Why are heavy shopping carts much harder to turn than they are to push forward?
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When you’re pushing straight ahead, you’re only working against the inertia of the cart (its resistance to change in speed). When you’re changing direction, you’re also working partially against its momentum (the force of its current motion). Most other things on wheels have at least some that don’t swivel freely, and can transfer momentum to the ground for free.
Edit: also, many use the wrong technique by trying to turn them like a car. The most efficient way is to actually twist the entire cart more than you want it to turn, and start pushing straight on it until you’ve achieved the desired direction. Then you turn it to point in the actual direction of travel again, and push straight. If you’ve ever played Asteroids, it’s the same technique you use for steering the space ship.
When you go forward you push with you legs, legs are strong.
When you turn you use your arm to direct the cart, arms are weak.
Try to feel what muscle you are using next time you use a shopping cart.
Because it’s a lever.
Pushing your cart forward transfers 100% of the force in direction of movement, over 100% of the way you pushed.
Turning it means you move the tip of the cart around you while (more or less) standing in place and moving the handle very little. For the two sides of a lever, the product of force and length is equal, so when you move the closer end just a little and the far end moves a lot, only a fraction of the force you applied gets transferred. That means in order to accelerate the tip of the cart by an amount you deem appropriate you need a way stronger force compared to just pushing the cart forward.
This works the other way as well: if you keep the tip of your cart in place and turn it by moving the handle around that turning point, you will find it easier to move the cart at all, for the tradeoff of having to apply the force over a longer distance, i.e. you have to walk along the circle with your carts handles.
Reason 1: Work
The amount of energy needed to move the cart (‘work’) is greater for its rotational movement (torque) than it is for its linear movement on wheels (basically a frictionless surface).
Reason 2: Human Body
In pushing the cart forward, you’re utilizing your legs, butt, triceps, and self-weight to transfer energy. To rotate the cart, you’re basically just using the small muscles in your wrists when grabbing the cart handle.
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