# Tag Archives: force

The reason for using a simple or complex machine to do work is to use less effort or force to do the same amount of work. How much force a machine will save is its mechanical advantage. Question: What is the mechanical advantage of an inclined plane?

Materials:

2 boards, one twice as long as the other

stack of thick books, at least four 5 cm or more thick

spring scales

Procedure:

Set up the stack of four books

Measure the height of two, three and four book stacks

Lift your block using the spring scale to the top of the stack The blocks are being lifted to the top of the book pile. No matter how the blocks get there, this is the work being done. Lifting a heavy weight is easier using a ramp. Mechanical advantage determines how well the ramp works.

Record the force needed

Measure the length of the two boards

Set up one board as an inclined plane to the top of the stack of four books

Pull the block up the ramp recording the force needed Pulling a weight up a ramp takes less effort than lifting the weight straight up. How efficient is the ramp?

Remove one book from the stack

Pull the block up the ramp recording the force needed

Remove another book from the stack

Pull the block up the ramp recording the force needed

Replace the ramp with the other board

Pull the block up this ramp recording the force needed

Add a book to the pile

Pull the block up the ramp recording the force needed

Add the last book to the pile

Pull the block up the ramp recording the force needed

Analysis:

Calculate the work done lifting the block up the height of two, three and four books using the formula W = Fd. The short ramp is much steeper and takes more effort. It is also shorter. Is it more efficient? Is its mechanical advantage greater?

Calculate the mechanical advantage of the inclined planes using the formula M.A. = R/E where R is the force needed to lift the block up the pile of books and E is the effort or force needed to pull the block up the ramp.

Another way to calculate the M.A. of an inclined plane is to divide the length of the plane by the height it goes to. Use these measurements to calculate the M.A. of your ramps.

Conclusions:

The scale reading when you lifted the block up is the mass of the block, the force needed to lift it and the resistance for calculating mechanical advantage. How can it be all three?

How do the M.A. you calculated using R/E and using L/H compare? Should they be the same? If yours are not, why not? The shorter the height and/or the longer the ramp, the less effort is needed to get the blocks up onto the books. Do these increase mechanical advantage of the ramp?

Does a short or a long inclined plane have more mechanical advantage?

Does the height of the ramp end matter for the mechanical advantage?

The total amount of work done by each ramp for each pile of books was the same. What was not the same? The work done is the same so, why use a ramp? How do you get the most mechanical advantage out of a ramp?

What I Found Out:

My scale read 200 g lifting the block up the pile of books.  Two books were 14.5 cm high making the work done 2900 g-cm. Three books were 21.5 cm tall making the work done 4300 g-cm. The tallest pile of four books was 29 cm high making the work done 5800 g-cm.

The mass of the block is the pull of gravity on it. Lifting the block requires enough force to counter gravity, equal to the mass. Since gravity is pulling on the block, it is resisting being moved by the mass amount making the force needed to lift the block equal to the resistance from gravity which is the mass of the block.

The two boards I used were 74 cm and 105 cm long.

My long board was 1.5 cm thick so I added that to the height of the stacks. The short board was .5 cm thick.

For the short ramp the force needed was 100 g [2 books], 130 g [3 books] and 150 g [4 books]. For the long ramp the force needed was 80 g [2 books], 110 g  [3 books] and 130 g [4 books].

Using the first formula the mechanical advantage for the long ramp was 2.5 [2 books], 1.8 [3 books] and 1.5 [4 books]. For the short ramp the mechanical advantage was 2 [2 books], 1.5 [3 books] and 1.3 [4 books].

Using the second formula the mechanical advantage for the long ramp was 6.6 [2 books], 4.6 [3 books] and 3.4 [4 books]. For the short ramp the values were 4.9 [2 books], 3.7 [3 books] and 2.5 [4 books].

My calculated mechanical advantages by the different formulas were very different. I had expected them to be similar. Perhaps my measurements were not as accurate as they should have been.

The special formula for calculating mechanical advantage for an inclined plane is the second one so I would prefer using those values. Another reason I would favor those is that my spring scales are not easy to read and inaccurate whereas my meter stick and rulers are easy to read and much more accurate.

Both ways indicate the longer ramp has a greater mechanical advantage. This value went down as the height the ramp went to became greater.

In all cases, the height of the book pile was the same for the 2, 3 and 4 books. The amount of work done was the same. What really changed was the distance the blocks had to be moved to get to the top of the book piles.

# Physics 6 Meet the Inclined Plane

You are going to visit a friend and run up to the porch. How are you going to get onto the porch? You can jump up or you can walk up the steps.

Jumping up may be more fun. Walking up takes less effort. Those stairs are one kind of inclined plane. Hailyann Workman’s help was greatly appreciated on this project. She seemed to think this was fun to do.

Question: How does an inclined plane work?

Materials:

3 Boards or pieces of stiff cardboard 10 cm wide and 0.5 m, 1 m and 1.5 m long

3 Bricks or 3 books about 5 cm thick

Spring scales

Meter stick

Block with loop

Procedure:

Set up the pile of books

Measure the height of the pile of books

Stand the block next to the pile of books

Use a spring scale to lift the block onto the books recording the force in grams

Remove the block

Measure the length of the boards A short ramp is steep. Since work is force times distance, the longer distance makes the amount of work much higher.

Lean the short board on the pile of books to form an inclined plane or ramp

Set the block just on the edge of the board

Use the spring scale to pull the block up onto the books recording the force needed

Repeat this for each of the other boards

Observations:

Height of book pile:

Length of short board:

Length of medium board:

Length of long board:

Force needed:

To lift the block

Short board

Medium board

Long board

Analysis:

Calculate the work needed to get the blocks onto the books by multiplying the force on the scale times the height of the books. This is W = Fd or Work = Force times distance.

Using a simple machine is supposed to reduce the force needed to get the same amount of work done. Now that we know how much total work is needed, we can calculate the force needed for each of the inclined planes by rearranging the formula so W/d = F or work divided by distance equals force.

Calculate the force needed for each inclined plane.

Go back to the Procedure to complete the Project

Conclusions:

Compare the force you measured for each inclined plane with the force you calculated.

Compare the force needed for each ramp with the force needed for the others and to the force needed to lift the blocks. A longer ramp has less of a slope making it easier to pull the blocks up.

What happens to the distance you must pull the block to use less force?

Would it be better to lift or use a ramp for a lightweight object? Why do you think so?

Would it be better to lift or use a ramp for a heavyweight object? Why do you think so?

What is the advantage of using a ramp?

What I Found Out:

This week I found Hailyann Workman to help me do this project. She is five and in kindergarten. She thought pulling the blocks up a ramp fun to do.

My stack of books was 15 cm tall. The scale registered 200 g lifting the blocks up. The work done was 3000 g-cm.

The short ramp was 44.5 cm long. The scale showed 150 g needed to pull the blocks up the ramp. I calculated 67.4 g-cm.

Next the blocks went up a 74 cm ramp using 100 g of force. I calculated needing 40.5 g-cm. Remember finding out about friction last week? My long ramp was rough making lots of friction. Covering the ramp with paper made it smooth.

The long ramp was 109 cm and rough. It was hard to pull the blocks up so I taped paper onto the ramp to make it smooth. The blocks pulled up easily with 70 g of force needed. My calculated amount was 27.5 g-cm.

My measured forces were much higher than my calculated forces. Perhaps I misread the scale. My block was smooth but not slick. My ramps were not slick so there was friction.

The needed force did decrease as the ramp got longer. The medium ramp took half the force of lifting the blocks.

The distance increases as the force needed decreases.

A lightweight object can be lifted up to move it the shortest distance. A heavyweight object should be moved up a ramp. This takes more distance but requires less force and is easier on you than lifting something heavy.

A ramp is a way to decrease the force needed to move an object even though it increases the distance needed to move it.

# Physics 4 Exploring Work in Physics

Picture yourself helping to push a car. You shove. You turn around and push with your back. The car doesn’t move. Did you do any work?

According to your muscles you did a lot. According to the physics definition you did none.

In physics work is defined as moving something over a distance. Since the car did not move, you did no work. In physics this is written as: W = FD or work equals force times distance. Question: How much work do you do?

Materials:

Spring scales [My set of 3 has a sensitive scale, a medium scale and a harder scale.]

Several blocks of different masses

Note: Another solution is to have stackable blocks.

Ruler

Procedure:

Each block will need a small loop to hook the spring scale to. An easy way to make such a loop for lighter blocks is to take a length of masking tape, attach one end to the block, crimp a length of the tape and attach the other end beside or over the other end on the block. The masking tape loop will only work for pulling light objects. It does a good job for that and is easy to make.

Place a block on a smooth table top

Set the ruler so you can see how far you will move the block

Hook a spring scale to the block It is important to pull steadily on the spring scale but reading the force can be difficult. Be sure to read it when the blocks are moving along.

Pull the block steadily for 30 cm

Observe the amount of force on the scale in grams

Note: If the scale barely moves, try a scale with a more sensitive scale on it.

Repeat this for each block or additional block

Observations:

Record the distance and force for each block My wood scrap blocks were mostly flat pieces making them easy to stack for pulling.

Analysis:

Multiply the grams times the distance in centimeters for each block to get the work done for each block.

Conclusions:

Which block has the most mass? Why do you think so? [You can check this by massing the blocks.]

Do you do more work moving a block with less mass or more mass? Each block added to the stack increased the amount of force needed to pull the stack.

If you pulled a block 15 cm, would you do more or less work? Why do you think this?

What I Found Out:

I used some scrap wood pieces for blocks so they came in various sizes and shapes. The biggest one was the one I chose to put the loop on. the others were piled on top of it one by one to increase the mass pulled by the spring scale.

The first block took 6 g to pull it the 30 cm. This made the work done 180 g-cm.

Two blocks took 40 g to pull the same distance. Now the work done was 1200 g-cm.

Three blocks took 49 g to pull. Now the work done was 1470 g-cm.

Four blocks took 52 g of effort making the work done 1560 g-cm.

Five blocks took 75 g of force making the work done 2250 g-cm.

Six blocks took 85 g of force making the work done 2550 g-cm.

Seven blocks took 90 g of force making the work done 2700 g-cm. The last block was added to the stack. The pile was pulled for the 30 cm so work was done.

The more blocks on the pile, the more force it took to pull the pile across the table. That makes me think a heavier block will take more force than a lighter one.

Looking at the increases in effort, the first block took 6 g but the second took an additional 34 g so the second block must have more mass than the first one.

The third block increased the force 9 g and the fourth a mere 3 g. These are lighter than the second block.

The fifth block increased the force needed 23 g. The sixth added 10 g and the last one 5 g. The second block was the biggest block with the fifth block next.

Pulling the first block 30 cm required work of 180 g-cm. If I had pulled the block only 15 cm, it would be 6 g times 15 cm or 90 g-cm. It takes less work to move half the distance.

# Physics 23 Balancing Force and Mass

Find yourself a half gallon juice or milk plastic jug and fill it two thirds with water. You can use a gallon jug but only half fill it. Set it on a table or stool so it is about waist high.

First stand next to the jug. Grab it and lift it up a foot or so.

Next stand at arm’s length away. Grab the jug and lift it up a foot or so.

What happened to the jug? Did anything change about the jug? What did change? Question: How can you balance force and mass?

Materials:

Several different masses like a set of masses

Unknown mass like a wood block

Scale

Slat board 30 cm to 40 cm long

Wedge 5 cm tall

Metric ruler

Procedure:

Mass all your different masses unless they are a marked set

Set the wedge on a table top pointing up

Balance the slat on the wedge I used a piece of scrap wood for a slat so it was not smooth. that made it harder to balance on the wedge. Having more of a flat area on the tip of the wedge would make balancing the slat easier too.

Note: This is easier if the wedge tip is a little flattened. Work slowly moving the slat back and forth a speck at a time finally until the slat seems to balance. It will probably never balance levelly on the wedge. Get as close to the balance point as you can.

Mark the place on the slat where it balances, the mass center point

Set a lighter mass on one end of the slat Placing a mass on one end of the slat or lever makes that end sink to the table.

Take a heavier mass and move it around on the other length of the slat until it balances

Note: Again the slat will probably not balance levelly on the wedge. Move the heavier mass until you find the point closest to balancing. A 50 g mass must be closer to the wedge or fulcrum to balance with a 20 g mass on the other end.

Measure the distances from the center point of the slat to each mass

Do this again with another set of masses

Mark the slat halfway from the balancing point to one end of the slat

Place the slat on the wedge so the new place is on the tip of the wedge

Place a heavy mass on the short end of the slat

Take a lighter mass and move it up and down the other end of the slat until the two masses balance

Measure the distances from the mark on the slat to each of the masses

Do this again using other masses

Place the unknown mass block on one end of the slat

Use a mass to balance the slat

Measure the distances

Mass the unknown block The wood block had a mass of 55.29 g on the scale.

Observations:

Masses of the masses:

Distances to the masses (mark down which mass is where each time):

Distances for the unknown mass:

Mass used to balance the unknown block:

Mass of unknown block:

Analysis:

Multiply the distance times the mass for each pair of masses to get the force each mass exerts Getting the two masses to balance takes careful nudging of one mass until the slat or lever slowly shifts from one side to the other.

You are using the formula Md = Md where M is mass and d is distance. Use this formula to find the mass of the unknown block.

Conclusions:

Compare the forces exerted for each pair of masses.

Compare your calculated mass for the unknown block to its actual mass.

How could you get the calculated mass and actual mass to be the same?

Why did the effort needed to lift the jug of water change?

If you are playing on a teeter totter with a small child who weighs much less than you do, where would you sit so the child could move up and down instead of being stuck in the air? A 10 g mass must be twice as far from the wedge as the 20 g mass to balance.

The slat could be considered to be a lever. The wedge is the fulcrum. If you wanted to move a very heavy rock with a lever, where would you place the fulcrum to use the least effort? Why?

What I Found Out

My slat was not smooth. Its thickness varied so the balancing point was not quite in the middle of the slat. My wedge did not have a flattened tip so the slat never really stayed level. I moved the slat then the masses until the slat tipped slowly from one end up to the other end up.

I tried to time my pictures for when the slat was slowly shifting. That is how I got the slat to look so balanced in some of them. I had to get the masses very close to balancing so the slat moved slowly for the picture.

When I used the 50 g and 20 g masses, the distance for the 20 g mass was 21.3 cm and the 50 g distance was 7.7 cm. The forces were 426 g-cm and 385 g-cm.

When I used the 10 g and 20 g masses, the distance for the 10 g was 21.1 cm and the 20 g distance was 11.5 cm. This gave forces of 211 g-cm and 230 g-cm.

Balancing the 10 g and 5 g masses, I had distances of 9.3 cm and 19.8 cm. this gave forces of 93 g-cm and 99 g-cm. Because my slat was so short, I had to use the 50 g mass to balance the wood block. Could I use a smaller mass if I moved the wedge or fulcrum closer to the block end of the slat? Probably.

I used the 50 g mass to balance the wood block. The mass had a distance of 21.4 cm. The block had a distance of 17 cm. This gave me a calculated mass of 63 g. The mass on the scale was 55.29 g.

It took a lot of time to keep moving the mass a speck at a time to get the balance really close. I got impatient and tried to hurry so I wasn’t as accurate as I should have been.

Having a better wedge with a flat spot so the slat would balance better would help too.

I forgot my metric ruler and used the meter stick. This was long and clumsy making it hard to read the distances accurately.

The jug of water did not change. What did change was the distance from the jug to my shoulder. The force exerted downward by the jug was the distance from me to the jug times the mass of the jug. Increasing the distance increased the force and made the jug seem heavier even though it wasn’t.

Playing on a teeter totter is only fun if both people can go up and down. The heavier person can slide forward so the beam balances better.

A lighter force farther from the wedge or fulcrum can exert more force on the other mass. So I would want the fulcrum close to the rock. that way I can push down a little to push hard on the rock.

# Physics 22 Force and Mass

Newton’s Second Law of Motion is Force = Mass x Acceleration. He says the three are related. According to this Law, increasing the acceleration and leaving the mass the same should increase the force. Does it? How are force and mass related? Question: How are acceleration, force and mass related?

Materials:

Spring scale

2 Small plastic cups (like those from apple sauce or fruit servings)

String

Stopwatch

Tape

Meter stick

Procedure:

Mark off a 1 m + 10 cm course on a smooth table (if the table is rough, use smooth cardboard or a smooth board)

Put two small holes directly opposite from each other in each plastic cup near the rim

Cut a length of string two times as wide as the cup

Put the ends through the holes in the cup and knot them to form a handle

Put the masses in the cups

Zero the spring scale A regular scale is much more accurate for finding the mass of an object.

Use the scale to mass Cup 1 and Cup 2

Remove the mass from Cup 1

Pull one end of the string out of the hole it is in

Knot the string and pull it through the other hole until the knot holds the end

Put a loop in the loose end of the string

Put the cup 10 cm before the starting line

Put the mass in the cup

Hook the spring scale to the loop

Practice pulling the cup down the course using a constant force on the spring scale Pulling a mass with a spring scale shows the force used. It is hard to read the force accurately especially when pulling the force quickly.

Time how long it takes to pull the cup down the meter course

Record the force needed

Try pulling the cup faster recording the time and force

Remove the mass from Cup 2

Change the string the same way for Cup 2 but attach it to Cup 1

Put the second mass in Cup 2

Time how long it takes to pull the two cups down the course

Record the force needed

Pull the cups faster recording the times and force needed

Observations:

Masses Hanging a mass on a spring scale reads more force than pulling the same mass across a table. The mass is the same but the acceleration of gravity is much greater so the force is greater.

Cup 1:

Cup 2:

Times:

One mass:

Force needed:

Both masses:

Force needed:

Conclusions:

Should you start pulling the cup before the starting line? Why?

Compare the force needed for one mass and for two masses.

How are force and mass related?

Does the force needed seem to change if you pull the cup fast or slowly?.

Why do you think this is the case?

Did you expect to need more force to make the cups go faster? Why?

How could you use greater force to move the cups?

Newton’s Second Law of Motion says increasing the acceleration should increase the force. Did the force increase? Is a spring scale a really accurate way to measure this acceleration force? Doubling the mass being pulled does double the force needed.

What I Found Out:

I used four smaller lead wheel weights used for balancing tires for masses. One cup of weights was a little heavier than the other one. I tried using smaller masses but found it too hard to read the forces on the scale. The ones I used were 68 g and 48 g.

As was the case in the last Project, it took more force to start the cup moving than it did to keep it moving. Starting to pull before the starting line let me measure the steady force only.

When I pulled Cup 1 the first time, I did it slowly in 5.15 sec and had a force of .1 N. I expected to use more force when I pulled the cup faster.

When I pulled Cup 1 in 3.34 sec, it still took .1 N. I tried it at several different speeds. As long as the cup moved steadily, the force remained the same.

When I tied Cup 2 to Cup 1, the force needed to pull the two increased to .2 N. Again this didn’t change if I pulled fast or slowly.

Force and mass are related as increased mass increases the amount of force needed to move the mass. Once the pulling force gets the mass moving, that is all the force needed to keep it moving regardless of how fast or slow the force is applied.

I think I could increase the force used to move the cups if I pushed on them. Possibly I could use a collision to apply the forces like with the projectiles but that would not be sustained over a distance.

The spring scale didn’t stay steady very well. It is difficult to read accurately. Trying to read the scale and time pulling the cup was difficult. I don’t think the force increased but it may have. Having a much heavier mass might have made the force easier to see too.

# 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.