# Back and forth motion

1. Dec 5, 2013

### viciam

Hi guys,

I'm not a physicist or a mathematician but I have a question which will probably sound silly to you all

I need to know what that motion is called when you have an object and it moves up and down. But it gains more momentum every time it reaches the top so when it drops it goes down further and faster and when it reaches the maximum bottom it comes back up even faster and further and this cycle just keeps repeating. On every completion of up and down movement it goes further and further down and further and further up

In one word please tell me what this is called so I can google it. I inertia and reciprocating but this is not it. I know there's one word to describe this but I just can't remember it.

Thanks

2. Dec 5, 2013

### darkxponent

In a periodic motion object moves back and foirth about its mean position.

The kind of motion you are describing can be more precisely called a 'Simple Harmonic Motion'.

Now you can google those two terms i described above

3. Dec 5, 2013

### AlephZero

Try "resonance".

But don't get sidetracked into crackpot websites about perpetual motion machines, "free energy", etc.

4. Dec 5, 2013

### Staff: Mentor

Undamped forced oscillation:

http://www.nptel.iitm.ac.in/courses/Webcourse-contents/IIT-KANPUR/engg_mechanics/ui/Course_home_26.htm [Broken]

Last edited by a moderator: May 6, 2017
5. Dec 5, 2013

### viciam

These all sound pretty complicated.

I thought it was as simple as a pendulum swing or something

6. Dec 5, 2013

### Staff: Mentor

You need to be putting energy in with each swing, to get bigger oscillatory motion. And there needs to be very little damping, so the energy being added in does not get dissipated by the damping.

7. Dec 5, 2013

### nasu

Yes, you can do this with a swing. Either by pushing someone from outside or by yourself, flexing your legs at the right times. The amplitude increases at each cycle, up to a limit.
Can be modeled as a forced oscillator, as already mentioned.
But the math is not so simple, even for a "simple" pendulum, at large amplitudes.

8. Dec 6, 2013

### viciam

This is exactly what I'm trying to find out. You see how the waves become larger and larger as they go along? I typed in increasing oscillations in Google to get this image.

http://www.nptel.iitm.ac.in/courses/Webcourse-contents/IIT-KANPUR/engg_mechanics/lecture26/26_1_clip_image059.gif [Broken]

Last edited by a moderator: May 6, 2017
9. Dec 6, 2013

### Jilang

10. Dec 6, 2013

### voko

A one-word description for this motion would be "impossible". Or "nonphysical". Or "imaginary". It does not exists in nature. The amplitude of oscillations may increase for a while, but then it will either reach some maximum level, or "bifurcate" into some other motion, which is a fancy way of saying "break into pieces".

11. Dec 6, 2013

### Staff: Mentor

One example of this oscillatory buildup which is very physical and non-destructive is the startup of a crystal oscillator. It starts with noise deviating the output of the oscillator amp a small amount, and the combination of positive & negative feedback builds up the oscillation slowly over many cycles until the oscillation limits itself at the nominal oscillator output amplitude.

12. Dec 6, 2013

### voko

So this reaches a maximum level, just like I said. It does not grow infinitely.

13. Dec 6, 2013

### Staff: Mentor

Yes, absolutely. It just doesn't break into pieces like the Tacoma Narrows Bridge!