REPRESENTATIONFree-body diagrams

WHAT IS REPRESENTATION?

Definitions Formulas

Diagrams

Graphs

Language of Physics

LEARNING OUTCOMES Identify forces acting on an object and

draw free-body diagrams Solve problems for a static-point massRecall and apply the equation F = ma

to new situation or to solve related problems

FREE-BODY DIAGRAMS A diagram which shows all the forces

acting on a body Consists primarily of a sketch of a

body and arrows representing the forces acting on it

Weight Contact force Tension

EXAMPLEDrawing

Free-Body Diagram

WEIGHT Weight of a body is the gravitational force

exerted by the Earth on the body. The weight acts at the center of gravity of a body. It is always directed towards the center of the Earth

The free-body diagram of a body falling without air resistance is shown below

CONTACT FORCE Contact force is the force exerted by

the contact surface on the bodyActs perpendicularly to the contact surface

Two forces act on the body: they are the contact force and the weight

RECALL: WEIGHT VS. CONTACT FORCE

Contact forces are forces that act on a body at the point of contact between two different bodies

Non-contact forces is a force that acts on a body such as the weight which acts through the center of gravity (CG) of an object

Contact forces Non-Contact forces

WHAT ARE THE FORCES ACTING ON THE BODIES?

The pair of action and reaction forces is the gravitational force by the Earth on the man and the gravitational force by the man on the Earth

There are 2 free-body diagrams : One for the man and the other for the Earth

FREE-BODY DIAGRAM

The free-body diagram of the man shows the force acting on him only

The free-body diagram for the Earth shows the force that acts on the Earth only

WHAT ARE THE FORCES ACTING ON THE BODIES?

The first pair of action and reaction forces is the gravitational force by the Earth on the man and the gravitational force by the man on the Earth

The second pair of action and reaction forces is the contact force by the man on the Earth and the contact force by the Earth on the man

FREE-BODY DIAGRAM

The free-body diagram shows two forces : F1 and F2 acting on the man.

Since the man is at rest , the two forces are equal and opposite in direction

FREE-BODY DIAGRAM

The free-body diagram for the Earth shows two forces , F1 and F2 acting on the Earth

Since F1 = F2 , the Earth is also at rest

THINK!

Which are action-reaction pairs?

WORKED EXAMPLES Gymnast 2 boxes Lift

GYMNAST

How many free bodies?

How many action-reaction pairs?

What are the forces acting on each body?

FREE BODY DIAGRAMSGymnast Beam

FREE BODY DIAGRAMSEarth

ACTION-REACTION PAIRS

TENSION The tension is directed along the length of

the string and pulls equally on the support and the pendulum bob The diagram below is not a free-body diagram as it

shows forces acting on three bodies: the pendulum bob, the string and the support. It will not be useful in analysis.

TENSION Isolate the pendulum bob as a free-

body Only contact force acting on the bob is

the tension Free-body diagram of the pendulum

bob consists of the tension and weight only

PUTTING IT TOGETHER

Weight and contact force are equal. No resultant force acting vertically

Acceleration of the crate is due entirely to the tension force

2 BOXES

2 BOXES

IN A LIFT – DAILY EXPERIENCES (VIDEO)

LIFT PROBLEM – AT REST

LIFT PROBLEM – ACCELERATING UPWARDS

LIFT PROBLEM – CONSTANT VELOCITY

LIFT PROBLEM – ACCELERATING DOWNWARDS

VECTOR DIAGRAMSRecap

VECTOR DIAGRAMS

2 scenariosUnbalanced

forcesBalanced

forces

UNBALANCED FORCES

When 3 forces are applied to a point of an object , the resultant force (due to it being unbalanced) acting on the object can be obtained by adding the force vectors

Vectors A,B and C represent three forces

UNBALANCED FORCES

BALANCED FORCES• When 3 forces acting on an object is

balanced , resultant force is ________.

• There is no _________.

• An object will either ________ or ____________.

BALANCED FORCES Consider a picture hanging on the wall. What are the forces acting on the

picture frame?

FREE-BODY DIAGRAM

Weight

Tension Tension

VECTOR DIAGRAM – BALANCED FORCES

Weight

Tension Tension

Tension

Tension

Weight

UNBALANCED FORCES Consider a boat being towed by 2

other boats.

TensionTension

Boat

Tension

Tension

ResultantForce

COMPARE:Picture on wall(Balanced forces)

Moving boat(Unbalanced forces)

Tension

Tension

Weight

Tension

Tension

ResultantForce

CASE STUDY

VECTOR DIAGRAMUnarranged Arranged

FRICTIONBoon or bane?

LEARNING OUTCOMES Explain the effects of friction on the

motion of a body

WHAT IS FRICTION?

AIR RESISTANCE

SPOILERS

LORRIES

EFFECTS OF FRICTION ON MOTION As a useful force

To move forward, you place one foot in front of you and push your other foot backwards on the ground. The friction between your shoe and the ground prevents you from slipping.

EFFECTS OF FRICTION ON MOTION

As each wheel turns, friction is created at the point of contact between the wheel and the road

At the point of contact, the wheel is moving backwards

Friction acts in the opposite direction and provides the forward force that drives the car

WHEN THERE IS NO FRICTION

Without friction, the wheels would spin without helping the car to move forward.

An example of this is when a car gets stuck in mud or fine sand as shown above

FACTORS THAT AFFECT FRICTION Depends on the nature of the surfaces

in contactThe rougher the surface the greater is the

friction Is proportional to the force pressing

the surfaces together (in the case of horizontal movement, the pressing force is the weight).Pushing heavy vs. light objects

• Is independent of the area of contact

SUMMARY