Velocity
D. Acceleration
E. Force and Mass
No class on Monday: Martin Luther King Day
Scalars
Scalar quantities are those which are described solely by their magnitude
Some examples are:
Mass e.g. 14 [kg], 36 [lbs], … Time e.g. 10 seconds, 40 minutes, … Volume e.g. 1000 cm^3 , 4 litres, 12 gallons Temperature e.g 14 oF , 25 oC, … Voltage e.g. 9 Volts, etc
docsity.com
Vectors
Vector quantities are those which need to be described by BOTH magnitude and direction
Some of the most common examples which we will encounter are:
Velocity e.g. 100 [mi/hr] NORTH
Acceleration e.g. 10 [m/s^2 ] at 35o^ with respect to EAST
Force e.g. 980 [Newtons ] straight down (270o)
So, the main difference between vectors and scalars is that for vectors, you must also indicate the direction!
Distance
q The separation between two locations.
q Distance can be measured in many types of units. We will mostly use:
qYou should be comfortable with converting from [cm] to [m], [mm] to [km], and so on. We’ll do a few examples later…
q We often use the symbolic notation ∆d to mean a change in the position. The symbol ∆ should be read as “the change in”
MKS Units millimeters [mm] centimeters [cm] meters [m] kilometers [km], etc
FYI : 1 [km] = 0.6 [mi]
docsity.com
Velocity II
0
10
20
30
40
50
60
0 1 2 3 4 5 6 Time (sec)
Distance (m)
(^) Car 1 Car 2 Car 3
Distance vs Time A graph of “distance” versus “time” is another way of exploring velocity.
The graph shows the distance covered as we increase time for 3 different cars.
The “slope”=∆y/∆x, and in this case, the slope is:
Slope = ∆ (Distance) / ∆ (time) = ∆d/∆t == Velocity!
So, the slope of the distance vs time graph is equal to the velocity !!!
Velocity III
0
10
20
30
40
50
60
0 1 2 3 4 5 6 Time (sec)
Distance (m)
(^) Car 1 Car 2 Car 3
How fast is car 1 going? A) 50 m/s B) 30 m/s C) 10 m/s D) 5 m/s
How fast is car 2 going? A) 25 m/s B) 5 m/s C) 1 m/s D) 2.5 m/s
How fast is car 3 going? A) 2.5 m/s B) 12.5 m/s C) 10 m/s D) 5 m/s
docsity.com
Velocity IV
If a train’s velocity is 50 [m/s], how far will it go in 10 minutes?
a) 500 [mi.] b) 500 [m] c) 3000 [m] d) 30 [km]
If a train advances 0.3 [km] in 10 [s], what is its average velocity?
a) 3000 [m/s] b) 3 [m/s] c) 10 [m/s] d) 30 [m/s]
Velocity vs Speed
Velocity is a vector !!!!!
Speed is a scalar.
If a car goes around in a circle at constant speed, is it’s velocity changing? A) YES B) NO
v
v
v
The velocity is actually continually changing !!
docsity.com
Acceleration III
Acceleration can be negative also!
q If the acceleration is in the same direction as the velocity, the object has positive acceleration (it speeds up).
q If the acceleration is in the opposite direction as the velocity, the object has negative acceleration or deceleration (it slows down).
Acceleration IV
0
5
1 0
1 5
2 0
2 5
3 0
0 1 2 3 4 5 6 Time (sec)
Velocity (m/s)
Car 1 Car 2 Car 3
What is the acceleration of Car 1? A) 2.5 m/s^2 B) 5 m/s^2 C) 125 m/s^2 D) 25 m/s^2
What is the acceleration of Car 2? A) 0.5 m/s^2 B) 5 m/s^2 C) 12.5 m/s^2 D) 2.5 m/s^2
What is the acceleration of Car 3? A) -5 m/s^2 B) -2.5 m/s^2 C) -1 m/s^2 D) -25 m/s^2
docsity.com
Acceleration and Force
How do you make objects accelerate?
***** You MUST apply a force *****
If there is no force being applied on an object, it cannot accelerate. That is, it must have: acceleration = 0
Can you think of a counter-example (where an object accelerates without a force being applied to it)?
Since a = ∆v/∆t, a = 0 Ë ∆v=0 … So velocity cannot change unless a force acts on an object.
What is a Force? (Vector)
Force is simply:
A PUSH or A PULL
Forces are vectors Ë they have both magnitude and direction
docsity.com
