The primary reason that it is difficult to push a shopping cart in a straight line is that the wheels are not fixed, but rather are able to rotate on their own axes. This makes it difficult to control the direction of the cart, especially when starting from a stationary position. One solution to this problem is to limit the ‘swiveling ability’ of the rear wheels, so that the front wheels can be better controlled.
It’s common knowledge that for a product to sell, you need a good marketing strategy to advertise your product to the masses, a good sales team, an effective distribution system and many other things. But hey! Did you ever realize that there’s one inconspicuous, utterly common thing that might also play a role, even a tiny one, that helps products get picked from the supermarket shelves?
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The Importance of Shopping Carts
This may sound a bit odd at first, but shopping carts have played an important role in revolutionizing the transaction of commodities in stores and shopping marts. If you’re a person who’s spoiled by the comforts and luxuries of the 21st century, ask yourself this; if checking out different varieties of the same product meant that you had to traverse the entire length of a section in a grocery store, would you really bother? Of course, some people would ‘go the distance’ to fetch the items of their favorite type or brand, but the average shopper, who doesn’t care if the toothbrush they’re using is made by Oral-B or Colgate, would just pick the one nearest to the billing counter.
The Design
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What’s the problem with shopping trolleys?
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What’s the reason behind it?
The set of four wheels dictate the terms when it comes to steering a shopping cart. In such carts, all four castors are ‘floating’, which means that they swivel. This is a good thing when it comes to the amount of force the user needs to apply to get the trolley ‘moving’, but it’s bad news for steering and overall maneuverability. Since all four wheels can rotate on their respective axes, it becomes much more difficult to move the trolley in the desired direction, especially when you start moving it from a stationary position.
What’s the solution?
Since the problem lies in all the castors moving in random directions of their own accord, you want to make sure that their random movement is somehow mitigated. Also, you don’t want to impact the maneuverability of the trolley (i.e., it should still turn easily). The solution is fairly straightforward: limit the ‘swiveling ability’ of the pair of castors on the rear. This way, you can push the trolley with ease, while exercising better control over the direction of the trolley’s movement by steering the front castors. Many countries (e.g., the United States) already have trolleys designed in this way. Thankfully, the ‘fixed-wheel’ design is becoming increasingly common in other parts of the world too, thanks to its popularity in shopping carts for grocery stores. 
In terms of applied force on shopping carts in grocery stores, it turns out that a simple design tweak can save a lot of ‘Newtons’ on a daily basis!
