Ordering slip or grip forces

What the Activity is for

To help children describe the world by:

• relating physical experience of exerting force to representing that force with arrows
• connecting the roughness of the surface to the size of the force

What to Prepare

• friction hand puppet pairs: slippy and grippy (grippy can be made of reversed leather, slippy of parachute nylon – see what materials are to hand)
• three pairs of rectangles of sandpaper, we'd suggest 400, 240, 120 grade
• some large diameter tins or plastic bottles to lift – glass jars are tempting, but are just a disaster waiting to happen

What Happens During this Activity

Invite children to drag the matched pieces of sandpaper over each other, and label them with the three slip forces. Encourage the children to relate their labelling to the roughness of the surfaces, ascertained by rubbing their finger over the sandpaper and perhaps inspection of the surface by using a magnifying glass. You could profitably relate this to the roughness of the finger puppets. It's important to make links to the warming effect of the slip force, and this is something that can be felt rather well when you drag the grip or slip puppet over most surfaces.

Now repeat with the grip arrows. Here the trick is to order the sandpaper pairs by which are best at preventing relative movement. You can relate this to many examples where we do not want things to slide past each other: getting cars going – its best if the tire does not slip on the road. similarly for walking: you don't want the sole of the shoe to slide over the pavement.And for lifting tins, you don't want movement between the surface of the tin, and your hand.

We'd suggest not using more than three matched pairs of sandpaper surfaces, and not mixing and matching the surfaces, as this makes the labeling harder to do. However children could be allowed to explore by themselves, but may have to invent their own labelling arrows. They might find it hard to discriminate reliably, but so long as they are reasoning about the surfaces, and are moderate in their expectations, such explorations are likely to be worthwhile.