1. Free Body Diagrams and Vector Diagrams

3.  Recap from ICT Package  Free Body Diagrams  Types of Component Forces  Vector Diagrams

4.  A force is a push or a pull  A force is a vector  Force has both magnitude and direction  Unit for force is Newtons (N)

5.  When a more than one force acts on a body at the same time, each individual force is called a component force  The total sum of all the component forces acting on an object is the resultant force  For any one object, there can be several component forces, but only be one resultant force.  Sometimes, resultant force is also called “net force”

6.  A component force is always exerted by one object onto another object.  Be careful to distinguish which is the object exerting the force, and which is the object receiving the force.  Force XXX by Object A on Object B  This only applies to component forces, and not resultant force.

7.  When there is no resultant force (or resultant force = 0), the object is said to be balanced  When there is a resultant force, the object is said to be unbalanced.  We will discuss more about balanced and unbalanced forces in this chapter, and later in chapter 5.

8.  A free body diagram is a diagram used to describe the forces acting on an object  It only describes the forces acting on ONE single object, and not other objects connected to it  Forces are represented by arrows.  Direction of arrow represents direction of force  Length of arrow represents magnitude of force  Each arrow must be labelled

9.  Objects are typically represented either using a silhouette of the object itself, or a simplification thereof.  E.g. free-body diagram of a cow in free fall Gravitational Force (Weight) COW Gravitational Force (Weight)

10.  Forces can be classified into 2 broad categories: contact forces & long range forces.  Contact forces require an object to physically touch something else in order for the force to take effect  Non-contact do not require physical contact for the force to take effect

11. Contact ForcesNon-Contact Forces Normal Contact ForceGravitational Force (Weight) FrictionElectric Force TensionMagnetic Force Air Resistance (Drag Force) Electromagnetic Force Thrust, Lift (in airplanes) Nuclear Force

12.  Breaking The Magician’s Code - Levitation Tricks  1) Levitating Girl 1 (https://www.youtube.com/watch?v=OAHsY_w4lc M)https://www.youtube.com/watch?v=OAHsY_w4lc M  2) Levitating Girl 2 (https://www.youtube.com/watch?v=F64L4CtGshw)https://www.youtube.com/watch?v=F64L4CtGshw  3) Flying (https://www.youtube.com/watch?v=O0MOVgzSU_ E)https://www.youtube.com/watch?v=O0MOVgzSU_ E  4) Walking on Water (https://www.youtube.com/watch?v=z0ukAescNC0)https://www.youtube.com/watch?v=z0ukAescNC0

13.  Strictly speaking, any two objects with mass exert attractive gravitational forces on each other (i.e. gravitational force is always a pull force)  However, this force is usually too small to observe in everyday objects  It is only observable for very large masses, i.e. the size of planets or larger.  It is the gravitational force by Earth on the Moon which keeps the moon in orbit around the Earth  Similarly, the gravitational force by Sun on the Earth keeps the Earth in orbit around the sun

14.  Weight is simply the gravitational force exerted by the Earth on an object  Formula: Gravitational Force = mg,  m = mass of the object (which the force is exerting on)  g = gravitational field strength. This is also identical to the acceleration due to free fall, which is 10 ms -2.  It is advisable to use “mg” instead of “W” or any other symbol to represent weight or gravitational force.

15.  To draw gravitational force of a free body diagram, locate the center of gravity of the object (typically the center of the object)  Draw an arrow downwards (towards the center of the Earth)  Label the arrow “mg” mg

16.  Sometimes called “normal reaction force”  The normal contact force is the force objects exert on each other when they press each other  Normal contact force is always a push force  Usually represented by symbol “N” or “n”  “Normal” here refers to “perpendicular”  The direction of normal contact force is always perpendicular to the surface of contact

17.  To draw normal contact force on a free body diagram, locate the surface of contact  Draw a force originating from the middle of the contact surface, going perpendicular and away from the surface  Label the arrow “N” or “n” Object on the ground Object on an inclined surface N N

18.  Tension is the force exerted by a taut string, rope, spring, etc. pulling on an object  Usually represented by symbol “T”  Tension is always a pull force  To draw tension in a free body diagram, locate the point where the string (for e.g.) is pulling the object  Draw an arrow originating from that point, in the same direction as the string  Label the arrow “T”

19.  Consider this scenario:  Ball suspended from the ceiling with a string  Free Body Diagram of the ball: mg T

20.  Consider this scenario:  Object being pulled along the ground by a string  Free Body Diagram: mg N T

21.  Friction is the contact force that opposes of tends to oppose motion between surfaces in contact  There is no symbol for friction. Either spell the word out in full, or use “F friction “  To draw friction in a free body diagram, identify the contact surfaces, and draw an arrow parallel to the surfaces, in the direction which would oppose motion

22.  If an object is moving in one direction, friction acts in the opposite direction  If friction is holding up an object which would otherwise slide away, friction acts in the direction opposing the sliding moving to the right Friction

23.  Air Resistance is similar to friction in that it is a force which opposes motion  There is no air resistance if the object is not moving.  The faster the object is moving, the greater the air resistance.  We will discuss more on air resistance in Forces (Part 3)

24.  Step 1: draw the object first. Do NOT draw other objects connected to it  Step 2: identify the center of gravity, draw and label the mg of the object  Step 3: draw all other component forces. The length of the arrows should be proportional to the magnitude of the force.  Note: NEVER draw the resultant force on a Free Body Diagram!! If there is a need to indicate the resultant force, draw it outside the FBD.

25.  Draw the FBD of an object which is being pulled by a rope up a rough slope mg T Friction N

26. Contact ForcesNon-Contact Forces Normal Contact ForceGravitational Force (Weight) FrictionElectric Force TensionMagnetic Force Air Resistance (Drag Force) Electromagnetic Force Thrust, Lift (in airplanes) Nuclear Force

27.  Free Body Diagrams are to display all the component forces acting on an object  Vector Diagrams help to determine the resultant force, given all the component forces  Note: Vector Diagrams are drawn to scale  Vector diagrams use the “head to tail” method

28.  Step 1: Draw any one component force  Step 2: Draw the next component force starting from the “head” of the previous component force.  Step 3: Repeat Step 2 until all component forces are drawn  Step 4: The resultant force is draw from the tail of the first arrow, to the head of the last arrow.

29. 500 N

35. A picture is hung on the wall with 2 strings from a nail. (a) Draw a free body diagram of the picture. (b) Sketch the vector diagram and determine the resultant force acting on the picture.

36.  Force is a push or pull, unit: Newtons (N)  Force is a vector (magnitude & direction)  Free Body Diagrams  Types of Component Forces  Gravitational Force (weight)  Normal Contact Force  Tension  Friction  Air Resistance (Drag Force)  Vector Diagrams