Wheels are everywhere in our world today—in very obvious places (on cars, trucks, and airplanes), but also hidden inside everything from computer hard-drives and clothes washers to electric toothbrushes and dishwashers. Six thousand years ago, there weren’t any wheels at all. The rise of the wheel, from a basic turntable that helped people mould clay pots to a key component in hundreds of important inventions, owes everything to the simple and effective way it helps us capture and harness energy and transform forces. Let’s take a closer look!
Wheels help vehicles to move along by transferring and reducing friction (Friction is the force that opposes the motion of an object).Wheels also work as levers, as in the case of ship wheel .
Why we need wheels?
Dogsled on ice
In modern times, we assume there have to be roads for wheels to travel on. But wheels were first used on carts precisely because there were no smooth tracks to use for reliable transportation. Before carts were invented, people dragged loads on sledges and frames hauled behind animals such as horses and dogs. Sledges were an effective way to move heavy loads before wheels were invented, but friction slows them down. Frames, where a load is part dragged and part carried, help to solve this problem. The A-shaped dragging frame, known as a travois, is thought to have been invented thousands of years ago and Native Americans used it up until the 19th century. Even with animal power to help, friction between the rough ground and the frame made the going difficult.
How do wheels work?
Dragging a load using a wheeled cart is far easier than dragging it on the ground—for two reasons:
Wheels reduce friction. Instead of simply sliding over the ground, the wheels dig in and rotate, turning around sturdy rods called axles. That means the only friction the animals have to overcome is at the point where the wheel and axle meet—between the relatively smooth inner surface of the wheels and the equally smooth outer surface of the axles around which they turn.
Wheels provide leverage (in other words, they are examples of force multipliers or simple machines). A cart with bigger wheels is easier to push because its greater-diameter wheels work like bigger levers, multiplying the pulling or pushing force and making it easier to turn the wheels around their axles—in exactly the same way that a long spanner makes it easier to loosen a nut.
Let’s look at both these things in more detail.
1. Shifting friction to the axle
When you push a box on the ground, there’s a lot of friction between the bottom of the box and the ground below for two reasons: 1) the ground is rough; 2) there’s a large area of box in contact with it:
How friction slows a cart dragged on the ground
When you push the same box loaded onto a cart with four wheels, there’s much less resistance. The box no longer has to slide along the ground so that part of the friction disappears. However, wheels don’t eliminate friction entirely, as some people think—far from it! There must be friction between the four wheels and the ground or they’d simply slide along (like something being pushed on ice). Friction between each wheel and the ground helps it “dig in” so the wheel can rotate.
Carts are easier to push because the only real friction you have to work against is between the four wheels and their axles. As you push a cart, the relatively smooth inside surfaces of the wheels rotate and slide around the relatively smooth outsides of the axles. There’s also relatively little area of each wheel rubbing around its axle. These two things together mean there’s much less friction compared to pushing the box straight along the ground—and that’s why the cart make loads easier to move:
2. Providing leverage
Wheels on carts help in another important way too: they work like levers. The rim of a wheel turns a greater distance than the axle so, in the case where you’re pushing a cart from behind or pulling it from the front, there is more force at the axle than at the rim. That means it really helps if your cart has big wheels because they give you more leverage, magnify your pushing force, and help you overcome the force of friction at the axles.