Copyright 1995, Mark Stockman (Double click for Copyright ...
Copyright ( 1995, 1996 Mark Stockman
[pic]
Washington State University
Department of Physics
Physics 101
Lecture 2
Chapter 2-1 through 2-7
Kinematics
Contents
1. Subdivisions of Mechanics
2. Coordinate Systems and Reference Frames
3. Displacement and Average Velocity
4. Instantaneous Velocity
5. Vectors vs. Scalars
6. Acceleration
1. Subdivisions of Mechanics
Kinematics studies mechanical motion irrespectively of the cause of the motion.
Typical problem: Given velocity and distance, find time elapsed.
Dynamics studies effects of forces on the motion.
Typical problem: given the force, find the trajectory of a projectile
Study of speed, velocity and acceleration, that we are dealing with, is kinematics
2. Coordinate Systems and Reference Frames
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Solar system: Trajectories depend on observer
[pic]Double-click to activate the demonstration package. Interactive Physics by Knowledge Revolution should be installed
View in the Sun-fixed system
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Observation from Earth
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3. Displacement and Average Velocity
Displacement
Displacement is the difference between the final and the initial coordinates of the body.
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Displacement:
[pic]
In our example
[pic]
In this example
Thus, minus sign indicates motion against the direction of a coordinate axis.
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Average velocity:
If, the last example, [pic], then [pic].
4. Instantaneous Velocity
[pic]
5. Vectors vs. Scalars
Vector is a quantity which is characterized by its direction as well as its magnitude
Examples: velocity, displacement, acceleration, electric field, force,...
Notation for a vector: [pic] or [pic].
In contrast, a scalar is completely characterized by its magnitude
Examples: speed, distance, time, mass, energy, power,...
Attention: There is another principal difference between velocity and speed.
Speed is defined in terms of the distance traveled.
In contrast:
Velocity is defined in terms of the displacement, i.e., change of the position of the body in space.
Example:
Time elapsed was 1 h
Speed was [pic]
Average velocity was 100 km/1 h=100 km/h
6. Acceleration
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[pic]
Instantaneous acceleration:
[pic]
Example: A car decelerates from 100 km/h to 0 in 10 s, Find acceleration.
Solution:
1. Convert:
[pic]
2. Initial conditions: Thus, initial velocity [pic], final velocity was 0; time elapsed was 10 s.
3. Calculate:
[pic]
4. Discuss: Note that the minus sign shows that this actually is deceleration. Both acceleration and deceleration are called in physics as acceleration and distinguished by their signs.
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MECHANICS
KINEMATICS
DYNAMICS
STATICS
x=2.1 m
Y
y=1.5 m
X
Motion is relative and it is always described with respect to a reference frame. When characterizing the motion, this frame should always be specified
40 km/h
Observer
45 km/h
0 km/h
-5 km/h
Observer
0 km/h
-45 km/h
Mercury
Earth
Mercury
Earth
Initial position
Final position
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Initial position
Final position
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[pic]
Braking
Speed-up
Average
velocity
Graphic representation
v
Highway cruising (v=const)
t
Initial
position
Final
position
D=100 km
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