TO SEE THE BUILT- IN ANNIMATION EFFECTS, THIS SLIDE SHOW MUST BE DOWNLOADED

Force and

MotionForce is a push or a pull

By Moira Whitehouse PhD

The strength of a force can be measured. How strong the push or pull is measured with a spring scale in units called newtons.

One newton is equal to about a quarter of a pound.

This is a newton scale. By hooking itonto an object and pulling it along, one can read the force that is required to move that object under those conditions.

There are four kinds of forces; some scientists also add the fifth, friction.

Weak nuclear force:

Strong nuclear force:

Electromagnetic force:

Gravitational force:

Short-range force responsible for binding atomic nuclei together. The strongest of the four fundamental forces of nature. Important force in certain decaying functions within the atom, but way beyond me.

a force between objects exerted by positively and negatively charged particles.

the force of attraction between all masses in the universe; especially the attraction of the earth's mass for bodies near it.

Electromagnetic force

We know that everything is made up of tiny particles called atoms.

Although atoms are much too small to be seen, scientists have figured out that they are made of even smaller particles that have electrical charges. They are called protons and electrons.

Let’s start with....

Protons and electronsEverything is made up of atoms and every atom is made up of protons in the nucleus with positive electrical charge (+) and electrons swirling around the nucleus each with a negative electrical charge (-). There are also neutrons in the nucleus but they have no electrical charge.

http://www.windows.ucar.edu/

Because atoms have the same number of positive (+) and negative (-) charges, most things are in electrical balance.

But when the atoms of an object get out of balance electrically, strange things happen.

They can get out of balance when the swirling negatively charged electrons are knocked loose from their atom. This can happen rather easily and helps explain static electricity.

Sometimes when two electrically balanced objects rub against each other, electrons from the one are rubbed off onto the other. The object that received the electrons would then have extra electrons and an overall negative charge.

The object that lost the electrons would no longer be balanced having too many protons for the remaining electrons and thus becoming positively charged.

Even though we say that “strange things” happen depending on the balance or unbalance status of the electrons, the reactions are actually very predictable.

If the both objects have excess negative charges:If the both objects have excess positive charges:

Like charges repel

If the objects are balanced:

Uncharged

If one object has positive and the other has negative excesses:

Opposite charges attract

Shoes and carpet, like most everything else, are made of atoms that are electrically balanced. But when shoes rub against the carpet, electrons are transferred from the carpet to the shoes and the shoes become negatively charged.The carpet which loses electrons to the shoes becomes positively charged.

As you proceed about your business with all those extra electrons, you do not notice anything until......you touch a metal doorknob?

Those electrons that moved up from your

shoes are now ready to

get “in balance” again.

The metal doorknob which is a good conductor

of electricity, is neither positively or negatively charged.

When your negatively charged finger approaches the metal doorknob the attraction becomes greater until...

ZAP!!!

You may have noticed that you do not build up static electricity when you walk across a concrete floor. That is because some atoms hold on to their electrons more tightly than others do.

Examples of materials that are more apt to give up electrons are: fur, glass, human hair, nylon, wool, silk.

Examples of materials that are more apt to capture electrons are: styrofoam, Saran Wrap, polyurethane polyethylene (like Scotch Tape) polypropylene vinyl (PVC).

Or, negative clothes are attracted to the positive clothes.

Often when you take clothes from the clothes dryer, they seem to stick together.This is because some of the clothes have gained electrons by rubbing against other clothing. The clothes losing electrons become positive and are then attracting those pieces of clothing that have gained extra electrons.

Objects with extra (+) charges push away away from each other.

Objects with extra (-) charges push away away from each other.

Objects with + and – charges attract one another.

The bottom line, once again:

It’s like the poles of a magnet, “likes repel and opposites attract.”

Now we will do an experiment to demonstrate what we have been discussing.

B

Here we have two pieces of tape with the ends wrapped around tooth picks.

These two pieces of tape are marked with a “B” to show that they are on the bottom and sticking to the surface.

B

Next we will stick two more pieces of tape on top of the first two. They are marked with a “T” for top.

T

B

T

B

Using your materials, set up the experiment by sticking (pressing) your “T” tape directly over the (on top of) the “B” tape while it is still sticking to the surface of your table.

Now peel the top tapes off. They both had electrons stripped away when they were peeled up.

So now the “T” tapes will have fewer electrons because of those they lost, but the same number of protons they started with, which makes them positively (+) charged.

T

B

T

B

Now you are going to “test” your two “T” pieces of tape by holding them close to each other to see if they repel or attract. Before you do, make a prediction. Next, peel up the “B” pieces of tape from your table and after making your prediction, test them in the same way.

Finally, test one of the “B” pieces of tape with one of the “T” pieces, but only after making a prediction whether they will repel or attract.

Another vivid demonstration of what happens when the electric balance of an object is upset is the Van de Graff generator.

The Van De Graff generator is a device that demonstrates the effects of unbalanced charges as can be clearly seenhere.

Van de Graff generators have several parts: a motor, a belt, two rollers, two "combs," and a metal sphere.

The bottom roller is made out of a material that loses electrons easily, and the upper out of a substances that readily captures electrons.A comb pulls electrons from the material on the bottom roller (which loses electrons easily) and transfers them to the rubber belt.

As the motor turns, the rubber belt first goes over the bottom roller.

The belt then travels to the top roller. The second comb

near the top roller collects the electrons from the belt and stores them on the metal sphere. The motor turns very fast, so the sphere quickly collects a lot of electrons and becomes negatively charged and so do you when you touch the dome.

Touching a charged sphere is truly a "shocking" experience!

When a person places their hand on the ball and the machine is turned on, electrons are transferred to and collected on the person touching the silver ball.

Why do you think this machine affects the hair of the children in the picture?

Magnetism and electricity are related.

If you run electricity through a wire, a magnetic field is set up around the wire-- the wire becomes a magnet as long as electrons flow through it.

Activity with circuit and compass.

An electromagnet is a magnet that runs on electricity.An electromagnet works because an electric current produces a magnetic field. Unlike a permanent magnet, the strength of an electromagnet can easily be changed by changing the amount of electric current that flows through it. The poles of an electromagnet can be reversed by reversing the flow of electricity. If a wire carrying an electric current is formed into a series of loops, the magnetic field can be concentrated within the loops. The magnetic field can be strengthened even more by wrapping the wire around a core of soft iron.

This business of an electric current running through a coil of wire and making a magnet opens all sorts of possibilities, like electric motors and electric generators.

Any electric motor is all about magnets and magnetism. A motor uses magnets to create motion.

We will use this simple Beakman motor for study. The armature or rotor (in this case the coil of copper wire) is an electromagnet.

The ends of the copper wire in the coil make contact with the pieces connected to the battery terminals.Current flows through the coil, making it into an electromagnet.

Inertia causes the coil to continue around and when the coil nearly completes a spin, the process repeats itself.

Since magnets attract, the coil is attracted to one pole of the ceramic magnet.

Activity with electric motors

We have just seen how electricity is used to make motion, now we’ll see how motion is used to make electricity.

It uses motion to generate electricity.

This is a generator.

www.energyquest.ca.gov

A generator has a long, coiled wire on a shaft surrounded by a giant magnet.

When the turbine turns, the shaft and rotor also turn.

As the shaft inside the generator turns, an electric current is produced in the wire. An electric generator converts mechanical, moving energy into electrical energy.

The generator is based on the principle of "electromagnetic induction" discovered by Michael Faraday, a British scientist in 1831 Mr. Faraday discovered that if an electric conductor, like a copper wire, is moved through a magnetic field, an electric current will flow in the conductors.

Consider the many things that we depend on daily that are powered by electricity, and then realize our debt to its discoverer. His name is Michael Faraday.

Activity with generator

Gravity

Next, we will examine the phenomenon that keeps our feet on the ground:

Gravity is a force.

Gravity is a force that pulls.Every object has gravity.

So every object pulls on every other object.

The more mass an object has, the harder it pulls.

We will use two hypothetical planets for our example. Both the blue and green planets are pulling on each other.

Which one pulls harder?

This should help us see that the more mass an object has the stronger its gravity.

The Earth obviously has more mass than the Moon and it pulls harder. So much harder that the Moon is held in an orbit around the Earth as though by some magically strong string.

But the Moon’s gravity is also pulling on the Earth. So hard that the oceans swell toward the Moon where ever it passes. We call this high tide.

EarthMoon

Let’s think about gravity on our favorite little planet, Earth.

Gravity on Earth pulls everything toward its center.

That’s why there is no top or bottom and no one falls off.

If you dug a hole right through the Earth and fell in, how far would you fall?

As soon as you passed it, you would be pulledback towards the center.

You’d fall into the hole and shoot right past the center because you would be going so fast.

So you would bounce back and forth like a bungie jumper till you finally stopped at the center.

On Earth it appears that not everything falls to the ground at the same rate.It seems to us that things with less mass or weight e.g. feathers fall slower than things with more mass or weight e.g. rocks.

Lighter objects on Earth fall slower due to our atmosphere which slows their descent.

This is a misconception.As demonstrated by Galileo in the 1500’s, all objects in a vacuum, fall at the same rate regardless of mass.

On the Moon, an astronaut dropped a feather and a hammer.

Since there is no atmosphere on the Moon the feather and the hammer hit the ground at the same time.

Demo dropping book and paper.

1st Drop a book and wadded up paper at the same time. (land together)2nd Drop a book with a sheet of paper touching the under side of the book. (land together)3rd Drop a book with a sheet of paper touching the top of the book. (land together)4th Drop a book and a sheet of paper separately but at the same time. (book lands before paper)

Use previous slides to explain why.

The weight of an object is a measure of how hard gravity pulls on it.

However, the amount of gravity on each planet differs. The Moon has only one-sixth as much gravity as the earth. Consequently, on the moon you would weigh only one-sixth of what you weigh on Earth.

This boy weighs 60 pounds on Earth.

On the Moon he would only weigh 10 pounds.

Since each planet has a different amount of gravity, this boy’s weight would change each time he went to another planet.

On Jupiter, this would be like carrying an extra 100 pounds around on your back all day.

The farther an object is from the center of a planet, the weaker the force of gravity.

So, would this apple weigh more in some place like Death Valley or on top of a very high mountain?

Weight of a one pound apple heading out to space.

Appleweighs1 pound here

Appleweighs1/4 pound here

Appleweighs1/9 pound here

Appleweighs1/16 pound here

It is because the spaceship, being pulled by gravity, is always falling from beneath you.

In a spaceship like the shuttle, you would be weightless. However this is not because you are so far from Earth that there is no gravity.

Both of these men are weightless, still they are both being pulled by gravity. They have weight

only when gravity pushes them against something like the floor or a scale.

What causes gravity?

Even the great Sir Isaac Newton couldn’t answer that one.

But it is a force that effects everything in the universe.

At his time and still today, what causes gravity is a mystery.

Force and

Motion

Motion is the process of an object moving.

An object’s motion changes when a force acts upon it.

Newton’s Three Laws of Motion

Newton’s First Law of Motion--Inertia

• an object at rest stays at rest unless acted on by another force

• an object in motion stays in motion unless acted on by another force

Motion is a relative term.

All matter in the universe is moving all the time, but the motion referred to in the first law is a position change in relation to surroundings.

We live on the Earth which is rapidly rotating and orbiting the Sun. But when we sit down we say we are at rest.

When you are sitting in your seat in an airplane flying through the sky, you are at rest.

But, if you get up and walk down the airplanes aisle, you are in motion.

In order to understand the first law it is important to understanding balanced and unbalanced forces. If you hold a ball in your hand and keep it still, the ball is at rest.

All the time the ball is held there, it is being acted upon by forces.

The force of gravity is trying to pull the ball downward, while at the same time your hand is pushing against the ball to hold it up.

The forces acting on the ball are balanced.

Let the ball go, or move your hand upward, and the forces become unbalanced.

The ball then changes from a state of rest to a state of motion.

If you are not in motion right now, chances are that you have balanced forces acting on you .

The first part of this law seems pretty obvious—an object stays at rest until a force acts upon it.

Now let’s get back to discussing the first part of Newton 1st Law of Motion.

A ball sitting on the ground is at rest and when it is rolling or flying it is in motion.

Furthermore, a resting ball stays resting until a force acts upon it—in this case a moving foot.

The second part of this law is less obvious—an object in motion stays in motion until a force acts upon it.This is a difficult concept because in our experience things do slow down and stop, they don’t keep moving in a straight line and at the same speed.

The reason, of course, is that there is a force acting on those things. The force is usually friction, which we will study later.

This second part of the first law of motion explains why we should wear seat belts.

The car and person are both in motion and when the car stops abruptly the person stays in motion flying out of the car.

Otherwise, when they push against the spacecraft, they would start moving away from the ship and continue moving out into space in a straight line until acted on by another force.

Astronauts who “walk in space” are tethered to the shuttle or space station so they do not float off into space.

Activity with car and clay: place a gob of clay on a toy car, run the car into something fixed, car stops clay flies forward. (inertia)

Another: place an index card on a beaker or cup, place a penny on index card then thump the card (card flies away, penny drops straight down (inertia).

Activity: stack of pattern blocks (or poker chips) and a blade with which to strike the bottom one. Bottom one flies away, other stay (inertia).

Newton’s First Law of Motion combined with the Law of Gravity explains why a planet or moon orbits another (and larger) object.

The planet or moon is actually moving in a straight line that would carry it away from the larger object it is orbiting.

At the same time, the force of gravity pulls the planet or moon towards the larger object.As a result of the two balanced forces, the planet or moon keeps falling into orbit around the larger object.

Newton’s Second Law of Motion

An object’s acceleration depends directly upon the net force acting upon the object, and inversely upon the mass of the object.

As the force acting upon an object is increased, the acceleration of the object also increases.

As the mass of an object is increased, the acceleration of the object decreases.

Acceleration is either a change in speed (speeding up or slowing down) or a change in direction.

Same speed, same direction, this is not acceleration.Speeding up or slowing down, is acceleration.A change in direction is acceleration

For example, if you are pushing on an object, causing it to accelerate, and then you push, say, three times harder, the acceleration will be three times greater.

First:An object’s acceleration is directly proportional to the force.

If you push twice as hard, it will accelerate twice as much.

For example, if you are pushing equally on two objects, and one of the objects has five times more mass than the other, it will accelerate at one fifth the acceleration of the other.

Second:

This acceleration is inversely proportional to the mass of the object.

If it gains twice that mass it will accelerate half as much.

Sometimes a picture can say more than words. Let’s see.

We have a large force and a small mass.

The large force is applied to the small mass.

The small mass accelerates rapidly.

Or, in the other case:

We have a small force and a large mass.

The small force is applied to the large mass.

The large mass accelerates slowly.

A speeding bullet and a slow moving train both have tremendous force. The force of the bullet is a result of its incredible acceleration while the force of the train comes from its great mass.

. Therefore the difference in forces would be caused by the different masses of the balls. Newton stated this relationship in his second law, the force of an object is equal to its mass times its acceleration.

A bowling ball has a lot more mass than a soccer ball.

If a bowling ball and a soccer ball were both dropped at the same time from the roof of a tall building obviously, because it has more mass, the bowling ball would hit the ground with greater force than the soccer ball.We know that gravity accelerates all objects at the same rate, so both balls would hit the ground at the same time.

Therefore, the differences in force would be caused by the different masses of the two balls.

Newton stated this relationship in his second law, the force of an object is equal to its mass times its acceleration.

Force 100 N

If the mass of an object doubles, you would need to exert twice the force to accelerate it at the same rate.

Force 50 N

Notice that doubling the force by adding another dog would double the acceleration. Oppositely, doubling the mass to 100 kg would halve the acceleration to 2 m/s2.

Right granted for use for noncommercial use How Stuff Works

When you plug in the numbers for force in the illustration above, (100 N) and mass (50 kg), you find that the acceleration is 2 m/s2.

It is the force of gravity that causes an object to move down a ramp or inclined plane.

The more mass an object has the greater the force of gravity pulling on it even in this situation.However, the acceleration of the objects be the same. They will move down the ramp at the same rate regardless of their mass.

Experiment to demonstrate Newton’s Second Law of Motion (balls of different masses)

Newton’s Third Law of Motion

For every action, there is an equal and opposite reaction.

The skateboard responds to that action by traveling some distance in the opposite direction. The skateboard's opposite motion is called a reaction.

In the Third Law, the stepping off the skateboard is called the action.

The rider steps off the skateboard.

When you compare the distance traveled by the rider and the skateboard are compared, it appears as if the skateboard has had a much greater reaction than the action of the rider. This is not the case.

The reason the skateboard has traveled farther is that it has less mass than the rider—the Second Law of Motion.

If two people, both on skateboards, push on one another (action), they move away in the opposite direction as the push (reaction) .

When this man on roller skates pushes on the car, the car doesn’t move because it has great mass but he who has little mass rolls backwards.

When a gun fires, the bullet moves forward (action) causing the gun to recoil (reaction).

When a balloon full of air is sealed, the air pressure on both inside and outside are balanced, same pressure.

When the balloon is not tied the air inside the balloon escapes and then the air pressure outside the balloon is greater than inside.As a result of the air moving out of the balloon in one direction, the balloon moves in the opposite direction—action, reaction.

In both the balloon and rocket engine shown above, gases rush downward (action) causing the balloon and rocket to go up (reaction).

Activity with balloon “rocket”

Along with Newton’s Laws of Motion, we now consider Friction.

Considered by some to be one of the basic forces, friction is the force that opposes motion when an object’s surface is in contact with other objects.

Although we seldom think about the role it plays, friction is crucial to many things we do....often making our lives more difficult and often making it easier.

For example, it is friction between the ground and the sole of our shoes that make walking possible and it is lack of friction that makes our feet slip on ice or highly polished surfaces.Without friction, the belts of machines would slip, nails and screws wouldn’t hold, wheels would spin without making things move. At the same time friction wastes energy and causes our machines to break down and to wear out.

force

Friction is the force that opposes motion.To move the blue bar over the orange bar, friction could be a problem.The greater the “load” the more “force” will be needed to overcome “friction.”

The two major types of friction are:

Sliding friction: The rubbing together of the surface of a moving body with the material over which it slides.

Static friction: the force between two bodies in contact that opposes sliding.

Sliding friction-can be easily demonstrated in the classroom.Put both of your hands together and move them back and forth. Push your hands together harder and move them faster. What do you experience? Are your hands warming up? Do you hear the sound of the hands moving against each other?Friction results from the surface of your hands moving in opposite direction over each other. Because your hands are in motion this type of friction is known as sliding friction.

Sliding friction

Many teachers have dealt with the problem of moving the “big” box of new books when all the carts were already taken.

Here it is in graphic form.

Sliding friction between the: the broom and the floor

the foot and the floor the hand and the hat

Now let’s look at static friction—the force between two bodies in contact that tends to oppose sliding.

In order to move something, you must first overcome the force of static friction between the object and the surface on which it is resting.

Football players understand static friction well.When they first hit this blocking sled, it very much resists moving (static friction).

Once moving, the sled becomes somewhat easier to push as sliding friction becomes the main force resisting movement.

Initially you have to push really hard to get the car moving. That is static friction.

Once you have the car rolling it is easier to keep the vehicle moving. That is sliding friction.

If you have every pushed a car you have experienced static friction.

It takes more force to get the block moving—static fiction than it does to keep it moving—sliding fiction.

This picture shows static friction, just before the block moves.

This picture shows sliding friction, while the block is in motion.

(Activity: Use scales to determine the force (in newtons) required to move a brick in basket.)

1. How smooth two surfaces are that are touching.

2. The weight of the moving body or the body you are trying to get to move.

The amount of friction encountered, either sliding or static, will depend on two things:

Even though a surface may look very smooth, friction occurs in part because no surface is perfectly smooth.

When two surfaces try to move past each other these little bumps collide and slow down the motion of the surfaces.

Rough surfaces have grooves and ridges which catch on one another as the two surfaces slide past each other.

The rougher a surface is, the more and bigger bumps it has--more friction. The smoother a surface is, the fewer and smaller bumps—less friction.

If you cover the block in sand paper (making it rougher) the block moves even slower because of the sandpaper’s rough surface.

For example if you slide a wooden block down a ramp it will be slowed by friction.

If you sandpaper the block to make it smooth, the block will be smoother and slide faster.

Even surfaces that are apparently smooth can be rough at the microscopic level. Under a microscope, no surface is really "smooth."

The ridges of each surface can get stuck in the grooves of the other.

No matter how smooth the surfaces may look to your eyes, there are many ridges and grooves.

Sandpaper viewed under a microscope

Cloth viewed under a microscope

Surface of a tile viewed under a microscope

Paper viewed under a microscope

Friction activity with different surfaces

1. How smooth two surfaces are that are touching.

2. The weight (or mass) of the moving body or the body you are trying to get to move.

Once more, the amount of friction encountered, either sliding or static, will depend on two things:

The more mass or weight an object has the more friction it has. Therefore it will take more force to get it moving and more force to keep it moving.

A dump truck has more mass than a Smart Car.

The affect of weight on friction:

If it takes 10 newtons of force to slide a block with a weight of 50 newtons, it will take 20 newtons of force to slide a block that weighs 100 newtons:

Friction does not depend on the amount of surface area in contact between an object and the ground, as demonstrated in Example B.

Very interesting!!!

So, is friction good or bad?

Sometimes friction works against us and sometimes it works for us. It depends on the situation.

The answer, of course, is YES.

How does friction works against us?

Friction between the moving parts of an engine resists the engine’s motion and turns energy into heat, reducing the the efficiency of the machine and causing it to wear out.

Friction also makes it difficult to slide a heavy object, such as a refrigerator or bookcase across the floor.

In others situations, friction is helpful.

We would be unable to walk if there was no friction between our shoes and the ground. It is that friction that allows us to push off the ground without slipping.

On a slick surface, such as ice, shoes slip and slide instead of gripping. This lack of friction, makes walking difficult.

Friction allows the tires on our vehicles to grip and roll along the road without skidding.

Activity with person walking on board on dowels.

Place a piece of plywood (could be about 3 x 3 ft.) over several aligned wooden dowels and have a students try to walk on the plywood. The board flies backward and the student stays put. (lack of friction)

Friction between nails, screws and beams prevents the nails and screws from sliding out stopping our buildings from collapsing.

We want tread on our tires so we can drive our cars and to prevent us from slipping around on wet surfaces.

We want tread our footwear so we can gain traction.

Often however, we wish to reduce friction.

Reducing the amount of friction in a machine increases the machine’s efficiency. Less friction means less energy lost to heat, less noise and less wear and tear on the machine.

Less friction makes it easier to move things.

People normally use three methods to reduce friction.

The first method involves reducing the roughness of the surfaces in contact.

For example, sanding materials lessens the amount of friction between the two surfaces when they slide against one another.

The second method is to use smooth materials which create less friction.

Or by putting a smooth surface under the rougher surface.

Lubricant: By adding a thin layer of oil or grease between two objects, you can reduce static or sliding friction and lessen wear on machines.

Rolling friction: Instead of sliding surfaces together, you can place rollers between them.

The third way to reduce friction is often the best way--replace static or sliding friction with rolling friction and/or add a lubricant.

Rolling friction

When a cylindrical or spherical body rolls over a surface, the force opposing the motion is called rolling friction. Adding rollers between two surfaces reduces friction.

Ball bearings are an example of rolling friction.

Friction can be reduced by adding a lubricant such as grease or oil between the two surfaces.

Lubricants reduce friction by minimizing the contact between rough surfaces. The lubricant’s particles slide easily against each other resulting in far less friction.

Activity with shaving cream as a lubricant and

Air Carts using air to separate surfaces and reduce friction.

Lubricants decrease the amount of energy lost to heat and damage to machine surfaces.

Oil Grease

Two common lubricants

That’s it for today...

MAY THE FORCE BE WITH YOU!