Heres Tim amp Mobey on Force Brainpop Suppose I asked you to move a heavy desk in the classroom How would you move it hmmmmmmmmmmm You might get on one side of the desk and start pushing Or you might grab the legs and start pulling ID: 429379
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Slide1
Balanced and Unbalanced ForcesSlide2
Here’s Tim & Mobey on Force
BrainpopSlide3Slide4
Suppose I asked you to move a heavy desk in the classroom. How would you move it?
hmmmmmmmmmmm.....Slide5
You might get on one side of the desk and start pushing, Or you might grab the legs and start pulling.
Either way, you would be using a
force
.
Any push or pull is a force.
The force you use in pushing or pulling the desk has a size.
You could use a small amount of force or a lot of force. Forces differ in size.
Another way forces differ is in direction.
A force can be left or right, up or down.Slide6
The desk will not move by itself.
We need to remember an object at rest tends to stay at rest and
an object in motion tends to stay in motion
with the same speed and in the same direction unless acted upon by an unbalanced force.
A force is needed to put the desk in motion. Slide7
But what exactly is meant by the phrase
unbalanced force
?
What is an unbalanced force?
In pursuit of an answer, we will first consider a physics book at rest on a table top.
Slide8
There are two forces acting upon the book.
One force - the Earth's gravitational pull - exerts a downward force.
The other force - the push of the table on the book (sometimes referred to as
a
normal force
) - pushes upward on the book.Slide9Slide10
Since these two forces are of equal magnitude and in opposite directions,
they balance each other
.
The book is said to be at
equilibrium
.
There is no unbalanced force acting upon the book and thus the book maintains its
state of motion
.
When all the forces acting upon an object balance each other, the object will be at equilibrium;
it will not accelerate.
Slide11
Consider another example of a balanced force - a person standing upon the ground.
There are two forces acting upon the person.
The force of gravity exerts a
downward force
.
The force of the floor exerts an
upward force.Slide12
Since these two forces are of equal magnitude and in opposite directions, they balance each other.
The person is at equilibrium.
There is no unbalanced force acting upon the person and thus the person maintains its
state of motion
.Slide13
Now consider a book sliding from left to right across a table top.
Sometime
in the prior history of the book
, it may have been given a shove and set in motion from a rest position.
Or perhaps it acquired its motion by sliding down an incline from an elevated position.Slide14
Whatever the case, our focus is not upon the
history of the book
but rather upon the current situation of a book sliding across a table top.
The book is in motion and at the moment there is no one pushing it to the right. Slide15
http://www.engineeringinteract.org/resources/parkworldplot/flash/concepts/balancedandun.htmSlide16
(Remember:
a force is not needed to keep a moving object moving to the right
.) Slide17
The force of gravity pulling downward and the force of the table pushing upwards on the book are of equal magnitude and opposite directions.
These two forces balance each other. Slide18
Yet there is
no force present to balance the force of friction.
Slide19
As the book moves to the right, friction acts to the left to slow the book down.
There is an unbalanced force;
and as such, the book changes its state of motion. Slide20
The book is not at equilibrium and subsequently accelerates.Slide21
Unbalanced forces cause accelerations
.
In this case, since
the unbalanced force is directed opposite the object's motion,
it will cause a deceleration (a slowing down of the object).
To determine if the forces acting upon an object is balanced or unbalanced, an analysis must first be conducted to determine what forces are acting upon the object and in what direction.Slide22
If two individual forces are of equal magnitude and opposite direction, then the forces are said to be balanced
.
An object is said to be "acted upon by an unbalanced force" only when there is an individual force which is not being balanced by a force of equal magnitude and in the opposite direction .Slide23Slide24
So remember, even when the desk is not moving, forces are acting on it.
The floor is actually pushing up
on the desk.
At the same time, the force of
gravity is pulling down
on it.
If the desk is not in motion, the upward push is the same as the downward pull.
The size of the forces is the same, but the directions of the forces are differentSlide25
. Whenever forces are the same in size but opposite in direction,
they are balanced forces
.
An object that is NOT in motion will never move if balanced forces act on it. Slide26
How can you move the desk?
You can move it because you unbalance the forces.
The size of your force is greater than the forces of air pushing on the other side of the desk.
Unbalanced forces
act on an object,
the object will always
move in the direction
of the greater force.Slide27Slide28
Forces & MotionSlide29Slide30