Speed and Velocity

 

Motion is defined as:the

change of position with

time.

The standard units used to describe motion are the meter (m) for measuring

change in position, and the second (s) for

measuring time.

Two quantities are often used to describe the rate of change of position: velocity and speed.

Velocity is a vector quantity, while speed is a scalar. What does this mean? Measures of velocity will have a magnitude, or numeric value, and a direction. Speed, on the other hand, has only a numeric value.

speed = 300 km h–1

direction = west

MTR drivers concern speed only.

Speed with direction

Pilots concern velocity (direction & speed).

speed = 90 km h–1

Speed is the quotient of total distance traveled and time, while velocity is displacement (total change in position) divided by the time elapsed.

Velocity, itself, comes in two flavors: average and instantaneous. Average

velocity is the quotient of displacement and time

elapsed, where the elapsed time is greater than 0. Translated into

the language of mathematics, the

definition of average velocity is:

where v represents average velocity,

s represents position, and t represents time.

The Greek letter (delta) means "change in."

Instantaneous velocity, on the other

hand, is defined as the limit of average velocity as t approaches zero:

The derivative nature of velocity (and acceleration)

will become more apparent in the discussion

of acceleration, next.

unitsSpeed and velocity are

both measured using the same units. Given that the SI unit of both distance and displacement is the meter and that the SI unit of time is the second, it should be intuitively obvious that the

unit of both speed and velocity would be a ratio of

two units. The SI unit of speed and velocity is the

meter per second. m/s 

AccelerationWhen a car moves faster and faster, its speed is increasing (velocity changed).

AccelerationWhen a car moves slower and slower,

its speed is decreasing (velocity changed).

When a car changes direction, its velocity changes too.

Acceleration

AccelerationAcceleration measures the change in velocity

Acceleration = velocity per unit timeAcceleration = velocity per unit time

direction speed

overall change in velocity

total time taken

= m s–2Unit: m s–1 / s vector quantity

=

Acceleration is the rate of change of velocity (or speed, but physicists tend to prefer velocity—it’s more descriptive). So average acceleration is the rate of change of the rate of displacement, the standard units for which are "meters per second per second", or "meters per second squared" (m/s2). Mathematically, this translates to:

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Test your Understanding:

1. The units for acceleration are:a) m/secb) m/sec²c) sec d) (m/sec)²

4. The units for velocity are:

a) m/sec²b) sec.c) milesd) m/sec

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The End

Web ที่��ค้�นค้ว้�า

1.www.kyc.edu.hk/studteach/teacher/hlt/.../05-1_speed_vel_acc.ppt – 2.www.physics4kids.com/files/motion_velocity.html

3.physics.info/velocity/ 4.library.thinkquest.org/15433/unit1/1-1.html