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Free Body Diagrams

A look at making 'Free Body Diagrams' through the lens of Newton's Laws of Motion.

Peter Bagnall

on 7 December 2013

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Transcript of Free Body Diagrams

Newton's First Law
Forces in Nature
(The "ROOT" of all movement)
Weak Nuclear
Strong Nuclear
The force holding neutrons and protons together
Provokes radioactive decay and hydrogen fusion in stars
The force which brings positive and negative electric and magnetic charges together
The force between any two objects with mass
Laws of
Are there reasons things move as they do?
What is the cause of motion?
Newton's Second Law
Newton's Third Law
Newton's First Law
An object in motion will stay in motion at a
constant velocity and an object at rest will
stay at rest unless acted upon by an
unbalanced force.
Net Forces
Unbalanced forces are known as 'net' forces; if the net force is zero then the object's velocity is constant.
"Free Body Diagrams"
Diagrams showing all the forces on an object are useful in determining if there is a net force on the object.
If F =0
then v=constant
and a=0
Newton's Second Law
The change in velocity of an object is parallel and directly proportional to the net force acting on the object.
Force and Acceleration
The force and acceleration of an object are also related to how heavy the object is.
F =ma
A free body diagram showing the forces on an object can help in finding the net force and its direction so that we can determine the acceleration.
Newton's Third Law
Forces come in pairs! For every action force there is an equal and opposite reaction force.
The Normal Force
The reaction force exerted when two objects come into contact is known as the normal force. This force is perpendicular to the contact surface and typically written Fn.
In a Free Body Diagram, all the forces on an object are shown including the forces resulting from Newton's Third Law. Thus any normal force must be included.
Sir Isaac Newton
Born 1642, Died 1727
Scientist and Mathematician
Wrote books on Calculus, Optics, and Mechanics
Father of classical mechanics (often known as Newtonian Physics)
A Free Body Diagram
The Force of Gravity
Draw from the centre of
the object in the direction
of the gravitational field
(Usually towards the centre
of the largest mass around)
The Force of Tension
Forces holding objects dangling
from ropes or cords (or apple
stems) are generally referred
to as 'tension' forces. These are
drawn from the centre of the
object in the direction of the
The Force Arrows
The length of the arrow shows the
magnitude of the force. If forces are
of equal magnitude and in opposite
directions, they effectively cancel
each other so that the net force is 0.
The Force of Friction
Don't forget to add the force of
friction to your diagram. If friction
and air resistance are not being
ignored, there is always a force
acting in the opposite direction of
the object's motion. Since our apple
is at rest, there is no friction.
Other forces
Other forces that are commonly
included in free body diagrams are:
-any applied force
-the normal force
Forces are VECTORS, that is, they have both magnitude and direction. Thus they can be added by breaking them down in their horizontal and vertical components on a free-body diagram.
Example 1
Example 2
Lesson Sequence:

Lesson 1: Forces in nature (gravity, electromagnetic, strong nuclear, weak nuclear, friction, tension, etc)
Lesson 2: Newton's 3 laws (intro) with emphasis on 3rd law and normal force.
Lesson 3: Drawing free body diagrams (arrows, representations, conventions, how are they helpful?)
Lesson 4: Newton's first law of motion (inertia)
Lesson 5: Newton's second law of motion (F=ma with investigation)
The Pedagogical
Curriculum Expectations:

(Grade 11. Physics)
C2.1 use appropriate terminology related to forces, including, but not limited to: mass, time, speed, velocity, acceleration, friction, gravity, normal force, and free-body diagrams [C]
C2.4 analyse the relationships between acceleration and applied forces such as the force of gravity, normal force, force of friction, coefficient of static friction, and coefficient of kinetic friction, and solve related problems involving forces in one dimension, using free-body diagrams and algebraic equations (e.g., use a drag sled to find the coefficient of friction between two surfaces) [AI, C]
C2.5 plan and conduct an inquiry to analyse the effect of forces acting on objects in one dimension, using vector diagrams, free-body diagrams, and Newton’s laws [IP, PR, AI, C]
Potential Student Difficulties:

There are several common errors in constructing a free body diagram.
-Types of forces that apply to the subsets of a system can often be left off or misplaced in the free body diagram.
-Students often are not clear which object is being analysed-Drawing a force as acting on the object, even though that force does not act directly on the body
-Students can forget to account for the opposite and equal forces between the analysed objects

Free Body Diagrams are used to visualize and organize all the forces that are acting on a particular object. They represent a simplified sketch of the object in which all the forces that are acting on it are shown. The object itself may be drawn as a dot or a rectangular shape. This will make it easier to see which forces act on a particular object and the relations between them.
In order to draw a correct Free Body Diagram, students need to know a few basic concepts like the types of forces in nature (gravity, electromagnetic, strong nuclear, weak nuclear, friction, tension, etc) and Newton’s laws of motion.

There are a few steps that need to be followed when drawing a Free Body Diagram.
-label all forces
-sketch a coordinate system
-break down forces that are not along the coordinate axes
Thank You!
Applications and Societal Issues/Implications

Newton’s laws of Motion can all be applied to everyday situations that happen around us. Students will discover what happens when they walk, when they ski, when they sit down on their chair, when a rock falls off a cliff and so on. Students will learn all these with the help of group activities, brainstorming, building 3D models and using real time and online demonstrations.
Solutions to Student Difficulties:

To address misconceptions, the teacher can:
-Make sure students have the necessary background to understand the lesson and if not the case, a short review is required before teaching the concept.
-Constantly assess students before, during and after class to make sure that there are no misunderstood concepts among the students using open ended questions, quizzes, activities, exit cards
-Examples that lead to these misconceptions should be introduced in the lesson and be cleared out from the beginning.
-Have a lot of varied examples that students can learn from
-Clear the theory and present different concepts in different ways (e.g. mass vs weight)
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