Transcript Notes

FORCES AND FREE BODY
DIAGRAMS
 http://vimeo.com/28965409
What is a Force?
• A force is a push or a pull.
• All forces have two properties:
–Direction and Size (magnitude)
• Units - Newton (N) It is equal to a kgs m
• A force acts ON one object FROM
another object (an object cannot exert a
force on itself!!)
2
Types of Forces
• Weight (gravity) – always acts down
(non-contact)
• Normal – a force that acts perpendicular
to a surface that an object is in contact
with
Normal force directions
• Up
– You’re standing on level ground.
• Sideways
– A ladder leans up against a wall.
– You’re against the wall on the “Round Up” ride
when the floor drops out.
• At an angle
– A race car takes a turn on a banked track
– An object on an incline
• Down
– You’re in a roller coaster at the top of a loop.
Types of forces
• Friction – the force exerted by a surface as
an object moves across it or makes an
effort to move across it – acts parallel to
surface and AGAINST motion
• Air Resistance – a special type of friction
that acts upon objects as they travel
through the air – acts up and is not
constant, changes with speed
Types of Forces
• Tension – a force that is transmitted
through a string, rope, cable or wire when
it is pulled tight by forces acting from
opposite ends – acts away from object
• Applied - a force that is applied to an
object by a person or another object
Types of Forces
• Spring - the force exerted by a
compressed or stretched spring upon any
object that is attached to it
Types of non-contact forces
• Gravitational - a force that attracts two
objects to each other (we’ll cover this
later)
• Electrical – the force that attracts or
repels two charged objects (this is
Physics 2)
• Magnetic – attraction or repulsion that
arises between electrically charged
particles because of their motion (this is
Physics 2)
The meaning of each of these
forces will have to be
thoroughly understood to be
successful during this unit.
Ultimately, you must be able to
read a verbal description of a
physical situation and know
enough about these forces to
recognize their presence (or
absence) and to construct a
free-body diagram that
illustrates their relative
magnitude and direction.
Free Body Diagrams
• A force diagram, which is also known as a free
body diagram, is a sketch in which all the force
vectors acting on an object are drawn with their
initial points at the location of the object.
When to use a free-body
diagram
• Always - every problem that has to do with
forces!
Steps to drawing a free body diagram
1.Pick one object to analyze
2.Draw a box or dot to represent the object
3.Draw an arrow to represent each force
acting on the object
4.Make sure the arrow is labeled, and shows
the direction and relative size of the force
Problem 1
A book is at rest on a table top. Diagram the
forces acting on the book.
Problem 1
The forces are balanced because the
book is not moving
Problem 2
An egg is free-falling from a nest in a tree.
Neglect air resistance. Draw a free-body
diagram showing the forces involved.
Problem 2
The forces are unbalanced, so the egg
will accelerate downward.
Problem 3
A flying squirrel is gliding (no wing flaps)
from a tree to the ground at constant
velocity. Consider air resistance. A free
body diagram for this squirrel looks like…
Forces are balanced
because constant
velocity
Problem 4
A rightward force is applied to a book at
rest, in order to move it across a desk.
Construct a free-body diagram for the
book.
Right and left
forces are
unbalanced
Up and down forces
are balanced (no
up and down
movement)
Problem 5
A man drags a sled across loosely packed
snow with a rightward acceleration. Draw
a free-body diagram of the forces acting
on the sled.
Problem 6
A football is moving upwards toward its peak
after having been booted by the punter.
Neglect air resistance. Draw a free-body
diagram of the football in mid-air.
Remember
force does
not determine
movement, it
determines
acceleration!!
Problem 7
A car runs out of gas and coasts to a stop
on flat ground. Draw a free body diagram
of the forces acting on the car.
Even though the
car is coasting,
there is still the
dragging friction
of the road (left
pointing arrow)
as well as gravity
and normal
forces. That is all
the car is in
contact with so
we are done!
What about the force of
the car’s motion???
There is no such thing as
force of motion. All
forces must be exerted
by an identifiable object
(gravity is exerted by
the earth)
Only focus on what is
happening NOW, not
what happened earlier
to get the car moving.
If the motor is running,
you will have a force to
the right.
Problem 8
A car parks on an incline. Draw a free body
diagram of the forces acting on the car.
Mary pulls on a lawnmower parallel to the ground. The mower
moves at a constant speed up the hill.
Which free body diagram best
represents the forces on the mower?

y
Nsurface on mower
1

x

Ff surface on mower
Fstudent on mower

FEarth on mower

y Nsurface on mower
3

Ff surface on mower

FEarth on mower
x

Fstudent on mower
2

Nsurface on mower
y

Ff surface on mower

FEarth on mower
4
y
 x
Fstudent on mower

Nsurface on mower

 x
Ff surface on mower
Fstudent on mower

FEarth on mower
Stacey pulls a mower at 40-degrees relative to the ground.
Which free body diagram best
represents the forces on the mower?
1

y Nsurface on mower

Ff surface on mower
40

FEarth on mower
3
y

Fstudent on mower
Fstudent on mower
400
x

Ff surface on mower
0
x

Nsurface
 on mower
Fstudent on mower

Ff surface on mower

FEarth on mower
400
2

Nsurface
on mower

y
x

FEarth on mower
4
y

Nsurface
 on mower
40 Fstudent on mower

x
Ff surface on mower

FEarth on mower
0
The motion of a car is represented with the motion diagram
below.

vfinal 

vinitial
a
o
x
Which diagram best represents the forces exerted on the car
by the Earth and the Earth’s surface?
1
y
2
y
3
x
x
4
y
x
5
x
y
y
x
Which FBD below best
represents the forces
exerted on the ball as it
moves UP (ignore air
resistance)?

1
Fperson on ball

FEarth on ball

3
Fperson on ball

FEarth on ball
2

FEarth on ball

4
Fperson on ball

FEarth on ball
Which FBD below best
represents the forces
exerted on the ball as it
moves DOWN (ignore
air resistance)?

1
Fperson on ball

FEarth on ball

3
Fperson on ball

FEarth on ball
2

FEarth on ball

4
Fperson on ball

FEarth on ball
Which FBD below best
represents the forces
exerted on the ball at
the TOP of its path?

1
Fperson on ball

FEarth on ball

3
Fperson on ball

FEarth on ball
2

FEarth on ball

4
Fperson on ball

FEarth on ball
5
No forces are exerted on
the ball