na dynamics - e-learning
TRANSCRIPT
Dynamics
Lesson ObjectivesAt the end of the lesson, you should be able to:a) Identify forces acting on an object and
draw free body diagram(s) representing the forces acting on the object (for cases involving forces acting in at most 2 dimensions)
b) Explain the effects of friction on the motion of a body
What are forces?• A push or a pull – represented by arrows• S.I. Unit: newton (N)
Contact Forces – force touching the objectNormal Reaction Friction Tension
Push exerted by a surface on an object pressing on it – always perpendicular to the surface.
Force that opposes or tends to oppose motion between surfaces in contact
Pull exerted by a stretched spring, string or rope on an object attached to it.
N
T
f
What are forces?Non-Contact Forces – force not touching the object
Gravitational Force Electric Force Magnetic ForcePull exerted by Earth’s gravity on any object (i.e. weight)
The attractive (pull) or repulsive (push) forces between electric charges.
The attractive (pull) or repulsive (push) forces between magnets.
W
+ -Attractive electric forces between unlike charges.
+ +Repulsive electric forces between like charges.
S N S NAttractive magnetic forces between unlike poles.
S NN SRepulsive magnetic forces between like charges.
Effects of Forces on MotionEffects of a force
Move a body at rest
Increase the speed of a
moving body
Decrease the speed of a
moving body
Change direction of a moving body
Throwing a ballKicking a rolling ball to make it move faster
Using your feet to cushion a rolling ball
slowly to a stop
Kicking a rolling ball back to its
original position
Representation of Forces
Lesson ObjectivesAt the end of the lesson, you should be able to:• Identify forces acting on an object and
draw free body diagram(s) representing the forces acting on the object (for cases involving forces acting in at most 2 dimensions)
Free Body Diagrams• A diagram which shows all the forces
acting on a body• Consists of a sketch of a body and
arrows representing the forces acting on it.WeightNormal reaction forceTensionFriction
Weight (W)• Weight of a body is the gravitational
force exerted by the Earth on the body.• Always acts at the centre of gravity of
a body.• Directed towards the centre of the Earth
(always vertically downwards)
W
Normal Reaction force (N)• Normal reaction force is the force
exerted by the contact surface on the bodyActs perpendicularly to the contact surface
• Two forces act on the body: the Normal reaction force and the weight.
W
N
Tension (T)• The tension is directed along the length
of the string and pulls equally on the support and the body.
• But this is not a free body diagram!There are 3 bodies – rod, string, object
T
T
Tension (T)• Isolate the body as a free-body• Free-body diagram of the body consists
of the tension and weight only.
T
W
Combining all 3 kinds of forces
• The free-body diagram of the crate would consist of• The weight of the crate,• The tension force exerted on the crate,• The contact force of the floor of the crate.
Combining all 3 kinds of forces
• Weight and contact force are equal. No resultant force acting vertically.• Resultant force of crate is due to the
tension force.
Useful or Useless?Friction
Lesson ObjectivesAt the end of the lesson, you should be able to:• Explain the effects of friction on the
motion of a body
Friction• Friction is the contact force that oppose or tends
to oppose motion between surfaces in contact.• Represented by small f • Result from irregularities of surfaces• Must be drawn along the surface of contact
• Slows down or stops an object in motion
Ff
Friction• Air resistance, or drag, is the friction
between a moving object and the air it is moving through.
• Slows vehicles down and uses more fuel• Streamlining helps overcome air resistance
Friction (Positive effects)
• To move forward, you place one foot in front of you and push you other foot backwards on the ground.• The friction between your shoe and the
ground prevents slipping.
Friction (Positive effects)• As the wheel turns, friction
is generated between the wheel and the road.
• At point of contact, wheel is moving backwards.
• Friction acts in the opposite direction, providing the forward force that drives the car.
Friction (Positive effects)
• Without friction, the wheels will spin without helping the car to move forward.E.g. Car stuck in mud
Friction (Negative effects)• Wear and tear of surfaces
e.g. Soles of shoes
Factors that affect frictional forceThe frictional force between two surfaces on a horizontal plane changes with several factors.
• Nature of the surfaces in contact• the rougher the surface the greater is the friction
• Proportional to the force pressing the surfaces together • in the case of horizontal movement, the pressing force is
the weight.
• Independent of the area of contact!• A larger area does not mean greater friction!
AssignmentComplete Worksheet 1in pencil.
We will discuss the answer in class.