Centripetal Motion: Physics Lab

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  • 0:01 Projectile Motion
  • 0:48 Physics Lab Steps
  • 4:04 Lesson Summary
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Lesson Transcript
Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

After watching this video, you will be able to explain what centripetal motion is, provide an example of centripetal forces, and use the equation for centripetal force in a real-life scenario to calculate the tension in a string. A short quiz will follow.

Projectile Motion

Uniform centripetal motion is the motion of an object moving in a circle at a constant velocity. This happens due to a force pointed towards the center of the circle, called a centripetal force. The exact nature of this force depends on the circumstances. For example, if you whirl a ball over your head on a string, the tension in the string provides the centripetal force. Or for the space shuttle in orbit, gravity provides the centripetal force.

In other lessons, we introduced the equation for centripetal force, which is:

mv^2 / r

Where m is the mass of the object moving in a circle, measured in kilograms, v is the velocity of the object moving in a circle, measured in meters per second, and r is the radius of the circle, measured in meters. But now it's time to get our hands dirty and investigate that motion with an experiment.

Physics Lab Steps

For this physics lab, you will need:

  • A plastic cup filled with water
  • Some kind of square or circular platform; the sturdier the better. But you must be able to make holes in the corners. If you have access to a drill, you could use some plastic or acrylic. Otherwise, a really sturdy piece of cardboard might do the trick.
  • A large amount of string
  • A stopwatch
  • And a tape-measure or ruler

Step 1: Create your platform. You should have several holes on the edges to attach strings. If the platform is square, a hole in each corner is fine. The platform needs to be sturdy enough to hold a plastic cup filled with water without deforming noticeably.

Step 2: Attach strings to your platform. One string should be tied to each hole securely, so that you can hold the platform in the air by the strings, kind of like a puppet. The length of each string should be a little shorter than the distance from your outstretched arm to the floor, though a lot of different lengths can work.

Step 3: Put the plastic cup of water on the platform and hold the platform cup by the strings. The cup of water should be held perhaps a foot above the floor, sat on the platform. If this is difficult to do, your platform might not be sturdy enough.

Step 4: Starting carefully, swing the cup of water back and forth like a pendulum. As you gain confidence, try spinning it in a full circle, so the cup is upside down. If you do it fast enough, the water will not spill! Well done, you've just seemingly defied gravity! But it's all due to the power of circular motion.

It turns out, the water doesn't fall out of the cup, because by the time it begins to fall you've already moved the cup to the side (and down). It's like you're catching the water before it has a chance to actually move.

Step 5: Play around in this way for as long as you like.

Step 6: Once you're ready to take some measurements, measure the mass of your mostly-full cup of water. Also, measure the mass of the platform, and add it to this figure:

shows to measure mass of platform and water

This is your value of m.

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