phy spa - dynamics (free body diagram & friction)
TRANSCRIPT
At 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
• A push or a pull – represented by arrows
• S.I. Unit: newton (N)Contact Forces – surfaces in touch with each other
Normal 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
Non-Contact Forces
Gravitational Force Electric Force Magnetic Force
Pull 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 N
Attractive magnetic
forces between unlike
poles.
S NN S
Repulsive magnetic
forces between like
charges.
Effects 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 ball
Kicking 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
• 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.
Weight
Normal reaction force
Tension
Friction
• 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
W
• Normal reaction force is the force exerted by
the contact surface on the body
Acts perpendicularly to the contact surface
• Two forces act on the body:
the Normal reaction force and the weight.
W
N
• 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: The support, the string and the body.
T
T
• Isolate the body as a free-body
• Free-body diagram of the body consists of the
tension and weight only.
T
W
• 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.
• Weight and contact force are equal. No
resultant force acting vertically.
• Resultant force of crate is due to the tension
force.
At the end of the lesson, you should be able to:
• Explain the effects of friction on the motion of
a body
• 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
F
f
• 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
• 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.
• 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.
• Without friction, the wheels will spin without
helping the car to move forward.
E.g. Car stuck in mud
The 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!