New draft Experiences and Outcomes
Our Physical World - Forces & Motion

Second Level - By carrying out investigations into friction I can explain how it affects movement, and can use my understanding of friction to design or improve a product.  SCN 222L

                     

Who Says Friction’s a Drag?

Children are often aware of friction because it is one of the forces that can slow or even stop moving things. Weel kent investigations are based on moving objects down a ramp [1].

As an alternative investigation try this :-

You’ve got a horizontal pole and set of masses tied to one end of a length of string. How can you suspend the masses plus string on the pole without tying a knot? The masses have weight because the force of gravity is acting downwards on them. See if any of the children come up with the solution below.

What force is holding the weight on the bar and opposing the downward force of gravity? Friction.

  
Figure 1 - Using friction to hold things up

Where can we see this method of tethering in action? Watch a cowboy film – this is how they secured their horses outside the saloon (see above) – no need for a knot!

Ever done abseiling or mountaineering? Check out the ways they use for getting down, and up, ropes. We can’t go abseiling in class or bring in a horse, but the simple apparatus above  lends itself to the investigation of a range of variables.

Possible variables to change in your investigation include the:

• type of string

• thickness of the string
   

• diameter of the supporting bar

• material the supporting bar is made from
   

• number of turns of the string round the bar.

Some schools will be able to borrow the clamp stands shown in Figure 1 from their local secondary. If you can’t, you should have something in your technology box that you can use, for example, G-clamps.

Example investigation using the apparatus shown in Figure 1

Investigate how the diameter of the bar affects the weight which can be held.

 Vary

Diameter of bar.

Measure

Maximum weight that can be held.

Keep all the other variables the same throughout the experiment.

Wooden dowelling of various diameters can be use as the bars.

Draw up a table to record your findings :-
 

 Note that if you have already taught the difference between weight and mass, you should refer to the “Mass Supported (g)”.

Look at Figure 2 (left). A small weight seems to be able to support a larger one. This principle can be extended to a tug-o’-war – in a contest where the rope is straight a teacher would have little chance against 3 or 4 pupils but loop the rope round a tree (or strong, fixed pole) and the odds change.

Alternatively, this principle can be used to move objects which are heavy – an example being capstans (Figure 3, right). A capstan is used mainly on ships to lift heavy loads. Originally worked manually, electric versions are now available but they both work by winding rope around a drum.

Figure 2 - A small weight supports a larger one.                                                                           Figure 3 - Capstan on a ship

Reference

[1] - http://www.bbc.co.uk/schools/ks2bitesize/science/activities/friction.shtml

Plants for Primary Pupils