# I need some serious help, really basic physics!

1. Oct 19, 2009

### kylelomba8

I am trying to design an apparatus which will hit a marble a horizontal distance of from 1.5-1.6m every time. I have to calculate the force I need to hit the marble, but I can't seem to find an equation which works. If it helps any, the mass of the marble is 3.56g and I'm using a hammer which is held up on a swivel which swings down and strikes the marble. As well I have to calculate the average horizontal velocity. I am just not sure where to begin and what to do.

2. Oct 19, 2009

### Staff: Mentor

What forces slow down the marble? That would seem to be a key part of this project, eh?

3. Oct 19, 2009

### kylelomba8

the only force that slows down the marble is gravity, but I cannot figure out how to calculate the force required to make the marble go that distance

4. Oct 19, 2009

### Staff: Mentor

No, gravity does not directly slow down a rolling marble. What is the surface that you need to use for this project? There will not be much rolling resistance if the surface is hard and smooth...

http://en.wikipedia.org/wiki/Rolling_resistance

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5. Oct 19, 2009

### kylelomba8

I have actually made it with a golf tee and there doesnt seem to be too much in terms of friction to move the marble

6. Oct 19, 2009

### Staff: Mentor

When I asked what surface, I meant what kind of flooring material is it? Concrete, linoleum, wood, carpet, etc.? Since the ball is rolling, friction will likely play a small roll in slowing it down. It will affect how it reacts to the hammer tap and spin-up in the beginning, though.

7. Oct 20, 2009

### kylelomba8

Yeah I know what you mean, but when i am calculating it, I am looking for the distance it will go before hitting the floor. So it then doesnt really matter what the surface is.

8. Oct 20, 2009

### Staff: Mentor

Oohhh! My bad, I misunderstood your original post (OP). I thought the marble was supposed to roll that distance, but now I understand that it needs to fly that distance and hit its first bounce at that distance. Ah.

You calculate the motion using the initial velocity and takeoff angle, just like for any kinematics problem involving projectile motion and gravity. You should be able to calculate the initial velocity from the striking velocity of the hammer and the hammer and ball masses, since it is an elastic collision...

9. Oct 21, 2009

### Jobrag

If you already have the swinging hammer set up trial and error looks like a good solution.