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Transcript
FORCES
FORCES
• Force is a vector quantity and is measured in
newtons (1N)
• There are different type of forces:
–
–
–
–
–
–
–
–
–
weight
friction force
normal reaction force
tension force
buoyancy (upthrust) force
lift force
thrust force
compression force
air resistance force
WEIGHT vs MASS
• Weight is is the pull of gravity on a body
W=mg
• If that body is on the surface of the earth,
weight is directed towards centre of the earth
(usually we say it is directed vertically down)
• Mass and weight are not the
same thing – weight
describes force, mass
describes amount of matter
in a body
TENSION
Tension (T) is the force that arises in any body when it is
stretched.
NORMAL FORCE
If a body touches another body, there is a reaction
force (R) between the two bodies. This force is
perpendicular to the body exerting the force
DRAG FORCES
Drag forces are forces that oppose the motion of a body through
a fluid (gas or liquid). They are directed opposite to the velocity
of the body and generally depend on the speed of that body.
Higher speed equals higher drag force.
UP THRUST FORCE
An object placed in a fluid medium will experience up
thrust. If the up thrust force on a body is equal to the
weight, the body will float in the fluid.
FRICTIONAL FORCE
Frictional forces (f) are forces that oppose
the motion of a body f.
FREE BODY DIAGRAM
• Problems involve more than one body and more
than one force.
• We will describe each body separately and only
the forces acting on that body, not the forces that
body exerts something else
Normal force
Weight
ADDING FORCES
• Force is a vector quantity
• If two or more forces act on the same body,
you must add them vectorially
70N
60N
150N
100N
BALANCED FORCES
• If the resultant force on a body is zero, the
forces are said to be balanced
• If a body is in translational equilibrium, it
means that the forces acting on the body, in
all three dimensions, are balanced
100N
100N
NEWTON’S LAWS
• 1st LAW
– The velocity of a body remains constant unless the body is acted
upon by an external force
Body will either remain at rest, or move with constant velocity, unless acted by
an unbalanced force
• 2nd LAW
– The acceleration a of a body is parallel and directly proportional to
the net force F and inversely proportional to the mass m
F=ma
The rate of change of momentum of body is proportional to the applied force
and takes place in direction in which the force acts
• 3rd LAW
– The mutual forces of action and reaction between two bodies are
equal, opposite and collinear.
If body A exerts a force on body B, then body B exerts an equal and opposite
force on body A.