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.

materials for project

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

block hanging from spring scale

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

starting and moving a block shows degrees of friction

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

pulling the sandpapered block up the ramp

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

pulling block down the ramp

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 block and board going up ramp

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.

sandpapered block pulled across board

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 block up ramp

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.