THIS DAY IN SCIENCE: First Untethered Space Walk in 1984

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# Centripetal Acceleration

## Introduction

Take your students on an amusement park rideâ€”for just a penny! Discuss how an object can be accelerating yet moving at
constant speed. Investigate how a change in direction (at constant speed) is acceleration; that is, centripetal acceleration!

## Discussion

Velocity is the rate of motion in a specified direction and acceleration is a change in an objectâ€™sÂ velocity. Both velocity and acceleration are vector quantities in that they are based on a magnitudeÂ AND a specified direction (i.e., the car is traveling north at 55 mph). Speed, on theÂ other hand, is an objectâ€™s rate of motion. Speed is a scalar quantity and is based only on aÂ magnitude (i.e., the car is traveling at 55 mph). Whenever there is a force, there is accelerationÂ according to Newtonâ€™s second law of motion (F = ma). A force is required to change anÂ objectâ€™s speed, and a force is required to change an objectâ€™s direction. Since velocity is a quantityÂ of speed and direction, a change in an objectâ€™s speed OR a change in its direction, or bothÂ cause the object to accelerate. The force can change the speed of the object without affectingÂ its direction (linear acceleration), or the force can change the direction of the object with orÂ without affecting its speed (centripetal acceleration).