Transcript Forces: Newton`s Laws of Motion

Forces: Newton’s Laws of
Motion
Chapters 6 & 7
Force
 Any push or pull exerted on an object.
 The object is the system
 The forces exerted on the system is the environment.
Contact vs. Long-Range Forces
 Contact: acts on an object by touching it.
 Long-Range: exerted without contact. Gravity
 Cause of a force is the agent.
Newton’s 1st Law of Motion
 “An obect that is at rest will remain at rest or an
object that is moving will continue to move in a
straight line with constant speed, if and only if the net
force acting on the object is zero” Sir Isaac Newton
 Law of Inertia.
 Inertia is the tendency of an object not to accelerate.
 Equilibrium: occurs if the net force on an object is
zero.
Newton’s 1st Law of Motion
 The object is at rest, or moving at a constant velocity.
 Net force is anything that disturbs a state of
equilibrium
 Table 6-2 pg. 123
Force Misconceptions
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A force is needed to keep an object moving.
Inertia is a force.
Air does not exert a force
When an object is thrown, the force of the hand that
throws it remains with the object.
Newton’s 2nd Law of Motion
 If an outside force is applied to an object, the object
will accelerate.
 a= F net/m
 Net force is the vector sum of two or more forces on
an object.
 Weightlessness: no contact forces are acting upon an
object.
Friction
 Friction is an opposing force acting on an object.
 Second force acting in the opposite direction of the
applied force on an object.
 Static friction: occurs when there is no relative
motion.
 Kinetic friction: occurs when there is relative motion.
Air Drag
 Frictionlike force exerted when an object moves
through a fluid.
 Force depends on the speed of the motion. Faster
the motion, the greater the force.
When dropping an object
 Initially, it has very little velocity and only a small drag
force
 Gravity is stronger than the upward drag force, so
there is downward acceleration
 As the object’s velocity increases, so does the drag
force.
 Drag force will equal force of gravity.
 No net force, no acceleration. Velocity becomes
constant. Terminal Velocity
Periodic Motion
 Simple harmonic motion: occurs when the force that
restores the object to equilibrium is directly
proportional to the displacement of the object.
 Period: time needed to repeat one complete cycle of
motion.
 Amplitude: maximum distance that the object moves
from equilibrium
Periodic Motion
 Pendulum: demonstrates periodic motion
 Mechanical Resonance: increase in amplitude, occurs
when small forces are applied at regular intervals to a
vibrating or oscillating object.
Newton’s 3rd Law of Motion
 “For every action, there is an equal, opposite
reaction”
 Interactive forces: two forces that are “equal and
opposite”
 F A on B = -FA on B
Four Fundamental Forces
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Gravitational Force
Magnetic
Strong Nuclear Force
Weak Nuclear Force