Which reaches the bottom of the ramp first

Try this activity to find out! Background When you lift an object up off the ground, it has potential energy due to gravity. When you drop the object, this potential energy is converted into kinetic energy, or the energy of motion.

Ignoring frictional losses, the total amount of energy is conserved. For a rolling object, kinetic energy is split into two types: translational motion in a straight line and rotational spinning. So when you roll a ball down a ramp, it has the most potential energy when it is at the top, and this potential energy is converted to both translational and rotational kinetic energy as it rolls down.

This leads to the question: Will all rolling objects accelerate down the ramp at the same rate, regardless of their mass or diameter? If two cylinders have the same mass but different diameters, the one with a bigger diameter will have a bigger moment of inertia, because its mass is more spread out.

Similarly, if two cylinders have the same mass and diameter, but one is hollow so all its mass is concentrated around the outer edge , the hollow one will have a bigger moment of inertia. Does moment of inertia affect how fast an object will roll down a ramp? Give this activity a whirl to discover the surprising result! Observations and results You should find that a solid object will always roll down the ramp faster than a hollow object of the same shape sphere or cylinder —regardless of their exact mass or diameter.

This might come as a surprising or counterintuitive result! A classic physics textbook version of this problem asks what will happen if you roll two cylinders of the same mass and diameter—one solid and one hollow—down a ramp. The answer is that the solid one will reach the bottom first. In that specific case it is true the solid cylinder has a lower moment of inertia than the hollow one does.

Although they have the same mass, all the hollow cylinder's mass is concentrated around its outer edge so its moment of inertia is higher. So, in this activity you will find that a full can of beans rolls down the ramp faster than an empty can—even though it has a higher moment of inertia.

It has the same diameter, but is much heavier than an empty aluminum can. Applying the same concept shows two cans of different diameters should roll down the ramp at the same speed, as long as they are both either empty or full. The same principles apply to spheres as well—a solid sphere, such as a marble, should roll faster than a hollow sphere, such as an air-filled ball, regardless of their respective diameters. This activity brought to you in partnership with Science Buddies. Follow him on Twitter BenFinio.

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Key concepts Physics Mass Gravity Kinetic energy Potential energy Introduction Imagine rolling two identical cans down a slope, but one is empty and the other is full. Materials Two soup or bean or soda cans You will be testing one empty and one full. A hollow sphere such as an inflatable ball A solid sphere such as a marble It does not need to be the same size as the hollow sphere. Cardboard box or stack of textbooks Flat, rigid material to use as a ramp, such as a piece of foam-core poster board or wooden board.

The longer the ramp, the easier it will be to see the results. If our intuitive sense of this situation is right, and heavier things fall faster than lighter ones, then they should also roll faster.

Putting them on a ramp only slows things down so it's easier to watch. Try this yourself, however, and you will see that the two roll at exactly the same rate. That is, both will roll faster and faster as they proceed down the ramp, but they will keep pace with each other and hit bottom at the same moment. Give Now ». Noon Edition. Home Archives About Contact.

By William Orem Posted December 21, Media Player Error Update your browser or Flash plugin. Which Hits The Ground First? This Seems Hard To Believe What Happens With A Ramp? You have demonstrated that all things fall at the same rate.