Force and Motion
How do we know about motion?
Much of what we understand today about motion comes from the careful
studies and observations of men such as Galileo Galilei (1564-1642) and Sir Isaac
Newton (1643-1727). Galileo made the telescope famous in 1609 when he began
using it to see parts of our solar system that before could not be seen. More
importantly to us at this moment, he discovered that when two different objects
with different weights fall from the same height, they fall at the same rate. Newton
discovered even more things about motion. For example, from Newton we learned
that things move only when something causes them to move. Something has to
cause an object to speed up, and something also has to cause an object to slow
down. It is also thanks to Isaac Newton that we know about gravity. We are going
to look at motion and see for ourselves some of the things that Isaac Newton came
to understand more than 300 years ago. You will learn about motion the same
way other scientists before have learned things. You will observe it and
experiment with it. Maybe you will discover something that no one has ever
noticed before.
What is motion?
Motion refers to the way an object moves.
What are some examples of motion?
 Up
 Down
 Forwards
 Backwards
 Sideways
 Zigzag
 In a circle
In these examples, we are describing the direction in which motion is
occurring. For example, a plane flies up into the sky. Then, when the plane lands,
its motion is down towards the ground. A child can jump up on a trampoline and
then fall back down. A car drives backwards in a driveway and forwards on the
street. Submarines sink down under the ocean. A rocket zooms up towards the
moon.
You can have motion as well. Can you move your body sideways? Can you
run in a zigzag or a circle and walk up or down stairs?
You can also cause motion. You can throw a ball up, push a toy truck
forward, pull a door open, or lift up your favorite cookie to your mouth. When you
cause something to move you are using force.
What is force?
A force is anything that is applied to an object to cause it to be in motion, to
change motion, or to stop motion. Another word for a change in motion is
acceleration.
See video clip: What is force?
What does this mean?
This can be anything such as a push or a pull that causes an object to start or
stop moving. It can also cause an object to move faster or slower. Pushing and
pulling are two examples of commonly used forces. Two other well known forces
are gravity and friction, as we will discuss in greater detail later.
What are some examples of how we use force?
Imagine you are on a swing and your mom pushes you, she forced you to
begin swinging. If she does not push you anymore you slow down, but if she
keeps pushing you the swing goes higher and faster. Your mom’s push is the
force; the action of the swing moving back and forth is the motion.
Here is another example. Your little sister wants a ride in the wagon, so you
have to pull it to make it go. You are applying force to the wagon by pulling. The
movement of the wagon in response to your pulling is the wagon’s motion.
Force can also change motion. Suppose you want it to go faster. Then you
have to pull harder and run! You have increased the amount of force to change the
amount of motion. Suppose now you want to go slowly. You walk slower. You
again change the amount of force needed to move the wagon.
See video clip: children and wagon.
What are some other types of forces?
 Compression/Squeezing. This is a pushing action. This means that
something is squishing something else. Imagine squishing aluminum
cans flat so you can fit more in the bag to take to the recycling center.
See video clip: compression
 Tension/stretching: This is a pulling action. Something is being
pulled very long or thin. For example, we stretch our muscles before
exercising to make them longer and reduce our risk of injury.
Another example is stretching a rubber band before releasing it to fly
across the room. Do not do this in class!
See video clip: tension/stretching
 Bending: This is a deforming action. The ends of something are being
pushed or pulled together. Bend down and touch your toes. Bend your
pencil too much and it will snap in half.
See video clip: bending
 Shearing/sliding: Things are being rubbed together. This is an
example of friction. For example, the two blades of a pair of scissors
rub together. The edges of the blades work together by rubbing to cut.
See video clip: shearing/sliding
 Torsion/twisting: This is a turning action. Something is being turned
to make it tighter or looser. For example, twist off a bottle cap to get
to the soda inside. Twist the lid back on to keep the soda fizzy.
See video clip: torsion/twisting
What is gravity?
Gravity is that really cool force that keeps everything from flying off the
earth. Gravity is a force exerted by earth on objects on and around the earth.
Gravity is always pulling things down towards the earth. Have you ever seen
anyone fall up a flight of stairs? Gravity pulls you to the ground at the bottom of
the stairs. It never pushes you up the stairs.
All objects fall towards the ground at the same rate. Really. We can
demonstrate this using a cup with two holes in it and some water. If we punch two
holes in a cup, fill it with water and hold it up, the water will fall through the holes
due to gravity. The cup remains at rest because our hand is resisting earth’s
gravitational pull. Next, if we hold the cup with holes and water and drop the cup
with the water the water will not fall through the holes. This is because the cup
and the water are falling at the same rate. Water will not fall out of the cup until
the cup stops moving when it finally hits the ground.
See video clip: water and cup gravity demonstration
What is inertia?
Inertia is the force an object uses to resist motion. All objects have inertia.
The more inertia an object has, the harder an object is to move. Once an object
begins to move it will continue to move because its inertia will now act as a force
to keep it moving. Inertia and mass have a lot in common.
Now then, what is mass?
Mass is a body of matter. Matter is the material substance that makes up
the universe. Mass also resists changes in motion. Everything and everyone is
made of matter. Everything and everyone has mass.
How do mass and inertia relate?
The more mass and inertia an object has, the harder it is to move. A bowling
ball has more mass than a basket ball. It is harder to throw a bowling ball than it is
to throw a basket ball. A bowling ball also has more inertia than a basket ball.
Therefore, a bowling ball will roll a lot farther than a basket ball if they both start
rolling from the same point.
How do force and mass relate to matter?
Look at this: We will take the laundry basket and add objects (the mass) to
it. We will try to push the basket individually and then with two or three partners.
Add more mass to the basket. Now try and lift the basket again following the steps
mentioned above.
What happens?
What happens when more mass is added to the basket?
Does more mass make it harder or easier to push?
Is it harder or easier to push it with partners?
Did we use force to push the basket?
Did we use force the lift the basket?
Why is it easier to push or lift the basket with partners?
It is easier to push or lift the basket with partners because the more force
there is acting on a mass the easier the mass will move.
Why is it harder to push or lift the basket when more mass is added?
It is harder to push or lift the basket when more mass is added because force
and mass are indirectly related. This means that if there is more mass it will need
more force to get it moving. Heavier objects (masses) require more force. Lighter
objects (masses) require less force.
See video clip: force/mass demonstration.
For every action, there is an equal and opposite reaction.
Let’s see how this works. You will need two rolling chairs and a partner. Sit
in the rolling chairs and face each other. Both of you hold your hands up. Put
your palms against your partner’s palms. Have your partner push your hands with
his hands while you do nothing. What happens? Even though you did nothing the
energy your partner exerted in pushing your hands transferred back to him causing
him to move in his chair.
See video clip: patty cake
What did we learn from this?
We learned that for every action there is an equal and opposite reaction.
When one of you pushed the other, both of you moved! Why is this? This is
because the objects that are having force applied to them are applying a force back.
This force is equal in strength and opposite and direction. This is why the person
that did the pushing moved backwards. The person that did not push actually did
push back!
Now let’s look at rocket balloons. You will need two partners for this. You
will also need a very long string; long enough to reach from one end of your
classroom to another. Next you will need several balloons of several different
sizes, such as long skinny balloons and fat round balloons. You will also need
some masking tape. Mark the string at one foot intervals. Have each of your
partners take an end of the string and stand on opposite ends of the classroom.
Thread the string through a piece of a drinking straw. Next have them hold the
string so that the line is taut. It might make it easier to wrap the ends of the string
around Popsicle sticks giving them a handle to hold than simply wrapping the ends
around your hands. Now, mark a starting point on the sting. Then inflate a balloon
and pinch the nozzle shut. Tape the inflated balloon to the straw so that nipple
faces the starting point and the nozzle faces the student holding the string. Release
the balloon and record the distance it travels on the string. Repeat the process with
balloons of different sized balloons and compare the results.
Notice that balloons with greater air volume travel further than balloons with
less air volume. The balloon moves forward in response to the air rushing out of
the nozzle of the balloon. The greater the force of the air leaving the balloon, the
farther forward the balloon travels. When we reduce the amount of air that will be
released from the balloon, we notice the balloon does not travel as far forward.
See video clip: rocket balloon demonstration.
We can show this another way as well. Inflate a round balloon and tape a flexible
straw to it while holding the nozzle to prevent the air from escaping. Release the
nozzle. As the air escapes from the balloon the bend in the straw controls the
direction in which the balloon spins. Re-inflate the balloon. Bend the straw in the
opposite direction. Now release the balloon. Notice the balloon now spins in the
opposite direction. The balloon pushes on the air forcing it out through the straw
which in turn controls the direction of spin based on the direction it is pointing.
See video clip: balloon and straw.
What do you know?
Now you have had an opportunity to see some of the things that Newton
saw. You have seen that things can move (have motion) in many different ways.
You have also learned that something called force has to cause an object to move
and objects with different mass require different amounts of force to move them.
Newton taught us about gravity and we used a cup and some water to see gravity
work. We also had an opportunity to see that for every action there was an
opposite reaction. A person moved backwards when she pushed on her partner’s
hands and a balloon traveled forwards while air moved backwards. Share with
your teacher and your classmates the many different types of forces and motion
you experience each day.