Forces (Part 1)Free Body Diagrams and Vector Diagrams

Scene from what movie?

Topics

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

Recap from ICT Package – What is a Force?

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)

Recap from ICT Package – Balanced & Unbalanced Forces

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”

Take Note

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.

Recap from ICT Package – Balanced & Unbalanced Forces

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.

Free Body Diagrams

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

Free Body Diagrams

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)

Types of Component Forces

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

Types of Component Forces

Contact Forces Non-Contact ForcesNormal Contact Force Gravitational Force

(Weight)Friction Electric ForceTension Magnetic Force

Air Resistance (Drag Force)

Electromagnetic Force

Thrust, Lift (in airplanes)

Nuclear Force

Half-Time

Breaking The Magician’s Code - Levitation Tricks 1) Levitating Girl 1 (https://

www.youtube.com/watch?v=OAHsY_w4lcM) 2) Levitating Girl 2

(https://www.youtube.com/watch?v=F64L4CtGshw)

3) Flying(https://www.youtube.com/watch?v=O0MOVgzSU_E)

4) Walking on Water(https://www.youtube.com/watch?v=z0ukAescNC0)

Gravitational Force 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

Gravitational Force

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.

Gravitational Force

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

Normal Contact Force

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

Normal Contact Force

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

NN

Tension

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”

Tension

Consider this scenario: Ball suspended from the

ceiling with a string

Free Body Diagram of the ball:

mg

T

Tension

Consider this scenario: Object being pulled

along the ground by a string

Free Body Diagram:

mg

N

T

Friction

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 “Ffriction“

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

Friction

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

Friction

Air Resistance (Drag Force)

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)

Rules for Drawing FBDs

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.

Example

Draw the FBD of an object which is being pulled by a rope up a rough slope

mg

T

Friction

N

Recap – Types of Component Forces

Contact Forces Non-Contact ForcesNormal Contact Force Gravitational Force

(Weight)Friction Electric ForceTension Magnetic Force

Air Resistance (Drag Force)

Electromagnetic Force

Thrust, Lift (in airplanes)

Nuclear Force

Vector Diagrams

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

Vector Diagram

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.

Example

A car is being pulled by 2 ropes as shown below. The 2 forces are at an angle of to the horizontal. Find the resultant force acting on the car.

500 N

500 N

Example

500 N

500 N500 N

500 N

Where is the resultant force?

Where is the resultant force?

Where is the resultant force?

Vector diagram

Worked Example:

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.

Recap

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