Rub your hand on a rock. It feels bumpy, so it is rough. Now rub a window. It feels flat and slippery, so it is smooth.
1Icegoes farthest
2Rugslows down
3Grassstops soon
Rubbing slows things down
A rolling ball rubs on the ground. The rubbing slows it down. This slowing force is called friction. Rough ground has lots of friction. Smooth ground has very little.
Try it with a ramp
Roll the ball down the ramp below. Then pick a new floor and roll again. On a smooth floor it rolls far. On rough ground it stops sooner. A steeper ramp makes it faster.
Bumpy shoesgripSand on iceless slippingRub handswarm
Friction can help
Friction is not always bad. Bumpy shoe bottoms grip the floor, so we do not slip. Sand on an icy sidewalk helps the same way. Rub your hands fast. Friction makes them warm!
For parents
Goal: Your child learns that friction is the rubbing force that slows things down, that rough surfaces have more of it, and that steeper ramps make things go faster.
Touch things around the house and ask: is this rough or smooth?
Ask: why do you think your shoes have bumps on the bottom?
Before each test, ask your child to guess where the car will go farthest.
Away from the screen: Make a ramp with a book and a board. Roll a toy car down onto a smooth floor, then onto a towel or rug, and compare how far it goes. Then add a second book to make it steeper.