Speed is the rate at which an object covers a distance. How do you know if that rate changes or not? The video shows how to determine if the speed of an object is constant.
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Velocity is speed in a given direction. How is a change in velocity (acceleration) detected? Accelerometers. Most smartphones have accelerometers to detect changes in velocity to monitor certain tasks. The video demonstrates a very basic accelerometer and how it works.
A simple demonstration of centripetal force. A marble is moving in a circular path. The wall of the roll of tape is applying a force on the marble toward the center of the circular path. That is centripetal force. Once the force is removed, the marble, because it has inertia, moves in a straight line.
The more mass an object has, the more inertia it has. Inertia is the tendency of an object to resist a change in motion. There are two fishing weights. One has more mass than the other. A string applies an unbalanced force to the different weights. Which one resists a change in motion? Watch the video.
Another demo on inertia? Yup, and this time I have enlisted the help of one of my favorite science guys: Beaker. Beaker, books, and a rolling chair are moving at constant velocity until an outside unbalanced force is applied. Watch what happens!