Balanced and Unbalanced Forces - PowerPoint Presentation

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Balanced and Unbalanced ForcesSlide2

Here’s Tim & Mobey on Force

BrainpopSlide3
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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.

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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.Slide9
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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.

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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.

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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 .Slide23
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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.Slide27
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Forces & MotionSlide29
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