Contraption to apply constant force

1. Nov 9, 2013

johnnnnyyy

What is a contraption I can make at home somewhat easily to apply a constant force to a block? I am doing an experiment on how far the block will slide. I was thinking about making some sort of pendulum but would love to hear some of yours guys ideas.

Thanks

2. Nov 9, 2013

UltrafastPED

Springs are good ...

Or you can get a pulley, a wire, and a bag of sand; you may have to build a platform.

3. Nov 9, 2013

johnnnnyyy

Would a spring work better than a pendulum?

4. Nov 9, 2013

UltrafastPED

I don't see how you plan to use a pendulum as a force ... or do you plan to just use the bob? In that case it's like the bag of sand ...

For a spring the force is proportional to the extension/compression.

5. Nov 9, 2013

johnnnnyyy

I was planning on putting something on the end of the pendulum and raising it to a certain point every time and letting it hit the block at the bottom.

6. Nov 9, 2013

7. Nov 9, 2013

K^2

Not in general. The force that would actually be transfered 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.