Centripetal force is Newton's First Law in practice! Newton's First states that an object in motion (or at rest) will stay in motion (or at rest) unless acted upon by another force. In the instance of circular motion, centripetal force makes an object move in a circle rather than going straight.
One experiences centripetal force in many ways: turning a corner in your car, going upside down on that circular ride at the amusement park, and as shown in the lab below, keeping water in the cup as the cup is upside down.
To perform the demonstration below you will need a piece of stiff cardboard. I considered using plywood but if a student hit someone in the head that would not be good. Drill a hole in each corner of the cardboard. Attach a piece of string (we used acrylic yarn, strong but cheap) about 2.5 feet in length to each corner and knot together the other ends. Place two paper cups on the cardboard and fill each with a couple ounces of water.
Swing the apparatus back and forth a few times then swing in a circle a few times. Try swinging faster and slower. Stop and repeat the process after adding a few more ounces of water to each cup. Repeat again after filling each cup.
Do this outside as some students will not swing fast enough and the water will come flying out on everyone within range!
Students should be able to feel the change in centripetal force, as the tension on the string, which will be greater with more weight and also greater with more speed.
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