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Definitions: Speed
or Velocity? Speed
is the rate of motion, or the rate of change of position. It is expressed as distance
moved (d) per unit of time(t). Speed is a scalar quantity with dimensions distance/time.
Speed is measured in the same physical units of measurement as velocity,
but does not contain an element of direction. Speed
is thus the magnitude component of velocity. Velocity contains both the magnitude
and direction components. Common
speeds of moving
objects For
human beings, an average walking speed is about 3 mph (~5 km/h, 1.39m/s),
The
speed of long distance jogging for average persons is about 6 mph (~10 km/h, 2.7
m/s).
Top athletic sprinters can run at 23.03 mph (~36.85 km/h, 10.24 m/s)
within a short distance such as a 200 meters dash.
Cycling can average 12
mph (~20 km/h, 5.56 m/s)
Car can average 65 mph (~104 km/h, 28.9 m/s ) on
highway
A 747 Airplane
has an average speed 565 mi/hr Acceleration Acceleration,
(symbol: a) is defined as the rate of change of velocity. It is thus a vector
quantity with dimension length/time˛. In SI units, acceleration is measured in
meters/second˛. To
accelerate an object is to change its velocity, which is accomplished by altering
either its speed or direction (like in case of uniform circular motion) in relation
to time. Acceleration
can have positive and negative values. Any time that the sign (+ or -) of the
acceleration is the same as the sign of the velocity, the object will speed up.
If the signs are opposite, the object will slow down. Acceleration is a vector
quantity. When either velocity or direction changes, there is acceleration (or
deceleration). To
accelerate an object requires the application of a force. Graphing
Velocity and Acceleration A
car accelerates from rest. The following two graphs are created using a java applet
(see below). |  | | The
graph of velocity (m/sec.) vs. time (sec.) is a straight line for accelerating
objects. acceleration
= velocity / time What
is the slope of this graph? |
Please
enter your answer in the space provided:
 | |
The graph of
distance (x) vs. time (t) is a curve where the equation of motion is: x
= 1/2 at˛. What value of (a) would you expect for this graph? Hint:
For each value of t (in s), there is only one value for x (in m). Pick any ordered
pair for t and x and substitute them into the above equation. Then solve for a. |
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| Create
your own graphs for accelerating objects using the following java
applet: | |
This Java applet shows a car moving with constant acceleration.
The green control panel contains text fields where you can vary the values of
initial position, initial velocity and acceleration (don't forget to press the
"Enter" key!). By using the buttons at the top right you can bring back the car
to its initial position or stop and resume the simulation. The diagrams illustrate
the motion of the vehicle: Position x versus time t Velocity v versus time t Acceleration
a versus time t |
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