Understanding Forces and Free Body Diagrams in Physics

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Forces & Free Body Diagrams
Definitions
Dynamics
 -  the study of forces and
the effects they have on motion
Force
 - a push or pull that acts on an
object
Force is not energy or power!
Common Forces
Read
http://www.physicsclassroom.com/class/newtlaws/Lesson-
2/Types-of-Forces
Make a chart with columns: Force,
Definition, Direction of Action,
Type of Force (contact or action at
a distance) and real life examples
Force Body Diagrams
Shows all forces acting on the object
Use a square to represent object
Include a coordinate system
If the object is going up a hill, use the
surface as the x axis
Gravity should be broken down into
components (using the appropriate
coordinate system)
 
Example # 1:  Draw the free
body diagram of a car
accelerating forward
 
Example 2:  Draw the free
body diagram of a biker
moving up a hill while
accelerating down the hill.
Net Force
Also called: resultant force, total
force or sum of all the forces
To determine Net Force, find 
F
In 2-D problems, find 
Fx  and
Fy
Example # 3:  Draw the free body of a
biker going at a constant velocity up a
hill with the values of each force.  The
mass of the biker and bike is 60.0 kg.
The angle of the hill is 40 degrees from
the horizontal. The coefficient of
friction against the biker is 0.0030.
Example # 4:  Two tugboats are pulling a
barge.  Each tugboat pulls with a force
of 10 000 N each with angle of 20
degrees from the forward motion of the
barge. (Note: The tugboats’ 20 degrees
directions are on each side of the
straight forwards direction).  The barge
has a mass of 1.80 X 10
4
 kg.  What is
the acceleration of the barge?
Practice
1.
Draw FBD’s for each of the following situations for the object in bold.  Include all forces involved;
draw in the components for all vector and determine their magnitude using variables or greater
than, equal or less than statements if possible.
a)
You push your 
ruler 
westward  at a constant speed across your desk by applying a force from an angle of
25
o 
above the horizontal
b)
 A 
cup
 hanging from a hook
c)
A 
person
 standing in an elevator that is moving downwards
d)
A 
person
 standing in an elevator that is moving upwards
e)
A 
hockey puck
 sliding freely in a straight line down an ice rink
f)
A 
crate
 being dragged across a floor, with significant friction, by a person pulling on a rope at an angle of
23
o
 above the horizontal.
2.
The force of gravity on a textbook is 18N [down].
a)
What is the net force on the book if it is held stationary?
b)
What is the net force on the book if you drop it?
3.
A football is being thrown to a receiver.  Gravity is pulling down on the ball with a force of 10N
[down].  Air resistance is acting on the ball with a force of 3 N [43
o
 below the horizontal].
4.
Harjot pushes a book with a force of 16N [N34
o
E] while Amandeep pushes on a book with a force
of 23 [N65
o
W].  If Rohin wants to keep the book stationary, with what force does he need to push?
Draw an FBD with your solution.
From Nelson 12 Physics text
Pg. 76#1-6
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Explore the concepts of forces, dynamics, and free body diagrams in physics. Learn about different types of forces, how to draw free body diagrams, calculate net force, and solve example problems involving forces and motion.


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  1. Forces & Free Body Diagrams

  2. Definitions Dynamics - the study of forces and the effects they have on motion Force - a push or pull that acts on an object Force is not energy or power!

  3. Common Forces Read http://www.physicsclassroom.com/class/newtlaws/Lesson- 2/Types-of-Forces Make a chart with columns: Force, Definition, Direction of Action, Type of Force (contact or action at a distance) and real life examples

  4. Force Body Diagrams Shows all forces acting on the object Use a square to represent object Include a coordinate system If the object is going up a hill, use the surface as the x axis Gravity should be broken down into components (using the appropriate coordinate system)

  5. Example # 1: Draw the free body diagram of a car accelerating forward

  6. Example 2: Draw the free body diagram of a biker moving up a hill while accelerating down the hill.

  7. Net Force Also called: resultant force, total force or sum of all the forces To determine Net Force, find F In 2-D problems, find Fx and Fy

  8. Example # 3: Draw the free body of a biker going at a constant velocity up a hill with the values of each force. The mass of the biker and bike is 60.0 kg. The angle of the hill is 40 degrees from the horizontal. The coefficient of friction against the biker is 0.0030.

  9. Example # 4: Two tugboats are pulling a barge. Each tugboat pulls with a force of 10 000 N each with angle of 20 degrees from the forward motion of the barge. (Note: The tugboats 20 degrees directions are on each side of the straight forwards direction). The barge has a mass of 1.80 X 104kg. What is the acceleration of the barge?

  10. Practice Draw FBD s for each of the following situations for the object in bold. Include all forces involved; draw in the components for all vector and determine their magnitude using variables or greater than, equal or less than statements if possible. You push your ruler westward at a constant speed across your desk by applying a force from an angle of 25o above the horizontal A cup hanging from a hook A person standing in an elevator that is moving downwards A person standing in an elevator that is moving upwards A hockey puck sliding freely in a straight line down an ice rink A crate being dragged across a floor, with significant friction, by a person pulling on a rope at an angle of 23oabove the horizontal. The force of gravity on a textbook is 18N [down]. What is the net force on the book if it is held stationary? What is the net force on the book if you drop it? A football is being thrown to a receiver. Gravity is pulling down on the ball with a force of 10N [down]. Air resistance is acting on the ball with a force of 3 N [43obelow the horizontal]. Harjot pushes a book with a force of 16N [N34oE] while Amandeep pushes on a book with a force of 23 [N65oW]. If Rohin wants to keep the book stationary, with what force does he need to push? Draw an FBD with your solution. 1. a) b) c) d) e) f) 2. a) b) 3. 4.

  11. From Nelson 12 Physics text Pg. 76#1-6

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