Transcript Newton`s 2nd Law

Newton’s 2nd Law
Chapter 5
Acceleration and
velocity
• Remember v = d/t and a = v/t
• An object traveling at constant velocity
has zero acceleration
• An object at rest has zero acceleration
• In terms of force, the forces acting on
these objects are balanced
Acceleration and
velocity
• Balanced forces means that the net
force = zero.
• Zero net force = zero acceleration
• Net force is directly proportional to
acceleration
• F~a
Acceleration and
velocity
• Mass resists acceleration
• Acceleration is inversely proportional to
mass.
• This has to do with the inertia of the
object.
• Remember, inertia is the object’s ability
to resist change (Newton 1).
• a ~ 1/m
Newton’s 2nd Law
• Newton’s 2nd Law states: The
acceleration produced by a net force on
an object is directly proportional to the
magnitude of the net force, is in the
same direction as the net force, and is
inversely proportional to the mass of the
object.
Newton’s 2nd Law
• Restated, the law says F = ma
• The units for force:
F = 1N = 1kg•m/s²
Forces
• Forces occur in pairs.
• Gravity is paired with normal force.
• On an airplane, there are four forces
involved.
Forces - Plane
•
•
•
•
Thrust – moves the plane forward
Drag – pulls it backward
Lift – moves it up
Weight – holds it down
Example
• How much thrust must a 30,000 kg
plane develop to achieve an
acceleration of 1.5 m/s² ?
• F = ma
• F = (30000kg)(1.5 m/s²)
• F = 45,000 N
More forces
• Friction is a force that affects motion.
• It acts on materials that are in contact
with each other.
• It is dependent on surface irregularities.
• Friction always acts in a direction to
oppose motion.
Friction
• Friction occurs in solids, liquids, and
gases.
• In air, it is called air resistance.
• In gas, it is called fluid friction.
• Fluids include both gas and liquid. They
both flow.
Free-body diagrams
• Free body diagrams show only the
forces.
• They are merely vector diagrams of all
the forces acting on an object.
Pressure
• Pressure is the amount of force applied
over a unit of area.
• P = F/A
• The unit is the Pascal
• 1 Pa = 1N/m²
Terminal velocity
• When air resistance = weight, then net
force = zero.
• When net force = zero, acceleration =
zero.
• This is called terminal speed or velocity.
Terminal velocity
• For humans skydiving, terminal velocity
is roughly 150 to 200 km/hr.
• For a penny, terminal velocity is
approximately 60 km/hr.
Example
• What is the minimal force required to lift
a 5kg baby out of a crib?
• F = ma
• F = (5kg)(10 m/s²)
• F = 50 N