# Physics 19 How Friction Works

Up until now we have ignored the effects of friction on our balls and cars. The effect on them was so small, it did not affect our results.

Friction does have a big effect on our lives everyday. It keeps a car on the road and lets it stop. It creates static electricity.

Question: How does friction work?

Materials:

Block of wood [a piece of a 2×4 is great]

Sandpaper or rough cloth

Tape

Long smooth board [about 0.5 m long]

Meter stick

Procedure:

Rub the piece of wood on your hand

Describe how it feels

Rub the piece of wood back and forth on your hand really fast several times

Describe how it feels

Set the piece of wood on one end of the long board

The block should be set with the same side down and with the same side on the downside each time as a control of variables.

Lift that end of the board up slowly until the piece of wood slides down

Measure how high you lifted the end of the board

Describe how the block of wood acts

Since I taped the sandpaper on only one way, I had to place the block the same way on the board each time or the results changed.

Tape the sandpaper on the block of wood [only tape from the edges]

Place the block of wood on the end of the long board

Lift that end of the board up slowly until the block of wood slides down

The board had to be lifted twice as far before the block with sandpaper on it would slide down the board.

Measure how high you lifted the board

Describe how the block of wood acts

Tape sandpaper on the long board

The sandpaper had to be taped on the sides and the ends to lie flat on the board.

Place the block of wood on the end of the long board

Lift that end of the board up until the block of wood slides down

Measure how high you lifted the board

Describe how the block of wood acts

Take the sandpaper off the block of wood

Place the block of wood on the end of the long board

Lift that end of the board up slowly until the block slides down

Measure how high you lifted the board

Observations:

Describe how the block of wood feels on your hand

Describe how your hand feels when you rub it with the block of wood

The block with sandpaper on a board with sandpaper did not want to slide down. I had to lift the board up a lot. One time the block tumbled down instead of sliding.

Measurements:

Plain board:

Sandpaper on block:

Sandpaper on block and board:

Sandpaper on board:

Describe how the block of wood acts:

On the plain board:

With sandpaper on it:

With sandpaper on it and the board:

With sandpaper on the board:

Conclusions:

What creates the heat when you rub the board on your hand?

Why do you think the block with sandpaper on it didn’t slide down as soon as the block without the sandpaper did?

What is one way to increase friction between two objects?

A smooth block on a smooth board showed the least friction.

What I Found Out:

My piece of wood was varnished on one side so it felt very smooth. When I rubbed it back and forth on my hand, my hand felt a little warm.

The block slid down the board when it was 17.3 cm off the table.

After putting sandpaper on the block, I had to raise the board up 30.5 cm before the block slid down. The sandpaper was rough and held on to the board keeping the block in place longer.

When the block with sandpaper was wet on the board covered with sandpaper, I had to lift the board up 35.5 cm, almost vertical before the block slid down. The first time the block didn’t slide off, it tumbled off.

It didn’t matter whether the sandpaper was on the block or on the board, I lifted the board the same amount.

When only the board had sandpaper on it, I lifted it 30.5 cm to make the block slide down. This was the same as when the sandpaper was on the block and not on the board.

When both the block and the board were smooth, the friction was much less than when one or both had sandpaper on them. Roughness increases friction between two objects.