# Physics1 Defining Forces in Physics

The word force has lots of meanings. Most of them have nothing to do with physics. How are forces in physics defined?

The block is sitting not moving on the table. Is a force acting on it?

Question: What are forces in physics?

Materials:

Block of wood

Table

Wire with a crook at the end

Procedure:

Set the block of wood on the table

Observe the block for a minute

Push on the block

Pushing on the block made it move across the table. Pushing is another kind of force.

Use the wire with the crook to pull the block

Push the block off the edge of the table

Observations:

What the block of wood does sitting on the table:

What the block does when you push on it:

What the block does when you pull on it:

What the block does when you hit the table:

What the block does when you push it off the table:

Conclusions:

What makes the block move?

Is a force acting on the block as it sits on the table?

Why doesn’t the block move because of this force?

How do you know a force is acting on the block?

How does a force act?

How do you think forces in physics are defined?

I forgot to grab my wire to pull the block across the table. Fingers do work to apply force by pulling.

What I Found Out:

My block of wood did not move sitting on the table. It only moved when I pushed or pulled it.

A force is working on the block as it sits on the table. When the block is pushed off the table this force pulls it to the floor. The only reason the force couldn’t move the block before is because the table was in the way.

A force acts by pushes and pulls.

What Are Forces in Physics?

Physics defines force as a push or a pull. There are many kinds of forces but they all act by pushing of pulling to move an object.

Can you think of some other kinds of force?

Gravity is one. It pulls objects down to the ground.

Magnets have force. They can be used to push or pull each other around.

Electricity especially static is a force. Think about what happens when you rub a balloon and touch it to a wall.

For the next fourteen physics projects we will look at physical forces, pushing and pulling and how we can use this through simple machines to make work easier.

# OS9 Changing Water Pressure

A column of water presses down on its base. Each cubic centimeter adds another gram of mass changing water pressure on the base. We saw how that works last week.

When the siphon moved water from one jar to another, the water ran slower as the jars were closer in height or the water level went down in the jar.

If you put a hole in a can, any water in the can will run out. What if there is more than one hole in the can? How will this affect how the water runs out of the can?

Question: How does changing water pressure affect how water flows?

Materials:

2 very large juice cans with the top removed or soda bottles with the tops cut off

1 soup can or jar to set the large cans or bottles on

Large tray [not needed if you do this outside]

Drill with a 1/8 inch bit [Help to drill some holes in the cans]

Ruler

Tape

4 Nickels

Procedure:

Drill three holes 0.5 cm from the bottom of one juice can or soda bottle spaced around the can

It is possible to use a nail if there is a tight board inside the can. Drilling a hole is much easier and makes a better hole.

Use the ruler to make a line down one side of the other can or soda bottle

Mark a point 0.5cm from the bottom, 5.5 cm up, 10.5 cm and 15.5 cm

The line of holes is supposed to be straight. Mine wavered a little as the drill slipped a bit on the can.

Drill holes at each mark

Put pieces of tape over the holes in the cans. Be sure these are tight.

The piece of tape needs to be tight over the hole. The ends are left loose for easy grabbing.

Put the can with three holes on the small can in the tray, a bathtub or ground outside

Fill the can with water to the top or a mark so you can fill the can the same each time

The can set well on the upside down jar. I didn’t get it put back exactly the same every time but couldn’t be too far off or the can would fall off. I filled the can to the rim each time.

Pull off one piece of tape and put a nickel where the water first hits the tray or ground

Measure how far the water went from the can

I measured from the jar each time as the centimeters started a little out on the ruler accounting for the overhang of the can.

Describe how the water stream acts as the can empties

Take off all the tape from the holes and dry the outside of the can thoroughly

Put one piece of tape over all three holes

Masking tape will not stick to a wet can. Again the ends are loose for easy grabbing. Each part over a hole is rubbed down tightly.

Set the can back on the prop can and fill it with water to the same place as before.

Pull the tape off quickly and put a nickel where one of the streams of water hits the tray or ground

Observe how the three streams of water act as the can empties out

Measure how far the water went from the can

Set this can aside and put pieces of tape over the holes

Put the other can on the prop can

If each tape is on tightly on a dry surface, the pieces will hold even through refilling.

Fill the can with water

Pull off the top tape piece and put a nickel where the water first hits the tray or ground

Measure how far the water went

Dry the outside of the can and replace the piece of tape

Fill the can

Pull the tape from the second hole down and put a nickel where the water first hits

Measure how far the water goes

Dry the outside of the can and replace the piece of tape

Fill the can with water

Pull the tape from the third hole down and put a nickel where the water first hits

This can behaved differently as the third hole stream went as far as the bottom hole in the first can.

Measure how far the water goes

Dry the outside of the can and replace the piece of tape

Fill the can with water

Pull the bottom piece of tape and put a nickel where the water first hits

Measure how far the water goes

Remove the pieces of tape and dry the outside of the can

It helps to hold the top of the can steady while pulling off the piece of tape.

Put one piece of tape covering all the holes

Set the can on the prop can

Fill the can with water

Pull the tape off quickly and place nickels where each stream of water hits the ground

[You may have to do this more than once to mark all the streams of water.]

Observe how the streams of water act

Measure how far the water goes for each hole

Observations:

1st can:

Distance the water goes with one hole open

Opening one hole on the bottom let the stream of water go out 26 cm. It stayed that far for a time as the water level dropped then slowly moved closer to the can until it finally dribbled out as the water level reached the hole.

How the water acts as the can empties

Distance the water goes with all three holes open

How the water acts as the can empties

2nd can:

Distance for top hole

How the water stream acts as the can empties

Distance for top hole with all holes open

Distance for second hole

Each time the stream of water is the longest at first and ends when the water level is the same as the hole.

How the water stream acts as the can empties

Distance for second hole with all holes open

Distance for third hole

How the water stream acts as the can empties

Distance for third hole with all holes open

Nickels work well for marking the distances. They are easy to see. They are heavy enough the water stream can’t wash them away.

Distance for fourth hole

How the water stream acts as the can empties

Distance for fourth hole with all holes open

How the water acts as the can empties

Conclusions:

For the first can, compare how the water stream with one hole open acts with how the three act with all the holes open.

The three streams went out a shorter distance than for a single hole. The water level l was the same over all three but the water had more than one way to go so less went out each hole.

For the first can, is the water pressure the same for all the holes? Why do you think so?

Is the rate of changing water pressure the same for all the holes? Why do you think so?

For the second can, is the water pressure the same for all the holes? Why do you think so?

Is the rate of changing water pressure the same for all the holes? Why do you think so?

Does where a hole is placed in a container affect how water empties out of the container?

All four distances were a little less than for single holes.

For the second can, compare how the water stream for the third hole acts with only that hole open to when all the holes are open.

Describe the changing water pressure as a can empties out.

Use the changing water pressure to explain how the water streams act as a can empties.

Do you think changing the sizes of the holes would change how the can empties?

Do you think making the holes different sizes would change how the can empties?

[You can try this and compare your ideas with what happens.]

What I Found Out:

My holes were a little high around the can. I put the tapes pieces over the three holes, set the can up and filled it with water. One tape dripped a little.

I steadied the can with one hand and pulled one piece of tape off. The stream of water went out. I marked it. It stayed going that far for a long time then gradually moved in until it was a dribble down the side of the can.

It was hard to dry the can until I got a towel. Then the tape went over all three holes. This time I steadied the can and jerked the tape off. Three streams of water shot out.

I marked the distance for one stream but the streams moved in faster than the single stream did. The three were soon dribbled down the side of the can.

For the single stream the distance was 26 cm. The distance with all three streams going was 23 cm.

The three streams acted much the same as the single stream except for being a little shorter and losing distance much faster. Since all the holes were the same distance up from the bottom of the can and the water was as deep over all the holes, they had the same water pressure on them. That made the rate of changing water pressure the same for all of them because the water level dropped the same over all of them.

Having the holes lined up from top to bottom of the can made the water act differently for each hole. The top hole water stream went the shortest distance, only 16 cm. The water stream shortened to a dribble very quickly.

The hole next down put out a longer stream, 24.5 cm. This stream lasted longer too.

The third hole had an even longer stream, 26 cm, and lasted longer too.

The fourth hole had the longest stream, 29 cm, and lasted the longest too.

The water level dropped very quickly with all four holes open. This made it hard to mark all four streams at one test.

The water streams acted the same as for the three holes. Changing water pressure caused the streams to get shorter until they dribbled as the water level dropped to the hole level.

These holes had different water pressure behind them as the water column over each was different.

When all four holes were opened up, the streams of water were shorter. The changing water pressure made the streams change distance quickly. I had to refill the can to get all of them marked.

Making all the holes larger would let the water out faster. I think the streams would be shorter too because making the hole at the end of a hose makes the water go farther.

If the holes were different sizes, the water would go out the larger holes faster so the changing water pressure would make the streams get shorter faster.