Diagrams and Friction
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Transcript of Diagrams and Friction
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