Download January 24, 2011

January 24, 2011
Roller Coaster Problem
Work
Why work and energy?
how does it help us?
PHeT - energy of skater
Everyday Definition?
Physics Definition?
Equation: W = F
* Δx
in the direction of motion
Work
Important:
If the object does not move, how
much work is done? (no matter how
much of a force is exerted)
Vector?
Units
If the force is perpendicular to the
direction of motion, is any work done
(by this force)? explain
Everyday Definition?
Symbol
Physics Definition?
How do you find the component of
the force, in the direction of motion?
Causes a change in energy of a system
Work against gravity- visual
Is there any work done, while
moving an object horizontally
at a constant velocity?
sled moves forward
Explain: How you would find the following.
Work done by the person pulling?
Explain
Work done by friction?
Work done by the normal force or the force of
gravity?
The net work done on the "sled of fish"
January 24, 2011
F normal review for loncapa
**Force normal does not always equal force of gravity
In this case, what does the normal force equal?
FN
Fpull
F
Ffriction
pull vertical
Fpull horizontal
Fg
If the object moves 10.0 meters while being pushed, how
much work is done?
AP
Practice A #3
Work when an object goes around in a circle?
If an object is moving at a constant speed, is any work being done on it? explain
(assume no resistance force of any kind)
If an object changes speed due to a force acting on it, is work being done on it?
What happens to the speed of an object moving around in a circle in uniform circular motion? explain
Are there forces acting on an object to cause it to go in a circle? what direction are they? how are they
related to the instantaneous direction of motion?
Do these centripetal forces do work on an object undergoing uniform circular motion? why or why not
When would an object going in a circle experience work? explain
January 24, 2011
AP
AP
Area under a Force vs Displacement graph is work
What direction is the Tension Force?
What direction is the instantaneous
velocity
How do the directions described above
relate to each other?
Why is this import regarding work?
displacement (m)
Force Vs Displacement Graphs - Area under = Work (J)
Area under means what ?
What happens to the crate while being pushed?
Phet forces and motion visual
How much (net) work is done?
between 0 and 5 meters?
between 10 and 20 meters?
18
After being pushed what kind of energy does it have?
20
-1
if we neglect friction, if we don't neglect friction? is there a difference?
Push a crate - demonstration - applet
Calculate the energy used or work done according to this graph?
Teacher Only
Phet forces and motion visual
Kinetic Energy - is energy of ?
Suggestions
· Push crate with no friction first time
· remove walls, to get exactly -10 starting point.
· click on force vs time, and velocity vs time - zoom into either as needed
· Have students use work energy theorem to check number found on the screen.
· Add friction, Push dog because, dog has same static and kinetic friction
Create inquiry for this tool - todo
Ratio's? stopping distances
January 24, 2011
Example:
A cart of 50.0 kg initially moving at 5.0 m/s is pushed forward with a force (by an engine) of 100 N. The
engine operates causing the force to be applied while the cart moves 5.0 m. The cart continues to move without
friction. How fast is the cart moving after the engine pushes the cart forward?
The work done on an object can cause the object to have a change in kinetic energy.
Write out initial conditions
Apply the work-energy theorem
Example situations
· Exert a forward force on a car while moving on a horizontal road causing the car to speed up
· It may slow the car down
· In each of theses cases - the final energy of motion will be different than the initial energy of motion, because
work was done on the car. (energy was added to or taken away from the car)
Special circumstances
· The situation must not have a change in elevation involved in it. (if so we must include potential energy as well)
homework help sample problem
Example:
A cart of 50.0 kg initially moving at 5.0 m/s is pushed forward with a force (by an engine) of 100 N. The engine
operates cause the force to be applied while the cart moves 5.0 m. The cart continues to move without friction.
How fast is the cart moving after the engine pushes the cart forward?
?
January 24, 2011
Hint:
**in this special case: ΔKE = KEf
starts from rest
= KE f - KEi
2.0 x 103 kg
- 0 Joules
21.1 meters
AP -stopping distance ratio
When do we do work against gravity?
= F = m*g
g
h (height) or x
given, vi, vf, mass, coeficient - find stopping distance
discuss mass issues
what happens if you are going twice as fast - to your stopping
distance?
Find PEgravitational
height : measured from the reference point (zero level) ,
positive and negative are important.
Potential Energy Practice problems
January 24, 2011
AP
Hooke's Law
Spring Potential Energy
*applet needed
What would happen if you compressed a spring with an object that could slide without
friction on a horizontal surface and let go?
Spring potential energy depends up what?
Spring Potential Energy
Graph of Spring Potential Energy
What would have a larger K
value, a garage door opener
spring, or the spring in a pen
(spring loaded pen)?
Area under = 1/2 Fmax*x
Hookes law F = kx
relaxed length
Explain (units)
substitute in for Fmax
PEspring = 1/2 kx*x
2
PEspring = 1/2 kx
compressed length
homework help sample problem
Vertical Stretch of a spring
stretched length
relaxed length
January 24, 2011
Remember - Force Vs Displacement Graphs - Area under =Work (J)
x = stretch or compresion ** be careful
*see sample problem d for a visual
Energy Conservation
Work - Change in KE- example
from Homework help
If thrown with the same speed, which has most KE at bottom?
If the mass of the diver is 80.0 kg, what is the PEg at the top and KE at the bottom,
then find how fast they are going at the bottom (vf)
Energy Conservation with Spring Energy
January 24, 2011
Δx
D
Power
P = W/Δt
Roller Coaster Problem
Rate at which work is done or energy is used,
measured in watts (J/s)
Power = Work/time
Power can also be: P = F*v
Work = F*d
P = w/t = F*d/t or F*v
Work is done in raising an elevator upward. The power
consumption of the motor will be related to the rate at which
work is done or, in this case, the rate at which the elevator
(and occupants) is lifted.
P = N*m/s or J/s or w (watt)
Try practice F #1 (page 181)
Power example
Power Practice Problems
If Johnny complete 400 Joules of work in 10 seconds, what is your rate of power consumption?
+ efficiency
What if Suzie does 400 Joules of work, in 5 seconds?
Who does more work? explain?
Who develops more power?
January 24, 2011
Power - cost
review the kwh