Transcript FORCES

STATICS AND DYNAMICS
4TH ESO
Chemistry and Physics
IES AMES
LOOKING AT FORCES
“Peace cannot be kept by force; it can only
be achieved by understanding.”
Albert Einstein
“A mind at peace, a mind centered and not
focused on harming others, is stronger than
any physical force in the universe.”
Wayne Dyer
LOOKING AT FORCES
“FORCES ARE INVOLVED WHENEVER OBJECTS INTERACT”
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FORCES OF NATURE:
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GRAVITATIONAL: All objects with mass attract each other with a
gravitational force. WEIGHT= m x g
ELECTROMAGNETIC FORCE: They use attraction and repulsion
between positive and negative charges  Atomic structure.
NUCLEAR FORCES:
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WEAK NUCLEAR FORCE: These are the forces involved in the
radioactive decay of atoms.
STRONG NUCLEAR FORCE: Positive protons in the nucleus are held
together by the strong nuclear force.
LOOKING AT FORCES
What about simple pushing and pulling forces?
All the forces involved when objects are in direct contact are electromagnetic forces?
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FRICTION FORCES BETWEEN SOLID SURFACES
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The size of the frictional force depends on:
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The force of friction acts in the opposite direction to the motion
FLUID RESISTANCE (DRAG)
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Drag forces depend on:
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The faster the relative motion is
The viscosity of the fluid
Drag forces try to prevent motion between objects and fluids
NORMAL CONTACT FORCES
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The type of surfaces in contact
How hard the surfaces are pressed together
This is a force that exists wherever two solid surfaces are in contact.
Normal means at 90º to the surfaces.
TENSION
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A stretched wire or a string will exert a tension force that pulls on the object it is
connected to.
The tension force acts along the wire.
LOOKING AT FORCES
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FORCES are caused by the interaction of two
objects.
 They are vector quantities, so they are drawn
as vector arrows.
 A free-body diagram shows all the forces
acting on just one object.
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Balanced beams
Suspended objects
Objects resting against rough and smooth surfaces.
http://www.wisc-online.com/objects/index_tj.asp?objID=TP1502
LOOKING AT FORCES
ADDITION OF FORCES
http://www.walter-fendt.de/ph14e/resultant.htm
http://www.nhn.ou.edu/~walkup/demonstrations/WebTutorials/HeadToTailMethod.htm
SAME DIRECTION
If two vectors have the same direction, their resultant has a
magnitude equal to the sum of their magnitudes and will
also have the same direction.
DIFFERENT DIRECTION
LOOKING AT FORCES
RESOLUTION OF FORCES
A single force can be formed by combining two or more forces. It
can be replaced by, or resolved into two components.
http://www.walter-fendt.de/ph14e/forceresol.htm
LOOKING AT FORCES
THE EFFECT OF A FORCE
Force is an action or agency that causes a
body of mass m a deformation, a rotation,
an acceleration, or an increase in pressure
for the body.
LOOKING AT FORCES
statics
STRETCHING A SPRING
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Elasticity is the property of an object or material which causes it to
be restored to its original shape after distortion.
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A spring is an example of an elastic object- when stretched, it exerts
a restoring force which tends to bring it back to its original length.
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Hooke’s Law: the restoring force is generally proportional to
the amount of stretch.
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Measuring forces: Spring balance
LOOKING AT FORCES
statics
TURNING EFFECT OF FORCES
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CONDITIONS FOR EQUILIBRIUM:
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There is no net force acting in any direction: SF=0
There is no turning effect about any point  The size of the turning
effect is called the MOMENT OF THE FORCE: SM=0
CALCULATING MOMENTS:
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The turning effect or moment depends on:
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COUPLES: When two forces (equal in size, and opposite in
direction but not along the same straight line) are acting, we say they
form a couple.
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The size of force
The distance from the force to that point
MOMENT= FORCE x DISTANCE.
A Couple has no resultant force. It only produces a turning effect
M= F x d
PRINCIPLE OF MOMENTS
sum of the clockwise moments= sum of the anticlockwise moments
LOOKING AT FORCES
dynamics: forces and motion
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What is the link between force and motion?
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Force is needed to start things moving
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If an object is not moving there is NO RESULTANT FORCE acting
on it.
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But, does motion stops if the force is removed?
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ARISTOTLE (384-322 BC): a continuous force is needed to keep things
moving
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GALILEO (1564-1642): forces causes changes in an object’s motion. Forces
are needed:
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To start and stop motion
To change an object’s speed
To change an object’s direction
NEWTON (1642-1727): LAWS OF MOTION (1687)
LOOKING AT FORCES
dynamics: forces and motion
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NEWTON’S FIRST LAW: law of inertia
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If there is no resultant force acting on an
object:
 If it is at rest, it will stay at rest
 If it is moving, it keeps on moving at a
constant velocity (constant speed on a
straight line)
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INERTIA: It is the reluctance to change the
velocity.
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The inertia depends on its mass: a bigger
mass needs a bigger force to overcome its
inertia and change its motion
MOMENTUM: p= m . v
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The greater an object’s momentum, the more
force needed to stop.
Momentum is a vector quantity: it has the
same direction as the velocity of the object.
LOOKING AT FORCES
dynamics: forces and motion
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NEWTON’S SECOND LAW:
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The rate of change of momentum of an object
(acceleration) is directly proportional to the
resultant force acting on it.
F= D(mv)/Dt= m(v-u)/t
Resultant force= mass x change in velocity/time taken
If the mass is constant,
F= m . a
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The change in momentum takes place in the same
direction of that force.
1 N= it is the resultant force needed to give a mass of 1
kg an acceleration of 1m/s2.
LOOKING AT FORCES
dynamics: forces and motion
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NEWTON’S THIRD LAW:
action and reaction
principle.
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For every action, there is
an equal and opposite
reaction
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Forces always act in pairs.
If an object A exerts a force on
an object B, then B exerts an
equal but opposite force on A.
These forces don’t cancel out
because the two forces are
acting on different objects.
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LOOKING AT FORCES
dynamics: forces and motion
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Identifying Newton’s third law pairs
 Each force has the same magnitude
 Each force acts along the same line but
in opposite directions
 Each force acts at the same time
 Each force acts on a different object
 Each force is of the same type
LOOKING AT FORCES
dynamics: forces and motion
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IMPULSE
 The quantity ‘force x time taken’ is know
as impulse.
 It measures the effect of a force.
 It is measured in N s.
 F.t = m(v-u)
 Impulse = Change in momentum