Hey, it's Kaleigh
And in this lesson, we will learn to determine unbalanced and balanced forces on an object using Freebody diagrams
Remember that a force is a push or a pull on an object and
There are two different types of forces. Do you remember what they are?
That's right
contact forces and non-contact forces
some examples of contact forces are friction or applied forces like a push or a pull and
This means two objects are physically touching each other to apply the force
Examples of non-contact forces are gravity and magnets
This means two objects are still interacting
But they're at a distance from each other and they don't need to be physically touching to apply force
Can you recall what happens when an object is experiencing an unbalanced force
That's right it moves in fact as long as the unbalanced force is acting on it
It will accelerate and remember that to accelerate means that it will change
speed or direction or both we can simplify this by saying an object that is
Accelerating is changing its velocity. And do you recall the difference between the speed and velocity?
its
direction since velocity describes an object's speed and direction
we call it a vector quantity a
vector quantity is something that has magnitude like the amount of or how much of something there is and
the direction if
Something only has a magnitude we call it a scalar quantity
Some examples of vectors are things like force which has a magnitude the number of Newton's in a direction
up or down or left or right an
acceleration and velocity are vectors that have a speed the magnitude and a direction or a
Change in either one of those things
Some examples of scalars are things like speed which is the number of meters per second an object is moving
but no Direction is associated to it or mass which is the magnitude of
Kilograms an object has but obviously no direction
Sometimes it can be hard to tell if an object is experiencing balanced or unbalanced forces
We use a tool called the Freebody diagram to help us determine if an object will experience an unbalanced force
Why do you think that this would be important?
Architects and builders
Use Freebody diagrams when building houses and apartments
It's important for them to know if the walls or the roof will experience any unbalanced forces
Because that would mean the house would accelerate and that would be really dangerous
That would mean the walls or the roof could fall down in
Civil engineers and bridge designers also use Freebody diagrams to make sure bridges are experiencing balanced forces only
Even with lots of traffic and wind they've designed different types of bridges to withstand different forces
So they never accelerate
When we draw Freebody diagrams we represent the object as a box
Even if the object is a dog or person we still draw them as a box
And we use arrows to represent forests the arrow points in the direction. The force is acting and
We use big arrows to represent bigger forces and smaller arrows to represent smaller forces
Trying to do an example
Draw a Freebody diagram for a 20 kilogram ball at rest on the ground
So the first thing we can do is draw our object
They told us at one kilogramme ball, but since we draw all of our objects to the box, we can just go like this
Now let's determine what forces are acting on the ball
They told us the ball is at rest. So that means no acceleration. And that would mean the forces are balanced and
we know the ball has mass so we can assume gravity is acting on the ball and
Gravity points down so we can draw the down arrow just like this and the example says the ball is at rest not
Accelerating through the ground. So we need to draw the normal force arrow pointing straight up just
like this
Lastly we label the arrows so we can easily determine what forces are balanced and what forces are unbalanced
So here we labeled the bottom arrow FG for the gravity force and the top arrow FN for the normal force
So nice work. You just drew your very first Freebody diagram
Let's try more complex example now
Draw a Freebody diagram for a 20 kilogram lawnmower being pushed across a field
The lawnmower is accelerating across the field by 2 meters per second squared from the push
So the first thing we can do is our object and we'll just use the box to represent the lawnmower so it'll be like this
And now let's think about our forces
We know the lawnmower has masked so we can assume
gravity is acting on it so we can draw our graph the arrow like this and
the question says the mower is
Accelerating across the field but not through the ground so you can draw the normal force arrow acting up and balancing with the gravity arrow
So it'll be like this
finally we come to the force that is causing the acceleration a push is causing the mower to
accelerate 2 meters per second squared to the right
So we draw the arrow to the right of the box and show it direction. The force is acting
now we need to make sure that there is an unbalanced force in the direction of the mower
and
right now it looks like there is
The f.push is to the right and there is no arrow pointing to the left to balance it out
But what can we assume about the fields of grass? I think we can assume that there's friction and
the friction force
Opposes motion so it makes it harder to move
So we can draw the friction force arrow in the opposite direction of the pushing force
But should the friction force be bigger smaller or equal to the pushing force
It should be smaller we need the unbalanced forces to point in the direction of acceleration
Which is to the right the direction the pushing force is pointing
It's a great job
you just drew a Freebody diagram for an object with balanced force and
For an accelerating object keep practicing with the practice problems and remember to always be clever