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Speed, Velocity, and Acceleration

Speed, velocity, and acceleration are properties that describe the motion of an object. While these measurements are related, they are different in some important ways.

Speed

Speed measures distance over time. If you know that a object is moving at twenty miles per hour, you know its speed but you don’t know which direction the car is traveling.

Velocity

Like speed, velocity tells us how fast an object is moving. In addition velocity tells us which direction an object is headed. You need to know where an object started and ended to calculate velocity. Speed and velocity are sometimes used interchangeably, but in physics they don’t mean the same thing.

Acceleration

Acceleration measures the rate of change in velocity. Positive acceleration is an increase in velocity. A decrease in velocity is deceleration or negative acceleration.

What is a Scalar?

Speed is a scalar value. It has magnitude but no direction. For example if we know that a car traveled 100 miles per hour (its speed) for 3 hours (time) we wouldn’t know where it ended up.

What is a Vector?

A vector is a measurement that has magnitude and direction. A vector (like velocity) tells you not only how fast an object is moving through space, but also where it is headed. This allows you to calculate where and when the object will reach another point in space.

You Can Feel Acceleration!

If you travel at a constant velocity in a car, your body is moving but you don’t feel it. For example, riding in a car on a long road trip feels a lot like sitting on the couch. When velocity is constant, no forces are operating on you. By contrast, you may feel like you are being thrown around in different directions on a rollercoaster. This is because a rollercoaster is constantly changing velocity by accelerating or decelerating.

If a car accelerates gradually, you might not notice it. But, if a car accelerates quickly, you will feel the force pressing your body back into the seat. When the car decelerates quickly, the force of the deceleration pushes you in the opposite direction and you move forward. You might even bump your head if someone hits the brakes hard enough and you’re not wearing your seatbelt!

You can also feel a force acting on you if the car changes velocity without accelerating or decelerating. Imagine a car traveling at a constant rate of speed but turning sharply to the left. The change in velocity will move you to the right in your seat. This is because your body is still moving forward, while the car changes direction. Therefore, you are actually feeling the deceleration of your own body against the movement of the car.

Try this: Next time you’re riding in a vehicle, notice the forces of acceleration and velocity moving your body.

Written by Laura McCamy

Edited by Gabriel Buckley, MS Professional Natural Sciences

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