# Physics 5 Exploring Friction

Rub your hands past each other once. What do you feel?

My hands feel rough against each other. I can feel friction between them.

Now rub your hands back and forth a dozen times or so. What happens?

My hands warm up. I use this when my hands get cold in the winter.

Friction makes the heat. It does other things too.

Question: How does friction work?

Materials:

Smooth block of wood like a couple of inches of a 2 x 4

2 Pieces of Sandpaper

Tape

Smooth board about 0.5 m long

Meter stick

Procedure:

Rub the block of wood on your hand

Describe how it feels

Rub the piece of sandpaper on your hand

Describe how it feels

Push the block of wood down the smooth board

Describe how it feels

Set the block of wood on one end of the board

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

It didn’t take long for the block to slide down the varnished board. The two smooth surfaces had little friction between them.

Measure how high you lifted the board

Tape the sandpaper rough side out on the block

The sandpaper on the block felt very rough especially compared to the smooth sanded block surface.

Push the block on the board

Describe how it feels

Set the block on one end of the board

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

Measure how high you lifted the board

Tape a piece of sandpaper on the long board

I had a long piece of sandpaper used on a belt sander so it covered the length of the board.

Push the block on the long board

Describe how it feels

How high do you think you will have to lift the board this time?

Set the block on one end of the board

Lift the end of the board until the block slides down the board

Measure how high you lifted the board

Observations:

Describe how the block feels against your hand

Describe how the sandpaper feels against your hand

Describe pushing the block on the board

Describe pushing the block with sandpaper on it down the board

Describe pushing the block with sandpaper down the board with sandpaper

How high do you think you will lift the board the last time?

Measurements:

Plain block on plain board

Block with sandpaper on plain board

Block with sandpaper on board with sandpaper

Even the varnished board was rough enough for the sandpaper to hold to and keep the block from sliding down.

Conclusions:

Does the block or the sandpaper have more friction? Why do you think so?

Which of the three times you lifted the board was there the most friction? Why do you think this?

What did increasing the friction between the block and board do?

How could you decrease the friction between the block and board?

How could you increase the friction between the block and board?

Car tires use friction to keep and move the car on the road. What creates this friction?

Why is it important to not have smooth tires?

Why is driving on ice so dangerous?

The block seemed stuck to the board as I raised the board higher and higher. Shortly after taking this picture, the block tumbled down the board.

What I Found Out:

My block was a piece of a two by four so it was sanded smooth and felt smooth against my hand. The board was varnished on one side and that side felt slick. Rubbing them against my hand made my hand a little warm.

I put the block on the varnished side of the board and lifted the end. The end was 17.3 cm high when the block slid down.

The sandpaper was rough on my hand and made my hand hot rubbing it. I put the block on the varnished side of the board and started lifting that end. It was 30.5 cm high before the block slid down.

When the block with sandpaper on it was set on the board with sandpaper on it, I had to lift the end up 35.5 cm and the block tumbled off instead of sliding.

The block had a lot less friction than the sandpaper because it felt smooth and only warmed my hand up rubbing it. The sandpaper scraped my hand and made it feel hot.

Friction makes it harder for an object to move so the third time with sandpaper on both the block and the board had the most friction. I had to raise the board really high to get the block to move and then it fell off.

The smooth block and board had  the least friction so any way to make them smoother would decrease the friction even more. Using rougher sandpaper on both the block and board would increase the friction.

A car tire has treads. Roads are usually rough. Rougher kinds of tread and rougher roads would increase the friction. Smooth tires would have a lot less friction letting the tires slide on the road.

Ice is really smooth and slick. It would reduce the friction a lot so the tires would slide.

# Degrees of Friction

In the last Project we found that rough surfaces cause more friction between objects. There are several degrees of friction to explore. Force is often measured in Newtons which is mass in kg times distance in m divided by time in seconds squared.

You might have noticed I am not doing a lot of math but knowing what the units are can be important when you do.

Question: How does friction vary?

Materials:

Spring scale

Note: If you do not have a sensitive spring scale, you can use a thick rubber band. You will not be able to measure the friction but you can see what it does. Get a thick rubber band so the loop is about 5 cm long. Use a paperclip to make a hook on one end.

Wood block

Screw eye

Sandpaper

Smooth board

Procedure:

Put the screw eye in one end of the wood block

Attach the spring scale to the eye

Lift the block and get the measurement force in Newtons

Gravity is the force pulling the block down creating weight in grams or force in Newtons.

Place the block of wood on one end of the board

Watch the measurements on the scale as you very slowly pull on the block

Check the measurement on the scale as you pull the block down the board

There is friction even between the plain board and the plain block. It takes more force to start the block moving than to keep it moving across the board.

Repeat this two or three times observing what the scale does as you apply force on the block

Prop up the end of the board 10 cm and repeat your measurements

Move the prop to the other end of the board 10 cm and repeat your measurements

Remove the prop

Can you draw the force vectors for this? The hand is pulling upward. The sandpaper is pulling back. Gravity is pulling downward which splits for both down straight and down the ramp.

Tape sandpaper on the bottom of the block

Watch the measurements as you very slowly pull on the block then pull it down the board

Prop up the end of the board 10 cm and repeat your measurements

Move the prop to the other end of the board and repeat your measurements

Remove the prop

Tape sandpaper on the board

Watch the measurements as you very slowly pull on the block then pull it down the board

Prop up the end of the board 10 cm and repeat your measurements

Move the prop to the other end of the board and repeat your measurements

Note:

If you use the rubber band, watch how it stretches as you slowly pull on the block then pull the block along the board. Describe what the rubber band does.

Observations:

Force to lift the block

Greatest force before the block moves

Going down the ramp gravity helps pull the block down so less force is needed to move the block and keep it moving.

Plain block, plain board

No prop

Top prop

Bottom prop

Sandpaper block, plain board

No prop

Top prop

Bottom prop

Sandpaper block and board

No prop

Top prop

Bottom prop

Force to pull the block

Plain block, plain board

No prop

Top prop

Bottom prop

Sandpaper block, plain board No prop

Top prop

Bottom prop

Sandpaper block and board

No prop

Top prop

Bottom prop

Conclusions:

Did you need the same amount of force to start the block moving as you needed to keep it moving?

Why do you think this is the case?

Was the amount of force needed to pull the block the same as the force to lift the block?

Why do you think this is the case?

Sandpaper on both the block and the board causes more friction between them so it takes more force to move the block. Going up the ramp, gravity pulls the block down so even more force is needed.

Did you use the same force to pull the block down the board, along the board and up the board?

Why do think this is the case?

Tires use friction to keep a car on the road and moving down the road. How does ice on the road change things?

Does a car engine work harder to make a car go up a hill or down a hill? why do you think so?

What I Found Out:

I forgot my screw eye. I took a long piece of tape and made a loop with it. This worked fine as the block had a small mass.

My wood block had a mass of 49 g on the spring scale. This was the same as .49 Newtons.

The smooth block didn’t move until the scale read .14 N. It was really hard to get a good reading as the scale went up then suddenly dropped as the block moved. It only took .09 N to pull the block across the board. There were two degrees of friction: one to start the block moving and one to keep it moving.

Sandpaper is rough. Putting sandpaper on the block means more force is needed to move the block.

When I propped up one end of the board 10 cm and placed the block at the top, the block almost moved by itself. Only .06 N started the block moving and the scale dropped to less than 0 N to keep it moving down the ramp.

Pulling the block up the ramp was much different. This took .22 N to start the block moving and .18 N to keep it moving.

Putting sandpaper on the block made it much harder to move the block. On the level it took .48 N to move and .4 N to keep moving. Going down the ramp was easier with only .3 N to move the block and .1 N to keep it moving. Going up the ramp took. 56 N to move the block and .3 N to keep it moving.

Having sandpaper on both the block and the board was even more difficult. Now it took .52 N to start the block moving on the flat board and .48 N to keep it moving. Sliding down the board took .32 N to start the block moving and .18 N to keep it moving. Going up the ramp took .7 N to start the block moving and .36 N to keep it moving.

Pulling the plain block up the board takes more force than when the board was flat or the block was going down the ramp. It takes more force to start the block moving than to keep it moving. Different forces cause different degrees of friction.

Every time it took more force to start the block moving than it did to keep it moving. It was as though the block and board resisted the motion as long as possible then suddenly couldn’t hold still any longer. The block shot forward, jerked and then moved steadily.

I think friction held the block in place. Once enough force was applied, this friction was overcome. Then less friction was working on the block as it moved. The block also had momentum because it was moving and that helped overcome the friction.

When I held the block up, gravity pulled it down .49 N. Gravity always pulls down. When the block is resting on the flat board, gravity keeps it on the board so only the force to overcome gravity is needed to move the block. The sandpaper added enough friction so the force needed was as much as gravity or more with both pieces covered with sandpaper.

When the block was going down the ramp, gravity helped pull the block down. this is why it took less force to pull the block down a ramp than on a flat surface. Pulling the block up the ramp added gravity to the friction so more force was needed.

Ice is very smooth and slippery. Road pavement is rough. There is less friction on a smooth surface like ice so a car can slide instead of staying on the road.

It takes more force to go up a hill than down a hill because gravity pulls the car down the hill. The engine will work harder to go up the hill.

Friction changes for many reasons. It is a force that resists movement of an object. This project show many degrees of friction. Take a look for where these show up.

# 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.

# Physics 6 Friction

Rub your hands together. Do your hands get warm? Why do they get warm?

Two surfaces like your two hands catch at each other when they are rubbed together. It’s called friction.

Question: How does friction affect gravity?

Materials:

Room with carpet

Ball ramp

Light and heavy balls

Wooden ramp

Meter stick

Jar with lid

Water

Procedure:

Prop up the ball ramp 0.5 m

Roll the light ball down the ramp several times

Note: If the ball rolls farther than the room wall, start the ball lower down on the ramp. Both balls must start at the same place on the ramp.

Roll the heavy ball down the ramp several times

Roll the light ball down the ramp and mark where it stops

Roll the heavy ball down the ramp and mark where it stops

The red ball is lighter than the rubber ball. The heavier ball rolls farther before it stops.

Measure how far apart the two places are

Prop up one end of the wooden ramp 0.5 m

Roll the empty jar down the ramp several times

Even started half way down the ramp the jar is rolling very fast when it gets to the floor.

Note: Start lower down on the ramp if the jar rolls too far across the floor and hits the wall. The empty, half full and full jars must start at the same point on the ramp.

Roll the empty jar down the ramp and mark where it stops

Fill the jar half full with water and put the lid on tightly

Predict whether the jar will roll farther, the same distance or a shorter distance

Roll the jar in your hands and see what the water does inside the jar

Gravity pulls the water to the bottom of the jar and keeps it there even when the jar is rolled in the hands or the jar rolls down the ramp.

Roll the jar down the ramp several times

Roll the half full jar down the ramp and mark where it stops

Fill the jar with water and put the lid on tightly

Predict whether the jar will roll farther, the same distance or a shorter distance

Roll the jar in your hands and see what the water does

Roll the jar down the ramp several times

Roll the full jar down the ramp and mark where it stops

Compare where the empty, half full and full jars stopped

Observations:

Which ball rolled the farthest

How much farther this ball rolled

Describe what the water does when you roll the half full jar in your hands

Describe what the water does in the full jar when you roll it in your hands

Predictions:

How far the full jar will roll

How far the half full jar will roll

How far will this jar roll before stopping? Does putting water in the jar change how far it will roll?

What happened:

Which jar rolled the farthest

Which jar was in the middle

Which jar rolled the shortest distance

Conclusions:

Draw a ball rolling on the carpet. Add the vector arrows to show the forces acting on it

In the last Project we found that gravity pulls the same on a light and a heavy ball. Does friction act the same on a light and a heavy ball? Why do you think so?

Does your carpet have a lot of friction? Why do you think so?

When your jar rolled in your hands, did the water roll with it? Why do you think so?

Draw the empty jar on the carpet and the vector arrows showing the forces acting on it

Does this drawing look a lot like the one of the ball?

Draw the half empty jar on the carpet and the vector arrows showing the forces acting on it

Is there another force arrow on this drawing? [Remember what the water is doing.]

If the half full and full jars were just that much heavier than the empty jar, do you think they would roll farther?

How does the water change how far the jars roll even though the water adds weight?

The ball on the left marks where the half full jar stopped. The next ball is where the full jar stopped. The ball on the right, much further than the others, marks where the empty jar stopped.

What I found Out

I set my ball ramp up and rolled the light ball down from the top. It raced across the carpet and hit the far wall. I had to release the ball about half way down the ramp for it to stop on the carpet.

The heavy ball raced across the carpet a little over a meter farther than the light ball. Since both balls ran over the same piece of carpet, the friction on the balls would be the same. However the heavy ball rolled a meter further so friction took longer to stop the heavy ball than the light ball. Friction is affected by weight.

The carpet is indoor outdoor carpet with short tight stiff loops packed close together. It doesn’t seem to have a lot of frictional force as the balls roll easily across it.

The first time I released the jar from the top of the ramp, it rolled across the carpet and hit the wall. I had to release it about half way down the ramp just as I did the balls. I marked where it stopped with a ball against the wall so the other jars would not hit my marker and move it.

When I rolled the half full jar in my hands, the jar turned easily. The water stayed level with the floor unless I turned the jar very fast so the water couldn’t run off the jar sides fast enough. The water moved opposite to the way the jar was moving.

Heavier things roll further than lighter ones so the half full jar will roll further than the empty one. At least that is what I thought would happen.

The half full jar stopped long before the empty jar had. The water was pushing to stop the jar from turning along with friction from the carpet.

When I rolled the full jar in my hands, there were a couple of bubbles in it so I could see the water was acting the same as in the half full jar.

After seeing that the water’s friction slowed the half full jar, I thought the full jar would not roll as far as the empty jar even though it had more weight.

When I released the full jar, it did roll farther than the half full jar. This must be because it weighed more. It did not roll as far as the empty jar so the water slowed it down.

If the three jars had just had different weights, the heaviest jar would roll the farthest and the lightest one the shortest distance like the light and heavy balls.