IES Grant Module Format
Module – 9 Balanced and Unbalanced Forces
Grade Level- 8th
PA State Standard (# & short description)
S8.A.1.3.3: Examine systems changing over time, identifying the possible
variables causing this change, and drawing inferences about how these
variables affect this change.
Module Objective(s) Students will… Be able to explain the concept of
balanced and unbalanced forces and how these forces relate to motion.
Materials/Supplies- Rope (for tug of war)
Picture Prompts- Notes Slide Show
Big Idea:
An object’s motion is the result of all forces acting on it.
Essential Question(s):
What causes objects to move?
Module #1
1.
Do Now: Pre-test (10 minutes)
2. Video
Clip: http://www.engineeringinteract.org/resources/parkworldplot/flash/co
ncepts/balancedandun.htm
3.
Vocabulary:
·
Force- A force is a push or pull that causes an object to start moving,
stop moving, or change direction.
Bobbie used force to open the old rusty door.
·
Balanced forces- when forces are equal and opposite.
The science book rested on the table because of balanced forces.
·
Unbalanced forces- when forces are not equal or not opposite.
The blue team won the tug or war game because they were stronger and
created an unbalanced force on the rope.
·
Gravity- force that pulls all things toward Earth.
Rain falls from the sky because of gravity.
·
Friction- force acting in the opposite direction of motion.
The friction of the gravel track helped me run faster.
·
Magnitude- is the difference between the forces- measured in Newtons
(N).
The magnitude of the force used to open the door was 10 N.
·
Direction- refers to the way an object is traveling. (left, right, up, away
from, etc.)
The ball was rolling towards the wall.
4.
Lecture Part 1:
What is a force? Write this down. A force is a push or a pull
exerted on matter And a force causes an object to start moving, stop
moving, or change direction. What are some examples of forces? One
example of a force is gravity. You need to know that gravity forces all
object towards the Earth. When you drop a ball, it falls to the ground. That
is because the force of gravity. Something is pulling the ball towards the
Earth. Gravity is a force working on all objects on Earth. Pushing a cart up a
hill is another example of a force. When you push a cart up a hill, it might be
a little more difficult because you are applying a force against the cart and
are also working against the force of gravity and another force, the force of
friction. Take note that the force of friction works in the opposite
direction of a moving object. Friction occurs when two objects are in
contact with each other. Therefore pushing furniture on an old rough carpet
might be tough because there is a force working against you, the friction of
moving the furniture on the rough carpet. Imagine pushing the same
furniture on one of those smooth linoleum floors, its much easy- there is still
friction, its just less friction. Another simple example of a force is a
textbook resting on a desk. It may not seem there are any forces acting on
the textbook, but gravity is actually forcing the book down, while the table
is forcing the book up.
Now that we know some examples of forces, we can discuss how
forces can affect motion.
You need to know that forces acting on objects occur in pairs and are
either balanced or unbalanced. Forces also have magnitude and direction.
You must know that when a pair of forces have equal strength and act
in opposite directions, they are balanced forces. When we mention the
strength of a force we are referring to its magnitude. Write this downthe magnitude of a force is the difference between all the forces acting on
an object. Our unit for force is the Newton, because of Isaacs Newton’s
work with forces. It would be like if you and I are pushing this book towards
each other. So the direction of the forces in this case is towards each
other. And basically, the magnitude is the size of the force we are pushing
on the books.
You must remember the direction is the way the force and/or object is
moving. Lets try. If I pushed this book with a force of 3 N (magnitude)
towards you (direction), and you pushed the book with a force of 3 N
towards me, the forces would cancel out. The book wouldn’t move. The
difference between the forces is zero- the magnitude of the forces acting
on the book is zero. Three minus three equals zero. The word difference
means we subtract. Therefore these two forces cancel each other out and
the motion of the object they are acting on is unchanged. This book wasn’t
moving, and its still not moving.
You also must know that when one of the forces in a pair is greater than
the other, they are considered to be unbalanced. And an unbalanced force
results in a change in motion. For example, when an unbalanced force acts on
an object at rest, the object will begin to move. Watch what happens to this
textbook when I apply a greater force in this direction. The textbook
changes its motion. So if I am pushing (forcing) the book at you with a force
of 5 N, and you are pushing the book back at me with a force of 3 N, which
way will the book move? And what is the magnitude of the force while the
book is moving? Lets do the math. 5 N minus 3 N equals 2 N. Don’t forget
that an unbalanced force may also cause a moving object to stop, change
direction, or change speed.
Lets review balanced and unbalanced forces.
Balanced forces; the motion of object does not change, the forces are equal
in size and opposite in direction.
Unbalanced force; when the motion of an object does change in speed or
direction, the forces are unequal in size
Both gravity and friction are two forces that usually work against pushes
and pulls.
5. Activity: Balanced and Unbalanced Forces Activity- Tug of War with
students. Use rope and mix up student groups to show balanced and
unbalanced forces and magnitude.
6. Review Notes: Address Note-taking students, “Go to the last page of
your notes and write five lecture points, you may refer to your notes”,
Address all other students, “Fill in any gaps in your notes”.
7.
Post Test: (10 minutes)
8.
Homework/Follow-up: Balanced and Unbalanced Reading Guide Q&A