Why do some things roll?


Crayons and pencils roll off of desks. Tires spin and cars, bicycles and tricycles roll down the street. People can roll down hills (and probably get very dirty!)

If an object is the right shape and it gets pushed or pulled, it can roll! The most common shapes that will roll are the round ones like circles! Bowling balls, pennies, tires, oranges, marbles, and anything else round will roll pretty easily if we push them. Also, if they're on a ramp or a hill, they'll roll because gravity the force that pulls everything to the ground pulls on them.

The reason they roll so easily is because these shapes don't have a lot of edges to get in the way and stop them. Other shapes like rectangles, squares and triangles have many edges! If you were to push a book, not much would happen. It may slide a few inches, or it may tip over once, but it's not going to roll. The edges of the book act like brakes when the book hits the ground. It just lands flat on one side. It may still have some energy from your push, but not enough to tip it over to its next side. Round shapes don't have edges or sides, so they just keep turning and rolling until they hit something or they run out of energy!


Lucky Roll!

Sometimes, something that has a lot of edges can roll. You can see this the next time you're playing a board game that uses dice. Each of the dice is a cube with six sides and twelve edges. These edges do make it difficult for the dice to roll if you give them just a tiny tap, but if you toss the dice onto the game board with enough force, they will roll. The strong push that you give them makes the dice tumble edge over edge, until they eventually aren't strong enough to keep going, and they stop. It's a good thing they eventually stop, or else we wouldn't know how many spaces to move in the game!Think about that next time it's your turn to roll!