Transcript Lecture 8

Last week …
Collision means the action of
bodies striking or coming together
(touching).
Collisions involve forces which
cause is a sudden change in
velocity.
Collisions can be elastic, meaning
they conserve energy, inelastic,
meaning they do not conserve not
energy, or totally inelastic (or
plastic), meaning the object stick
together.
Work
Context: Do a work out. Carrying a chair upstairs.
Definition: Work = displacement * force
Discussion:
-Work = work done by a force on an object
-Displacement = displacement of the object
-Force = component of a force on the object in the direction of the
displacement
Discussion of work …
-Lifting a 2lbs ball by 2ft: Work = 4lbs-ft
-Lifting a 3kg object by 2m: Work = (weight force) * displacement =
(3kg*9.8m/s2) * 2m = 58.8 kg-m/s2 = 58.8 Joule
-Lowering a 3kg object by 2m: Work = (weight force) * displacement
= (3kg*9.8m/s2) * (-2m) = -58.8 kg-m/s2 = -58.8 Joule
- Joule is the unit of work
-Pulling a heavy, but frictionless cart: Work = zero
-Pulling a cart with a force of 2Newton: Work=2Newton*distance
Simple Machine: An Incline
Context: Humans can not produce large
forces, but they can apply a small force
for a long period of time.
Definition: An incline is a ramp for lifting
heavy objects to a given height.
Discussion:
-Force = Weight * height / (length of ramp)
-Small slope  long ramp  small force
-Work = Force * (length of ramp) = Weight * height
An incline
-Example: Lifting a 100lbs object by 1ft over a 10ft ramp force =10lbs
Simple Machine: A Lever
Context: Work in = Work out.
The work we do on the lever
is equal to the work the lever
does on the object.
A lever
Lect. Demo: Pliers, Hydraulic Jack
Definition:
A lever (from French lever, "to raise", c.f. a levant) is a rigid object that
is used with an appropriate fulcrum or pivot point to multiply the
mechanical force that can be applied to another object.
A lever has a long and a short arm. The short arm creates large forces but
moves short distances.
(Short arm force)*(Short arm distance) = (Long arm force)*(Long arm
distance)
Work in
=
Work out
Simple Machine: A Pulley
Context: Lifting heavy objects
without a ramp.
A pulley
Definition:
A pulley (also called a sheave or block) is a wheel with a groove
between two flanges around its circumference. The groove normally
locates a rope, cable or belt. Pulleys are used to change the direction of
an applied force, or realize a mechanical advantage .
Weight*lift = (pulling force)*(distance pulled)
Work out = Work in
Distance pulled = lift * (number of strings)
Pulling force = weight / (number of strings) mechanical advantage!
Work – Energy Theorem
Context: Newton second law
describes the effect of a force
on the motion of an object:
force = mass * acceleration.
What is the relation between
force and speed.
Illustration of Work-Energy Theorem
Definition:
- If the total work done on some object by all forces is positive, the object
speeds up.
-If the total work done on some object by all forces is negative, the object
slows down.
Work
Definition: Work = displacement * force
-Work = work done by a force on an object
-Displacement = displacement of the object
-Force = component of a force on the object in the direction of the
displacement