What is a simple way to slow down this cart? What can I do to slow the wheels down and make this 4 wheel cart harder to push? I'd prefer a fairly straight forward solution. It will be used for exercise. I am very naïve when it comes to physics.
 A: You'll want to increase rolling friction. You do that by adding/increasing factors that absorb energy.

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*Use non-ideal bearings that have friction. You can do so destructively by adding a sticky substance (honey e.g.) to the bearing (or dipping them in salt water and letting them rust, as a comment suggested).


*Use non-stiff tires. When the wheel material is compressed during rolling, work is done between rubber particles within which causes internal friction. Energy is absorbed within the rubber and converted to heat. You achieve this by letting air out of the tires.


*Add mass to the cart to increase the above rolling friction factors, as already mention by another answer.


*Add resistance to the axles. This could be a bar of rubber you screw onto the bottom of the cart above an/each axle which is in contact with the axle (the pole connecting two wheels). When the axle rotates, friction is generated, slowing down the cart.
Depending on your surface and surroundings you can use external methods such as:

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*Place the cart on soft ground. On the beach, for instance. Work must be done in altering the ground, pushing surface particles around.


*Add a "sleigh brake", which is a slab of material that scrapes over the floor/ground (or a lever that cuts into the ground for large resistance). Attach a slab of some rough material at one end to the frame with the rest of the slab hanging underneath the cart and sliding over the ground. Add mass to the slab to increase friction with the ground. For indoor use, rubber is again a good choice for high friction.
A: There are lots of things that can make the cart harder to push. All of them involve some kind of resistive force, which can be gravity, rolling resistance, or friction with the ground directly:
Push it uphill.
Remove the rear wheels entirely.
Add weight.
Push it on a soft or bumpy surface like sand, mud, or gravel.
Deflate the tires.
Add a wheel brake that rubs on the tire.
Add a ground brake that rubs on the floor.
A: You can deflate the tires and add weight, as others have said, or else add a friction brake like the type used in bikes and cars. The friction has to act against the wheel or the tire. Acting against the tire it might damage the tire. Acting against the wheel it will be hard to get right because the wheels are small so there is not much room to put something in there, but maybe a simple spring with a bike brake pad on the end will do.
But surely the easiest method is just to get rid of the wheels!
In other words, if you want it to be quite hard to push, then use a sled instead and if you are worried about damaging the floor then use it outside and enjoy the fresh air.
Nice cart by the way.
A: Add weights to the cart, this will increase the friction for the wheels and make it harder to push.
A: We do not know how much slow down you need, but I can propose a scalable method over a wide range.
I expect it may not be practical for your case, but it could be.
You could make the wheels roll on a layer of a sticky and highly viscous fluid.
You could vary the viscosity, but also the layer depth.
Drive through honey...
The fluid could be honey, for example. You can roll on a surface just wet with honey (or something similar), or in a 3 mm layer, or in 30 mm.
The sugar solution can vary from pure water to saturated solution, and could contain sugar crystals to increase viscosity. It is easy to imagine that it has different effects on the movement.
It changes it's viscosity and its saturation point with the temperature.
This scales from no braking with pure water to quite a lot with 20 mm viscous honey.
... or inverted sugar syrup
But if that is not enough, we need a fluid with higher Viscosity: You can use inverted sugar syrup instead of the sugar solution. That is a mixture or solution of glucose and fructose instead of the sucrose used before.
It has, depending on temperature and moisture, a viscosity from like honey to basically solid - still flowing, but too slow to see it.
That covers the scale from "no slow down at all" continuously down to "apparently stuck in place".
