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.

units Speed 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

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

Acceleration When a car moves slower and slower, its speed is decreasing (velocity changed).

When a car changes direction, its velocity changes too. Acceleration

Acceleration measures the change in velocity Acceleration =  velocity per unit time direction speed overall change in velocity total time taken = m s –2 Unit: 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/s 2 ). Mathematically, this translates to:

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Test your Understanding: 1. The units for acceleration are: a) m/sec b) m/sec² c) sec d) (m/sec)²

4. The units for velocity are: a) m/sec² b) sec. c) miles d) m/sec

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The End

Web ที่ค้นคว้า 1. cher/hlt/.../05-1_ speed_vel_acc.ppt – 2. otion_velocity.html 3.physics.info/velocity/ 4.library.thinkquest.org/15433/un it1/1-1.html