# I Need help with a calculation

1. Oct 10, 2016

### Kickboxer

I'm building a freestanding heavy bag. Since I live in an apartment I can't hang my bag, so I've decided to build a base to slip my bag onto. I have the materials but not the knowledge. Can someone please help me figure out what the base needs to weigh to keep the bag from toppling over?

Thank you!!!!

The bag is cylindrical; height is 6' and diameter is 16". It can be filled up to 150lbs.

The base is cylindrical as well; height is 10-11" and diameter is 30".

I'll be adding a 3/4" thick base underneath the base to help with balancing. I'm thinking a diameter of 45" should be enough.

I will also create a secondary base and stand that will be placed inside of the base for the purpose of slipping the bag onto.

The average generated force of my kick is about 650lbs.

2. Oct 11, 2016

### Hesch

From the mass center of the bag, you draw a vector representing the gravitational force.

When you kick the bag, you add a horizontal force vector.

Now, add these two vectors. When the direction of the resulting vector crosses the edge of the base, the bag will tilt.

3. Oct 12, 2016

### jbriggs444

This is nonsense. Force and displacement are two different quantities. You cannot add them.

4. Oct 12, 2016

### nasu

He was talking about a horizontal force vector and not a displacement.

5. Oct 12, 2016

### Staff: Mentor

Does the bag have any freedom to move when you kick it (such as a stiff spring for a support)? If not, it may be hard to keep it from tilting up off the ground when you kick it. But it is basically just a torque calculation:
The weight of the bag, support and base times the radius of the base equals the torque it can handle. And the torque you supply is the force of the kick times the height above the ground you hit it.

6. Oct 12, 2016

### jbriggs444

For a static or slowly changing torque, things are simple and one can do a force balance as @russ_watters suggests -- force of kick times height compared to weight of bag times offset of center-of-gravity from support rim. But a kick is the opposite of a slowly changing torque. Rotational impulse would seem to be the better measure of kick intensity.

Rotational impulse gives you rotational velocity by conservation of angular momentum. Rotational velocity gives you rotational energy. Rotational energy tells you how high the bag can lift as it rotates up into the air about the rim of its support. The balance point when the center of gravity passes over the rim tells you how far the bag can tip and, accordingly, how high it can lift before tipping over. Set up the equations and solve.

If we assume that oscillation (a series of kicks in rhythm with the back and forth motion of the bag) is not an issue, that general approach should solve the problem.

7. Oct 12, 2016

### Staff: Mentor

So yeah, different assumptions and goals lead to different paths. With impulse, the bag has to be allowed to move. Is it? As I said, absent a spring base and if we assume lifting is unacceptable, the static analysis works.

If lifting and motion are allowed, then impulse and momentum (energy?) is definitely better. But IMO if motion is allowed, the design needs a spring, which is common for commercial products like this:

http://www.target.com/p/freestandin...wRcBN6dsHIrfyyFIG6wyUaAtU98P8HAQ&gclsrc=aw.ds

Because I don't think it is a good idea to design this in such a way as to allow the base to lift off the ground.

8. Oct 14, 2016

### Kickboxer

Wow, thank you very much for the responses!

I'll have to get on some calculations this weekend.

I have been thinking of adding some kind of absorption mechanism. Maybe making the base bendable and attaching 8 heavy duty springs, equally spaced, around the bottom base connecting to the weighted base.

I'll do some more research this Sunday and do an update.

9. Oct 14, 2016

### Staff: Mentor

I'm sure you can find a sporting goods store to go kick around some ideas (see what I did there? ).