# Force on a surface

1. Jun 12, 2009

### James...

I was talking to my physics teacher the other day & he was saying about measuring the force exerted by an object on a surface if it is dropped onto it. The example we were using was dropping ball bearings onto a table (we are currently studying momentum)

If I have got the above correct, what measurements would you need to meauser the force. I may have got a mixed up explaining it but he said he has been looking for a way for a long time & has done alot of various ways but hasnt been able to yet.

Just wondering as I dont have him until for nearly a week & have been pondering over ways myself & wondering if one of them will work.

I may have got it worng but if anyone knows what I would need then i would appreciate it.

James

2. Jun 12, 2009

### diazona

Are you talking about the force of a single ball bearing dropped on to a table? Because that's highly time-dependent and pretty complicated to calculate - it depends on the elastic properties of the ball bearing and the table. But if you want to calculate the time-average force of a steady stream of ball bearings being dropped on a table, that wouldn't be too hard. All you'd need would be the rate of momentum carried by the stream of ball bearings.

3. Jun 12, 2009

### Andy Resnick

There is no such thing as a "forceometer". We cannot measure force directly. We can measure energy and momentum, and how much is imparted to a surface by a moving object.

4. Jun 12, 2009

### diazona

What about a scale?

5. Jun 12, 2009

### flatmaster

If you had a material that is easily dented such as play-dough, you could study an inelastic collision. I don't have the means to calculate it now, but if you have the velocity of the ball before it hits the play-dough (a function of the height from witch you droped it) and the depth of the dent, you can calculate the average force over the deceleration of the ball.

6. Jun 13, 2009

### granpa

look up 'impulse'. its much easier to measure than the force.

7. Jun 13, 2009

### James...

That will probably be what we would be measuring then I assume.

My idea was based on an experiment that we are going to be doing soon.

We were going to try & make a listening device to detect vibrations on objects. put something reflective on a low frequency speaker & fire a laser onto it, next put a microscope slide at 45 degrees in the path of the laser with a reciever one side & a fixed mirror the other side, both perpendicular to the beam in line with the slide.

Unless the object you measure moves, some light will arrive in phase with each other at the reciever, but it it moves slightly you can measure to 1/4 of a wavelength according to my teacher.

My theory was, if you could set up one vertically, reflecting light off a falling ball bearing you could connect it up to a computer & create a distance/time graph which could then allow to work out exectly when it hits, compression of the ball, deceleration, rebound etc.

Do you think this could work at all? I'm going to ask him when I have him on wednesday.

Cheers
james

8. Jun 13, 2009

### Andy Resnick

A scale does not measure force- in order for a scale to calculate force, there must be a constitutive relationship between force and deflection specified. Constitutive relations cannot be derived from first principles.

Ditto for springs (F = kx, where 'k' is determined experimentally).