Wheels help things roll — Physics, 4–6 years
A wheel changes sliding into rolling. Rolling usually makes it easier to move an object across the ground, because only a small part of the wheel touches the surface at one time.
Wheels help things roll
A box dragged across the floor rubs against a large area. Put round wheels underneath it and the box rolls instead, with much less rubbing. The wheel turns around its centre while its edge travels along the ground. That is why a wheeled object can move farther with the same effort.
Why make wheels?
Long ago, people had to move heavy loads without lifting them. Dragging worked, but it scratched the ground and needed lots of effort. Round logs gave a better idea: put a load on something that rolls. Wheels grew from this problem, although they still need a firm surface and do not make every journey easy.
Moving a heavy box
A 6-kilogram box must cross a room. First, slide it: it scrapes along the floor and needs a strong pull. Next, place four small wheels under a board and put the box on top. Push the board gently; the wheels turn, and the box travels across the same floor with less rubbing and less effort.
“Wheels always make it easy”
It is reasonable to think wheels should work on every surface, because they help so much on a smooth floor. But a wheel can sink into mud, slip on ice or stop at a step. Wheels reduce rubbing on a suitable surface; they do not remove bumps, slopes or the need to steer and stop.
Wheels around us
Bicycles, prams, skateboards, suitcases and cars use wheels to roll over the ground. Wheels also turn inside clocks, toy vehicles and some machines. Their size and shape must suit the job: a bicycle wheel rolls on a road, while a tiny wheel may work better under a drawer. Wheels are useful, but not the answer for every surface.
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