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Velocity, Speed, and Motion... Oh My!
Velocity and speed are very similar ideas. Velocity is the rate of motion in a specific direction. I'm going that-a-way at 30 kilometers per hour. My velocity is 30 kilometers per hour. It's also described as a measure of distance divided by time. Velocity can be constant, or it can change (acceleration). Speed with a direction is velocity.
Remember vectors? You will use a lot of vectors when you work with velocity and distance. Our real world example of navigation on the ocean used velocity for every vector. Velocity is a vector measurement because it has an amount and a direction. Speed is only an amount (a scalar). Speed doesn't tell the whole story to a physicist. 
One Moment in TimeThere is a special thing called instantaneous velocity. That's the velocity at a split second in time. Above, we were talking about your speed and direction over a long period of time. Why would you need to measure a velocity at one moment? Think about the moment you drove over the manhole. It's important to know if you were going 1 kmph when you drove over the manhole or 60 kmph. It doesn't matter if your average velocity was 30 miles per hour.The term "instantaneous" refers to something physicists call a limit. Scientists "limit" the amount of time they do the measurement. When the "limit" moves to zero, that limit is one tiny point in time. A physicist would measure your velocity as the "limit for a period of time", zero, to get the instantaneous velocity. 
Changing Your VelocityWhen velocity is changing, the word acceleration is used. A positive acceleration happens when you speed up. A negative acceleration happens when you slow down. When you accelerate or decelerate, you change your velocity over a specific amount of time.Just as with velocity, there is something called instantaneous acceleration. Instantaneous means scientists measure your acceleration for a specific moment of time. That way they can say he was accelerating at exactly this amount at this point during his trip. Constant Acceleration
There are a few special times when there is something called constant acceleration. This type of acceleration happens when the acceleration is constant in both amount and direction. The best example is gravity. Gravity's pull on objects is a constant here on Earth and it always pulls toward the center of the planet. The gravity of other planets is different from Earth's gravity because they have different masses. Even though the gravity may be smaller of larger, it will still create a constant acceleration.
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Go for site help or list of physics topics at the site map!©copyright 1997-2007 Andrew Rader Studios, All rights reserved. Current Page: Physics4Kids.com | Motion | Velocity and Acceleration |
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