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FORCE and MOTION
All of your life, forces have been acting on you even though you probably don’t notice. For
example, since you probably have never been in space, where there is no gravity, you have
gotten so used to gravity you don’t notice it. It’s there though, along with many other forces,
such as friction and inertia. By learning more about these forces, you will be able to explain
why a marble falls faster than a piece of paper and why you weigh different amounts on
different planets.
We tried to make our website simple by using the same example a couple of times.
You have to imagine that you are riding in a car with your friend, and your car runs
out of gas. You are trying to get home, but to get there you have to understand how
forces work. We’ll use this event to teach you a lot about forces and motion.
Force:
Every time you move something, use your pencil, or walk, you are using force. These forces
are examples of everyday life because that is what a force is, everyday life. Every day you use
force to make your life easier. Sometimes it is hard to understand how forces help you.
What is a Force?
The scientific definition for force is simply a push or a pull. For example, when you do
homework you exert a force on your pen or pencil because you push and pull it across the
paper.
Sometimes two forces act on something. Like if two people were pushing a shopping cart. If
they were pushing in the same direction, you would add the two forces together. The sum of
the forces is called the net force. In this case the net force is an unbalanced force. An
unbalanced force is a force that changes an object’s motion or causes it to accelerate. The
arrows show different forces and their direction, the wider the arrow the stronger the force.
Another way to look at this is you and your friend are riding in a car. It runs out of gas one mile
from a gas station. You need to push the car to the gas station. If your friend is smart, he will
push in the same direction you push, which is toward the gas station. If he does, your forces
are added together. With this larger force, the car will move faster.
You can also have two forces acting in different directions. When the forces are equal and
acting in different directions they balance each other out. When this happens, there is no net
force because it is a balanced force. If one person was at the front of the shopping cart
pushing backwards, and one person was pushing with equal force in the opposite direction, the
forces would be balanced, and the shopping cart would not move. Back to the car running out
of gas example, your friend might push the car in the opposite direction you are. If he does,
your two equal forces in opposite directions balance out to zero net force. This means the car
will not move.
Something else can happen when forces push or pull in opposite directions. If one force is
more powerful than the other, they will not balance out to zero net force. This is because one
force is stronger than the other, so the weak force is not strong enough to completely balance
out the stronger force. Suppose there is a person in the front of a shopping cart and one on
the other end. They are pushing in opposite directions, but one of them is pushing with a much
greater force. Because the forces are not balanced, the shopping cart is moving in the
direction that the person with the stronger force is pushing. Back to the car running out of gas
example again, it would be like if your friend was pushing in the opposite direction you were,
but this time you weren’t pushing with equal forces. If your friend was pushing harder than you,
the forces would not balance each other. The car would roll away from the gas station.
Newton’s First Law of Motion:
Newton’s first law of motion explains inertia, another force. Inertia helps explain why things
stay where they are. Think about how many times you would lose your papers if inertia didn’t
make it stay where you put it.
If you are playing hockey and the puck is sitting on the ice, it will stay there until you hit it or
another force acts on it. Why? The answer is inertia. Inertia is the tendency of an object to
resist change in its motion. This means the object does not want to move. Newton’s first law is
based on inertia. This is why it is sometimes called the law of inertia. It states that an object at
rest will remain at rest. Also, an object moving at constant speed will keep moving at constant
speed, unless acted upon by an unbalanced force.
When you hit the puck it moves across the ice. Inertia would cause it to keep moving at the
same speed until it hit the boards, but the friction between the rubber puck and the uneven ice
causes it to slow down. Friction is the unbalanced force that stops the puck from continuing at
the same speed.
You now have your friend pushing the car with you, not against you, and you’re a quarter of
the way there. After you got the car moving, you noticed it was easier to keep it moving than it
was to originally start it moving. This is because it has inertia. When it wasn’t moving, its inertia
made the car resist rolling. Once it was moving, the inertia made it resist slowing down.
Mass
Have you ever noticed how it is harder to move something heavy versus something light? Like
if you have a balloon filled with water, and a balloon filled with air, the one that is filled with
water is harder to move. This is because it has more mass. Mass is the amount of matter in an
object. Mass makes it harder to move the water balloon because the more mass an object has
the more inertia it has.
Now that you know this, you are happy you weren’t driving a semi truck in the car running out
of gas example. You remember how the car’s inertia made it hard to push it. If the amount of
inertia an object has depends on the mass of the object, the truck would have a lot of inertia
because of its great mass. It would be much harder to get the semi-truck rolling than it was to
get the car rolling. Newton’s Second Law of Motion:
Sir Isaac Newton thought that you could find the acceleration of an object, so he wrote this law
to explain how. It states that acceleration equals force divided by mass.
Force, mass, and acceleration are all related. Newton’s second law of motion explains how.
Acceleration = force divided by mass. If something is accelerating it is constantly gaining
speed. If you were driving and your foot was on the gas pedal, and you were constantly
gaining speed, you would be accelerating. Suppose your friend was driving the truck, and you
were driving the car. You both ran out of gas. To get to the gas station you both use the same
amount of force to push your vehicles. Which one will move faster? The car would, because it
has less mass. The number ten represents your equal forces. If five represents the mass of the
truck, and one represents the mass of the car, the car would have a higher acceleration. This
is because ten (the force) divided by five (the truck’s mass) equals two (the truck’s
acceleration). Ten (the force) divided by one (the car’s mass) equals 10 (the car’s
acceleration). You can see that the car accelerates five times as fast as the truck because it
has1/5 as much mass.
You can also rewrite Newton’s second law so it will be force equals mass times acceleration.
To find the amount of force acting on something we multiply the mass of what is producing the
force by the rate at which it is accelerating. The product of the two is the amount of force. This
amount is usually measured in newtons. A newton is equal to the force that is needed for a
one kilogram object to accelerate at one meter per second per second. If you traveling at one
meter per second per second, it would mean every second you were moving you would go one
meter per second faster than the previous second.
Mass and acceleration change in opposite ways. If you want something to accelerate faster,
you would need to decrease its mass. This also works the other way around, if you add to an
object’s mass it will accelerate slower.
Friction:
Friction is a force that slows things down. This force can be both useful and harmful. Friction
has two affects on a capsule reentering Earth’s atmosphere. The friction between the capsule
and the atmosphere cause extreme heat. This is the harmful part. After the capsule comes
through the atmosphere it uses parachutes to slow it down. The parachutes use air resistance,
a type of friction, to slow it down. Without fiction the capsule would not slow down.
If you have ever walked on slick ice, you might know how easy it is to slip and fall. Ice is very
slippery because it has little friction. The surfaces of things have little bumps and scrapes that
can be so small you can’t see them. These bumps and scrapes are called irregularities.
Friction is caused by these irregularities getting caught on each other as two surfaces rub
together. Some things like glass and ice don’t have many irregularities to get caught on, so
there is little friction. Without the friction, you slip. On concrete, there are many things for your
shoes to get caught on so you don’t slip.
Friction is a force that always acts in the opposite direction of the object’s motion. For instance,
if you were sledding down a hill friction would be opposing the sled.
The force of friction wouldn’t actually pull the sled up the hill. It would only bring it to a stop.
Once the sled stopped there would be no movement between the irregularities, so there would
be no friction.
Your friend wasn’t strong enough to move the truck, so he helped you get to the gas station.
Now both of you are back on the road, driving through some snow. As you go farther south,
the snow turns into freezing rain, and your tires start to slip. Your friend, who you let drive,
panics and jams the gas. You start moving forward a little faster, but without the friction it
wasn’t much. You slowly move far enough to get back on dry pavement, where your wheels
stop slipping because there is friction.
Sometimes friction would stop the sled much faster. This would happen if the force of the
friction would be stronger. Two things can cause friction to change its force. One is the type of
surface. If instead of the sled having a hard, smooth, plastic bottom it had something like
carpet for a bottom, there would be much more friction, so the sled would come to a stop
faster. The other thing is how hard the surfaces are pressed together. If a 200 pound man was
riding the sled there would be much more friction than if a 50 pound child was riding it.
Sometimes in the winter we put snow tires on our cars. This is because the surfaces on snow
tires cause more friction than regular tires do.
Friction is sometimes very useful, but sometimes it isn’t. Without friction it would be very hard
to move around. Walking would be kind of like walking on ice, but on ice there is at least some
friction. Without friction you wouldn’t be able to move. Sometimes friction is not useful. In a car
engine, there are many moving parts. These parts rub together, and they produce friction. The
friction produced when surfaces rub together is called sliding friction. The force needed to
overcome sliding friction is more powerful than the force needed to overcome fluid friction. This
is why we put oil in our engines. With the oil between the moving parts, the sliding friction
becomes fluid friction. With the parts oiled, it is easier to overcome the friction. Without friction,
we wouldn’t have to worry about putting oil in our engines.
Rolling friction is another type of friction. We use rolling friction to reduce the force needed to
overcome sliding friction. If you had two boards with marbles between them it should be pretty
easy to move the top board. Sometimes in machines there are ball bearings between moving
parts to reduce friction. Ball bearings are like marbles made of metal.
Remember when you were pushing the car. When it was rolling down the road, it was an
example of rolling friction. If the car was in park, and you tried to push it there would be sliding
friction. Now, instead of the wheels rolling on the road, they slide across it. This makes it much
harder to push.
Friction doesn’t just slow things down, it also produces heat. If you use sand paper to sand
some wood, there will be friction. The wood should feel warm after being sanded.
Gravity:
You may want to become an astronaut so you could float around, but do you know why you
would float? The reason is there is no gravity in space. Gravity is the force that pulls you
towards Earth.
If you push something, the law of inertia says it should keep moving, but friction stops it. If you
hold something in the air and let go, the law of inertia says it should stay there, but it falls. The
reason it falls is gravity. Gravity is the force that pulls things toward Earth.
If something is in free fall, it has no forces acting on it except for gravity. Gravity is an
unbalanced force. Because unbalanced forces can cause an object to accelerate, an object in
free fall accelerates. Every second something is in free fall, it accelerates by 9.8 meters per
second. So if an object free falls for two seconds, it would be traveling at 19.6 meters per
second.
If a piece of paper and a marble are in free fall, they will fall at the same speed, so they should
hit the ground at the same time. If you test this by just dropping a marble and a piece of paper
you will find it is not true. This is because the objects are not in free fall. To be in free fall,
gravity has to be the only force acting on the objects. When you just drop something, there is
also air resistance. Air resistance is a type of fluid friction. Because friction acts in the opposite
direction of the object’s motion, air resistance of an object falling downward is an upward force.
This is because a falling object is coming down, so the opposite direction is up. If air resistance
were equal for every object, objects would still fall at the same rate. Since we know they do not
fall at the same rate, we know air resistance is different for different objects. The amount of air
resistance acting on an object depends on the object’s surface area. If an object has a small
surface area, it will have little air resistance. Because the piece of paper has a larger surface
area than the marble, the marble will have less air resistance than the piece of paper. Just
because an object has more air resistance than another object doesn’t mean it will fall slower.
This is because of weight, the force of gravity on an object at the surface of the planet. Some
people confuse weight and mass, they are different. Mass is how much matter is in an object,
and weight is the force of gravity on an object. If the object with the high air resistance had a
high weight, it might fall faster than the object that has little air resistance, like if you had a
wrecking ball and a piece of paper. Even though the wrecking ball had more air resistance
than the piece of paper, it would fall faster because the wrecking ball weighed more than the
paper.
Terminal velocity
As the speed of a falling object increases, the air resistance does too. If an object falls long
enough, the force of the air resistance will equal the force of gravity. Now the forces are
balanced so the object will stop accelerating. When this happens it is called the terminal
velocity. If a skydiver jumped out of a plane, he would start accelerating. After falling a while,
the force of air resistance would equal the force of gravity. The forces are balanced, so the
skydiver stops accelerating. This does not mean he will stop falling, he will just stop
continuously gaining speed.
Universal Gravitation
The force of gravity acts between all objects. This is the law of universal gravitation. This
means you are attracted to everything else in the universe. You don’t notice it because the
force is not very strong. The strength of the force depends on the masses of the objects. You
are attracted to Earth because Earth has such a great mass. There are other things that have
great masses, such as the moon and the sun. Why aren’t you attracted to these? The reason
is the force of the gravity also depends on the distance between the objects. If you were on the
moon, you would be close enough for it to attract you. Still the force of gravity on the moon is
about 1/6 of what it is on Earth. On the moon you would weigh about 1/6 of what you do on
Earth. Because the different planets have different masses, you would have a different weight
on each.
Newton’s Third Law of Motion:
Newton’s third law says that for every action there is an equal and opposite reaction. Rockets
are propelled upward because of a reaction force. The exhaust gases cause a reaction force
that pushes the rocket into the air.
Newton’s third law of motion states that “for every action there is an equal but opposite
reaction.” An example of action and reaction is two basket balls. If you roll one ball at the other
ball, when it hits the other ball it will exert a force on the still one. In return, the still one exerted
an equal, but opposite, force on the rolling ball. This reaction force caused the ball you rolled to
slow down.
Before, your friend drove you off the ice by jamming the gas pedal. With the wheels spinning at
full speed, you slowly got back on the pavement. Once the tires reached the pavement, there
was enough friction for the wheels could grip the road, so you sped off at full speed. Your
friend, still panicking over the ice, drove right into the back of his old truck. If you knew he was
this bad with directions, you might not have let him drive. The force of the car hitting the truck
was the action force. Which sure enough was met with a reaction force! This reaction force
brought your car to an immediate stop.
I explained before that equal forces acting in opposite directions create a net force of zero.
Action and reaction forces are equal forces acting in opposite directions. The reason they don’t
cancel each other out is they are acting on different objects. When the ball you rolled hits the
still one, the force of the moving ball acts on the still ball. The reaction force acts on the one
you rolled. The two forces can’t be added together because they are acting on different
objects.
Momentum:
Momentum is a force that makes it hard to stop an object. If you were running, you would have
momentum. Because of this, if you tried to stop really fast, you might fall because the
momentum wants you to keep going forward.
Do you think you could stop a train moving at 50 miles per hour? No, you probably couldn’t.
Now, do you think you could stop a foam ball moving at 50 miles per hour? I think you could,
catchers in baseball stop 90 mph fastballs. One thing that would make it hard to stop the train
is its momentum. The ball has momentum, too, but much less of it. To find the momentum of
an object you multiply its mass by its velocity, or speed. The ball and the train had the same
velocity, 50 mph, but their masses were very different. The train’s mass was much larger than
the ball’s mass. Momentum can make it very hard to stop an object. We know what happens to
an object’s momentum if it has a high mass, but what happens if it has a high velocity? A bullet
fired from a gun is a good example of an object with a small mass and a high velocity. The
bullet would have a very high amount of momentum.
Your friend, being dumber than you thought, hadn’t worn his seat belt when the car crashed.
When your car stopped, his momentum carried him into the windshield. If he had worn his seat
belt, it would have connected him to the car. This would make it so when the car stopped, he
would have stopped, too.
Conservation of Momentum
If you are playing T-Ball and you swing the bat, the bat will have momentum. As the bat hits
the ball, some of that momentum transfers into the ball as the ball begins to move. If there is
no friction, or any other outside force, momentum will be conserved. This means that the
amount of momentum in the bat before contact is the same as the amount of momentum in the
bat and the ball combined after contact. In other words no momentum was lost. In this case,
conservation does not mean saving resources, it means an amount is the same before and
after an event.
In the waiting room of the emergency wing of the hospital, you find something called a
Newtonian Demonstrator. It has five metal balls hanging in a straight line on strings that are
tied to a piece of wood six inches off the ground.
You quickly figure out that if you pull one of the balls on one side and let go, it will swing back
to where it was and hit the ball it was next to.
The momentum of the first ball transfers into the second ball. Then into the third, and the
fourth. The momentum transfers from the fourth ball into the fifth ball.
This last ball then swings out to the side.
When the fifth ball comes back it restarts the chain. The momentum was transferred from one
ball to the other. Momentum is conserved.
Next time you plan on taking a road trip it might be a good idea to either ask a smarter friend,
or teach him about forces