Download Lecture 6

Last week …
Position, velocity, acceleration
- The position of an object is the location
of the center of the object. The position
of a person is a point between the eyes.
-Velocity is the rate of change in
position. Velocity describes how f ast
something moves.
-Acceleration describes how fast
does fast change. Acceleration describes
how rapidly an object is speeding up.
Acceleration the rate of change of the
velocity.
Force
Context: Move the cart. Move the car.
Throw the ball.
Definition:
-A force is a push, a pull, or a lift.
-A force is a tendency for change.
-A force makes a mass accelerate, if friction is small.
Examples: Weight force, tension in a string, spring force, pressure,
electric force (alternating),
magnetic force (between a current and a magnet,
between two currents),
nuclear forces
Newton first Law
Context: Taking the Aristotelian idea of
"natural states", the condition of constant
velocity whether it be zero or nonzero is
now considered the "natural state" of all
massive objects.
Definition: Newton’s first law states that
objects move with constant velocity unless there is an unbalanced
external force.
Lecture demos:
Discussion:
Teddy bear on cart, table
-Friction is often an unbalanced force.
cloth and dishes
-If an object stays at rest, if there is no net force.
-If it is at rest (in equilibrium), the net force is zero.
Newton’s second law
Context: How hard do I have to push,
to make a car move?
Definition: Newton’s second law states
Force = mass * acceleration, -i.e. F=ma.
Discussion: The intuitive Aristotelian belief that a
net force is required to keep an object moving with
constant velocity (therefore zero acceleration) is
objectively wrong.
- In many real world friction is the only force
acting on the object. Then the net force is not zero,
and the object decelerates.
Though Sir Isaac
Newton's most famous
equation is F=ma.
Lecture demo:
Sling shot, rocket
chair
Newton’s third law
Context: Pushes are equal and opposite.
Definition: Newton’s third law states that
all forces between two objects occur in
pairs, and these two forces are equal in
magnitude and opposite in direction.
Discussion: To every action there is always
opposed an equal reaction: or the mutual
actions of two bodies upon each other are
always equal, and directed to contrary parts.
— Whatever draws or presses another is as
much drawn or pressed by that other. If you
press a stone with your finger, the finger is
also pressed by the stone.
Newton's third law. The
skaters' forces on each other
are equal in magnitude, and
in opposite directions
Lecture demos:
Rope and skate boards,
Masses and springs
Force, Equilibrium & Newton’s Laws
Force = push or pull.
An object is in equilibrium if it is at rest and the net force is zero.
Newton’s first law:
An object moves with constant velocity if the net force is zero.
An object moves stays at rest if the net force is zero.
Newton’s second law:
The acceleration of an object is equal to the net force divided by
the mass, -i.e. Force = mass * acceleration.
Newton’s third law: Forces between two objects are equal in
magnitude and opposite in direction.