UltrafastPED said:
Gravity provides a constant force: it is always the same. The sand bag & pulley system provides a constant force for your block if the string is anchored to the block.
Not in general. The force that would actually be transferred to the block is tension in the string. While tension in the string is equal to m(g-a) of the weight. If acceleration of the block and the suspended weight are constant, then yes, the force applied will be constant. If for whatever reason, acceleration is not uniform, than neither will be the applied force.
If you want to look at it another way, no matter how much weight you suspend, you'll never have the block accelerate past g.
In all fairness, this is probably good enough for whatever OP is doing. But he should consider his exact experimental conditions and how precise things need to be.
In engineering applications, when it is necessary to provide a large, constant force to a moving object, hydraulics are used. For example,
arresting gear on aircraft carriers will typically use a hydraulic system. It is relatively easy to set the relieve valve to desired pressure, and that means the piston will not apply a force in excess of that set.
Of course, here, you also have to take into account mass times acceleration of the piston itself. But it is usually negligible compared to the force applied.
