Friction

Look out! It’s Newton I!

You know you’re a physics geek when the first thought that comes in to your head when watching all those car crashes on the news is: Now that’s what I call Newton’s first law of motion!

First law:
Every body remains in a state of rest or uniform motion (constant velocity) unless it is acted upon by an external unbalanced force.

Now most people think that this force simply slows the car down, but in the world of physics the word velocity covers two quantities; speed and direction. So it could be that the force merely changes the direction of the moving object, and has (little or) no effect on its speed. In fact that is what is happening when an object is moving in a circle (like a stone tied to the end of a rope circling over your head; the stone is moving at constant speed yet we still say that it is accelerating because its direction is changing). In this case the force is provided by the tension in the rope acting inwards.

We demonstrate this in class with an air-track, which is a bit like an elongated air-hockey table. A nice way to re-inforce the concept is to discuss why, when moon-bound rockets leave our gravitational field they can turn off their propulsion system and will remain moving at that speed untill they reach the moon’s gravitational field (although technically both gravitational fields are infinite – but that’s for another day).
Galileo himself (for it was he and not Newton who first promoted this) had great difficulty persuading others of the importance of this discovery.
The response of the students to the air-track demo is a reminder of how strange this idealised world of no friction actually is to us.

But every now and again we get to experience it for ourselves.

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Junior Cert Physics Investigation 2010/2011

The junior cert science investigations for 2011 were published recently and the phyiscs investigation is as folows:

Investigate the factors that determine the force of friction between a  wooden block and the surface on which it is resting.

As usual,  there are few spaces for science teachers to discuss how best to implement this investigation so for what it’s worth I’ve decided to throw out my thrupenny worth of ideas and resources.
Why should my students have an advantage over other students who perhaps have a biology specialist as their teacher? Hopefully a Biology/Chemistry teacher will reciprocate with their comments on the other investigations.

Alternatively maybe a student will happen upon this blog post as part of their research – if so good luck to you. Please let us know how you’re getting on and we will try to help.

Some points to consider:

  • You would imagine that the orientation of the block would make a difference: if the block is standing on edge then there is less surface-area and presumably less friction. But presumably isn’t good enough in physics; many (maybe even most) relationshiops in physics which were originally thought to be obvious turn out after suitable investigation to be actually wrong. Can you think of any?
    To see a demonstration of this see the link here (pull the tab at the bottom over to the 10 minute mark – thanks to colleague Dee Maguire for the link and the heads up on the approach above).

 

  • There is a another nice counterintuitive concept – if students  are playing with different types of sandpaper they may well find that  the friction force between the wooden block and the roughest type of sandpaper is actually less than the force of friction between the wooden block and the lab-bench.
    Hopefully the students will first notice this stange relationship and then figure out why for themselves (this should then form part of their report. Partial explanation – the grains in the very rough sandpaper are acutally acting like little ball-bearings (see link below for an animation).

 

  1. Friction between two rough surfaces from absorblearning.com
  2. Friction between two rough surfaces
  3. The effect of ball-bearings in reducing friction
  4. The effect of lubrication in reducing friction
  5. Rub the two books against each other and note the rise in temperature