B Coefficient of Friction With only Time and Distance

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1. Dec 24, 2017

YarnMonkey

Im a high school student and am in AP Physics 1. I just created a formula for calculating the coefficient of kinetic/static friction between two objects using only distance and time (I don't have a way to measure newtons). Is it correct and can it be used in the real world?

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2. Dec 24, 2017

scottdave

Everything looks good to me, with one exception. About midway you say "...and that the initial KE is 0", shouldn't it be "final KE is 0"? Since it comes to rest. You need to have a reliable method to start the clock and measure when/where you stop applying the motive force.

3. Dec 24, 2017

YarnMonkey

I see what you mean. I think it is just a typo, because right underneath I retain the negative indicating that it is initial kinetic energy that should be used in the equation, not finale. Thx for your help and if theres anything else you see that may not be realistic feel free to tell me!

Also, Im imagining that you start the clock as soon as the impulse is applied to the object giving it an initial velocity and the clock stops when the object has come to a complete rest. Then you measure the distance between the initial position of the object and the finale position after it has come to a rest.

4. Dec 25, 2017

scottdave

Hmm. I think the formulas will be more complex though. How much time is taken accelerating? How much distance while the force is applied?
The v used for Kinetic Energy is the maximum velocity (when you stop pushing). That should be when the clock starts for the time. If you want to start the clock when you start pushing, then you need to work out the forces acting during that time.

5. Dec 25, 2017

scottdave

From a practical standpoint, a device could be manufactured which would accelerate up to a certain location, then let go. Like a crossbow or ballista, perhaps.

6. Dec 25, 2017

YarnMonkey

To clarify, I am imagining that it would be like hitting a baseball, the amount of time during which the impulse is applied is insignificant and before it can even be measured the object already has a maximum initial velocity.
For example: I have a mass on a table, and i punch it. The time during which my hand is in contact with the mass is almost insignificant because the mass is done accelerating within fractions of a second. In other words, there is no force being consistently applied to the object other then friction and the impulse occurs within such a small fraction of a second that it is insignificant when calculated for.

7. Dec 25, 2017

scottdave

So if the distance is small enough and the time is small enough, then you should get a good approximation value for the actual coefficient. It would be good to have another merhod of measuring so you could verify the results.

8. Dec 25, 2017

YarnMonkey

Thanks a lot. As a matter of fact I did test this equation using some of my homework questions and it did check out although I still wanted to check on physics-forums cause my homework isn't always completely realistic and I didn't know if i was leaving out something that I hadn't learned. (In physics 1 we dont learn how to calculate air resistance so for a falling block we cant calculate is actual downward acceleration).

Thanks a lot for all of your help!

9. Dec 25, 2017

scottdave

If you do a lab experiment, like a falling object, the shape of the object affects how much air resistance there is. If it is the right shape and not falling for very long, the air resistance effect will be small compared to an object falling in a vacuum.

10. Dec 25, 2017

scottdave

Air resistance calculations are very complex. That is why they try to keep things to assuming negligible air resistance in introductory Physics classes.