Download Warm up - Milan Area Schools

Warm up
• 1) How much energy does a 45 kg rock
have when it is at the top of a 70 m cliff?
• 2) How much energy does a 20 kg tricycle
have when it is moving at 7 m/s?
Pass Back Quizzes
Questions? See me outside of
class or when we have free time
Work-Energy Theorem
• Work and Kinetic energy
– Work done = change in KE
– W = KE2 - KE1
Example
• A 900 kg car goes from 10 m/s to 23 m/s,
how much work is done on the car by the
road?
Practice
• If 300 J of work are done on a tricycle of
mass 8 kg, that is initially at rest, what will
it’s final velocity be?
Work-Energy Theorem 2
• Work and Potential Energy
– Work done = change in potential energy
• (only depends on initial and final positions)
– W = mghf – mghi
Example
• How much work is done lifting a 25 kg
object from the ground to a height of 2 m?
• This is the work done by gravity. Since
this work only depends on the initial and
final positions it tells us that gravity is
called a ‘conservative force’
– Conservative forces
• Gravity, elastic force, magnetic force,
electrical force
• Non Conservative Forces
– Stress, Friction, drag
Practice
• How much work is done (by gravity) when
a 30 kg rock falls 20 m?
• How much work is done by gravity when a
weightlifter picks up a weight, lifts it 2 m
over his head, then sets it down?
Total work done
• W = (Kf - Ki) + (Uf – Ui)
Work Lab
Lab
• Design a lab to measure the work done by
a force
• You may use anything in the lab
• One lab sheet per group
• Need: materials, procedure, data,
calculations, what force did the work, and
how much work was done
– Do not use a conservative force in such a way
that you get 0 work done
Homework
Worksheet
Warm Up
• An elevator lifts a person 10 floors (about 30 m).
If the man in the elevator has a mass of 80 kg,
and the elevator starts at rest and on the tenth
floor has a velocity of 9 m/s…
•
A) How much work is done bye the elevator
to change the man’s speed?
•
B) How much work is done by the elevator to
overcome the force of gravity on the man?
•
C) How much total work is done by the
elevator on the man?
Total work done
• W = (Kf - Ki) + (Uf – Ui)
Conservation of Mechanical Energy
• Usually the work done by non conservative external force = 0 so:
• Kf + Uf = Ki + Ui
• or
• Kf - Ki = - (Uf – Ui)
Example
Practice
Pg 191 (P 26, 32, 46, 48)
Homework
Pg 191 (CQ 9, 12, 13; P 25, 40)